Coffin-Lowry syndrome (CLS) is a rare X linked mental retardation syndrome characterised by severe psychomotor and growth retardation, distinct facial phenotype, and progressive skeletal malformations. It is caused by mutations in the RPS6KA3 gene located at Xp22.2. In this report we describe a family with CLS consists of three affected males, and two affected females, arising from c.898C>T mutation in RPS6KA3 gene. A 6 year-old, and a 3 year-old boy both had distinct clinical features of Coffin-Lowry syndrome; severe mental and motor retardation, microcephaly, prominent forehead, hypertelorism, large mouth, large ears, large soft hands, puffy tapered fingers, and pectus carinatum. In addition, they had multiple abnormal brain MRI findings. Other siblings presented with a mild and variable phenotype.

Clinical GeneticsVolume 85, Issue 1 p. 96-99 LETTER TO THE EDITOR RSK2 mutation co-segregates with X-linked intellectual disability and attenuated Coffin–Lowry phenotype in a three-generation family I Maystadt, Corresponding Author I Maystadt Centre de Génétique Humaine Correspondence: Isabelle Maystadt Centre de Génétique Humaine Institut de Pathologie et de Génétique B-6041 Gosselies Belgium Tel.: +3271447181 Fax:+3271347861 e-mail: isabelle.maystadt@ipg.beSearch for more papers by this authorA Destree, A Destree Centre de Génétique HumaineSearch for more papers by this authorV Benoit, V Benoit Département de Biologie Moléculaire, Institut de Pathologie et de Génétique, Gosselies, BelgiumSearch for more papers by this authorA Aeby, A Aeby Département de Neurologie Pédiatrique, Université Libre de Bruxelles-Hôpital Erasme, Brussels, BelgiumSearch for more papers by this authorD Lederer, D Lederer Centre de Génétique HumaineSearch for more papers by this authorS Moortgat, S Moortgat Centre de Génétique HumaineSearch for more papers by this authorD Jurkiewicz, D Jurkiewicz Department of Medical Genetics, The Children's Memorial Health Institute, Warszaw, PolandSearch for more papers by this authorM Krajewska-Walasek, M Krajewska-Walasek Department of Medical Genetics, The Children's Memorial Health Institute, Warszaw, PolandSearch for more papers by this authorA Hanauer, A Hanauer Translational Medicine & Neurogenetics, IGBMC, Illkirch, FranceSearch for more papers by this authorGM Thomas, GM Thomas Shriners Hospital Pediatric Research Center, Temple University Medical School, Philadelphia, PA, USASearch for more papers by this author I Maystadt, Corresponding Author I Maystadt Centre de Génétique Humaine Correspondence: Isabelle Maystadt Centre de Génétique Humaine Institut de Pathologie et de Génétique B-6041 Gosselies Belgium Tel.: +3271447181 Fax:+3271347861 e-mail: isabelle.maystadt@ipg.beSearch for more papers by this authorA Destree, A Destree Centre de Génétique HumaineSearch for more papers by this authorV Benoit, V Benoit Département de Biologie Moléculaire, Institut de Pathologie et de Génétique, Gosselies, BelgiumSearch for more papers by this authorA Aeby, A Aeby Département de Neurologie Pédiatrique, Université Libre de Bruxelles-Hôpital Erasme, Brussels, BelgiumSearch for more papers by this authorD Lederer, D Lederer Centre de Génétique HumaineSearch for more papers by this authorS Moortgat, S Moortgat Centre de Génétique HumaineSearch for more papers by this authorD Jurkiewicz, D Jurkiewicz Department of Medical Genetics, The Children's Memorial Health Institute, Warszaw, PolandSearch for more papers by this authorM Krajewska-Walasek, M Krajewska-Walasek Department of Medical Genetics, The Children's Memorial Health Institute, Warszaw, PolandSearch for more papers by this authorA Hanauer, A Hanauer Translational Medicine & Neurogenetics, IGBMC, Illkirch, FranceSearch for more papers by this authorGM Thomas, GM Thomas Shriners Hospital Pediatric Research Center, Temple University Medical School, Philadelphia, PA, USASearch for more papers by this author First published: 17 March 2013 https://doi.org/10.1111/cge.12122Citations: 2Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume85, Issue1Special Issue: BRCA1 and BRCA2January 2014Pages 96-99 RelatedInformation

The Coffin–Lowry syndrome (CLS) is a rare X‐linked semidominant syndrome characterized by severe psychomotor retardation, facial dysmorphism, digit abnormalities and progressive skeletal deformations. CLS is caused by mutations in a gene located in Xp22.2, RPS6KA3. This gene encodes for a growth factor‐regulated serine/threonine protein kinase, RSK2 (ribosomal S6 kinase 2), acting in the Ras‐mitogen‐activated protein kinase signaling pathway. Mutations in the RPS6KA3 gene are extremely heterogeneous and lead to premature termination of translation and/or to loss of phosphotransferase activity of the RSK2 protein. Screening for RSK2 mutations is essential in most cases to confirm the diagnosis as well as for genetic counseling. Here we present 44 novel mutations in RSK2 causing CLS. The overall number of CLS mutations reported now is 128. Thirty‐three percent of mutations are missense mutations, 15% nonsense mutations, 20% splicing errors and 29% short deletion or insertion events. Only four large deletions have so far been found. They are distributed throughout the RPS6KA3 gene, and the majority has been found in a single family. This study further confirms the high rate of new mutations at the RSK2 locus. It is important to consider the possibility of mosaicism when providing genetic counseling in CLS families.

Coffin–Lowry Syndrome (CLS) is an X-linked syndromic form of mental retardation associated with skeletal abnormalities. It is caused by mutations of the Rsk2 gene, which encodes a growth factor regulated kinase. Gene deletion studies in mice have shown an essential role for the Rsk2 gene in osteoblast differentiation and function, establishing a causal link between Rsk2 deficiency and skeletal abnormalities of CLS. Although analyses in mice have revealed prominent expression of Rsk2 in brain structures that are essential for learning and memory, evidence at the behavioral level for an involvement of Rsk2 in cognitive function is still lacking. Here, we have examined Rsk2-deficient mice in two extensive batteries of behavioral tests, which were conducted independently in two laboratories in Zurich (Switzerland) and Orsay (France). Despite the known reduction of bone mass, all parameters of motor function were normal, confirming the suitability of Rsk2-deficient mice for behavioral testing. Rsk2-deficient mice showed a mild impairment of spatial working memory, delayed acquisition of a spatial reference memory task and long-term spatial memory deficits. In contrast, associative and recognition memory, as well as the habituation of exploratory activity were normal. Our studies also revealed mild signs of disinhibition in exploratory activity, as well as a difficulty to adapt to new test environments, which likely contributed to the learning impairments displayed by Rsk2-deficient mice. The observed behavioral changes are in line with observations made in other mouse models of human mental retardation and support a role of Rsk2 in cognitive functions.

The 90 kDa ribosomal S6 serine/threonine kinase 2 gene (RSK2, U08316) has been recently identified as a disease-causing gene in an X-linked disorder, the Coffin–Lowry Syndrome (MIM 303600) characterized by severe mental retardation, facial dysmorphisms and progressive skeletal malformations. To investigate its possible role in cerebral cortex development, we performed RNA in situ hybridization at three stages of human development: day 32 (Carnegie 15), 9 weeks (Carnegie 23) and 13 weeks. RSK2 expression is detected in the embryonic anterior and posterior telencephalon (hippocampus anlagen), mesencephalon, rhombencephalon and cerebellum. RSK2 gene expression is also observed in dorsal root ganglia, cranial nerve ganglia, and sensory epithelium of the inner ear, liver, lung and jaw anlagen. This pattern of expression may be involved in cognitive impairment and facial dysmorphisms found in Coffin–Lowry Syndrome.

Coffin-Lowry syndrome (CLS) is caused by mutations in the RSK2 gene encoding a protein kinase of the Ras signalling pathway. We have studied two point mutations which cause aberrant splicing but do not concern the invariant GT or AG nucleotides of splice sites. The first, an A-->G transition at position +3 of the 5' splice site of exon 6, results in vivo and in vitro in exon skipping and premature translation termination. The natural 5' splice site, although intrinsically weak, is not transactivated under normal conditions. Consequently, replacement of an A/U by a G/U base pairing with U1 snRNA reduces its strength below a critical threshold. The second mutation, an A-->G transition 11 nt upstream of exon 5, creates a new AG near the natural 3' splice site. In vitro this synthetic 3' AG is used exclusively by the splicing machinery. In vivo this splicing event is also observed, but is underestimated because the resulting RSK2 mRNA contains premature stop codons which trigger the nonsense-mediated decay process. We show that a particular mechanism is involved in the aberrant splicing of exon 5, implying involvement of the natural 3' AG during the first catalytic step and the new 3' AG during the second step. Thus, our results explain how these mutations cause severe forms of CLS.

Coffin-Lowry syndrome (CLS) is characterized by cognitive impairment, characteristic facial and digital findings and skeletal anomalies. The gene implicated in CLS encodes RSK2, a serine/threonine kinase acting in the Ras/MAPK signalling pathway. In humans, RSK2 belongs to a family of four highly homologous proteins (RSK1-RSK4), encoded by distinct genes. RSK2 mutations in CLS patients are extremely heterogeneous. No consistent relationship between specific mutations and the severity of the disease or the expression of uncommon features has been established. Together, the data suggest an influence of environmental and/or other genetic components on the presentation of the disease. Obvious modifying genes include those encoding other RSK family members. In this study we have determined the expression of RSK1, 2 and 3 genes in various human tissues, during mouse embryogenesis and in mouse brain. The three RSK mRNAs were expressed in all human tissues and brain regions tested, supporting functional redundancy. However, tissue specific variations in levels suggest that they may also serve specific roles. The mouse Rsk3 gene was prominently expressed in the developing neural and sensory tissues, whereas Rsk1 gene expression was the strongest in various other tissues with high proliferative activity, suggesting distinct roles during development. In adult mouse brain, the highest levels of Rsk2 expression were observed in regions with high synaptic activity, including the neocortex, the hippocampus and Purkinje cells. These structures are essential components in cognitive function and learning. Based on the expression levels, our results suggest that in these areas, the Rsk1 and Rsk3 genes may not be able to fully compensate for a lack of Rsk2 function.

Coffin-Lowry syndrome (CLS) is a syndromic form of X-linked mental retardation that is characterized, in male patients, by psychomotor and growth retardation and various skeletal anomalies. Typical facial changes and specific clinical and radiological hand aspects exhibited by patients are essential clues for the diagnosis. CLS is caused by mutations in a gene that is located in Xp22.2 and that encodes RSK2, a growth-factor-regulated protein kinase. RSK2 mutations are extremely heterogeneous and lead to premature termination of translation and/or loss of phosphotransferase activity. Surprisingly, among a series of 250 patients screened by single-strand conformation polymorphism (SSCP) analysis, in whom a clinical diagnosis of CLS was made, no mutations were detected in 66% (165) of the patients. To determine what proportion of these latter patients have a RSK2 mutation that has not been detected and what proportion have different disorders that are phenotypically similar to CLS, we have, in the present article, investigated, by western blot analysis and in vitro kinase assay, cell lines from 26 patients in whom no mutation was previously identified by SSCP analysis. This approach allowed us to identify seven novel RSK2 mutations: two changes in the coding sequence of RSK2, one intragenic deletion, and four unusual intronic nucleotide substitutions that do not affect the consensus GT or AG splice sites. We have also determined the nucleotide sequence of the promoter region of the RSK2 gene, and we have screened it for mutations. No disease-causing nucleotide change was identified, suggesting that mutations affecting the promoter region are unlikely to account for a large number of patients with CLS. Finally, our results provide evidence that some patients have a disease that is phenotypically very similar to CLS, which is not caused by RSK2 defects. This suggests that there are defects in either additional genes or combinations of genes that may result in a CLS-like phenotype.

The Coffin-Lowry syndrome (CLS) is a rare X linked disorder in which affected males show severe mental retardation with characteristic dysmorphism, most notably affecting the face and hands. The typical facial features consist of a prominent forehead, hypertelorism, a flat nasal bridge, downward sloping palpebral fissures, and a wide mouth with full lips. Mild progression in facial coarsening occurs during childhood and adult life. The hands are broad with soft, stubby, tapering fingers. Other clinical findings include short stature (95%), a pectus deformity (80%), a kyphosis and/or scoliosis (80%), mitral valve dysfunction, and sensorineural hearing loss. The causal gene, RSK2, was identified in 1996 and contains 22 exons which encode a protein of 740 amino acids. Over 75 distinct pathogenic mutations have been identified in 250 unrelated CLS patients.

Mouse embryonic stem (ES) cells remain “pluripotent” in vitro in the continuous presence of leukemia inhibitory factor (LIF). In the absence of LIF, ES cells are irreversibly committed to differentiate into various lineages. In this study we have set up an in vitro assay based on the anti-apoptotic activity of LIF to distinguish pluripotent from “differentiation-committed” ES cells. We have examined the phosphorylation profiles of known (STAT3 and ERKs) and identified new (ribosomal S6 kinases (RSKs) and cAMP-responsive element-binding protein (CREB)) LIF-regulated targets in ES and in ES-derived neuronal cells. We have demonstrated that although STAT3, a crucial player in the maintenance of ES cell pluripotency, is induced by LIF in all cell types tested, the LIF-dependent activation of RSKs is restricted to ES cells. We have shown that LIF-induced phosphorylation of RSKs in ES cells is dependent on ERKs, whereas STAT3 phosphorylation is not mediated by any known MAPK activities. Our results also demonstrate that the LIF-dependent phosphorylation of CREB is partially under the control of the RSK2 kinase.

The ninth in this series of international workshops was held at “Le Bischenberg”, Obernai, Strasbourg, France, on 23–25 August, 1999, the same exquisite location of the fifth workshop organized by Jean-Louis Mandel with the exciting reports on the FMR1-gene discovery. The meeting was organized by the European XLMR Consortium (Jamel Chelly (Paris), Jean-Pierre Fryns (Leuven), Ben Hamel (Nijmegen), Claude Moraine (Tours), Hans-Hilger Ropers (Berlin), and Jean-Louis Mandel (Illkirch). Local organizers were Claude Stoll (Strasbourg) and Jean-Pierre Fryns (Leuven)), although major part of the work of organization was ably performed by Marleen Van Leemputten. The meeting was well attended, with 133 participants from 24 countries. The meeting opened with an invited lecture by Gillian Turner on “The History of X-Linked Mental Retardation.” The first part of the meeting was on X-linked mental retardation (XLMR) (36 platform presentations) with two clinical sessions chaired by Jean-Pierre Fryns, Lisbeth Tranebjaerg, Charles Schwartz and Didier Lacombe, a session on XLMR-mapping chaired by Laurent Villard and John Mulley, a session on new and candidate genes chaired by Hans-Hilger Ropers and Ben Hamel, a session on XLMR-molecular screening chaired by Jean-Louis Mandel and Claude Moraine, and a session on XLMR-new technologies (microarrays in gene finding and mutation analysis) chaired by Ted Brown and Hans Van Bolhoven with excellent contributions by Jean-Louis Mandel, Hans-Hilger Ropers and David Nelson. Ben Hamel presented the Update 2000 on syndromic XLMR genes [Hamel et al., 2000]. The XLMR part ended by a session on “Cellular Molecular Mechanisms in Neurodevelopment and -Function” chaired by Jamel Chelly and David Nelson, with two invited lectures by Ger Ramakers [Ramakers, 2000] and Ligun Luo, and the “Update 2000 on XLMR Linkage Data and New Genes” by Jamel Chelly [Chelly, 2000]. The Fragile X part of the conference (29 platform presentation) included a clinical session (chair: Gillian Turner and Michael Partington), a psychology session (chair: Martine Borghgraef and Claude Stoll), a session on population screening (chair: Angela Vianna-Morgante and James MacPherson), and two sessions on the molecular basis of Fragile X syndrome (chair: Ben Oostra, Peter Steinbach, André Hanauer, Paul Hagerman). The session on 37 posters, on XLMR and Fragile X syndrome, was chaired by Francis Rousseau and Gene Fisch. At the end of the meeting there was a lively discussion to determine the site of the next meeting, that will be in Italy at the Lago di Garda and will be hosted by Daniela Toniolo and Giovanni Neri. The session contained 6 presentations. Maarit Peippo described a family with variable severity of the same symptoms in a mother and her son manifesting microcephaly, hearing loss, epilepsy, ataxia and cerebellar hypoplasia. No genetic mapping-information was given. Muscle-biopsy and VEP were normal. The question was raised whether it could be X-linked or autosomal dominant inheritance. Based on the experience from a later presentation during the workshop, by Elke Holinski-Feder, the possibility of submicroscopic chromosomal telomeric deletion is worth exploring. Didier Lacombe reported 3 cases of the so called W Syndrome. He discussed the degree of clinical variation, that could lead to lack of complete ascertainment from previous estimates of the occurrence of the syndrome. Another two presentations by, respectively, Roger Stevenson, and Charles Schwartz, both from the Greenwood Genetic Center group, tried to analyze 2 clinical findings in greater detail: the hypotonic face and spasticity. Even if the hypotonic face is a nonspecific finding it is possible to analyze the upper, mid and lower third of the face of each patient in a more systematic fashion, but no scoring system was suggested. Based on this systematic approach, families with new syndromes had been picked as candidates for mutations in the ATRX-gene, that definitely has a hypotonic face as a very prominent finding. In the upper part of the face, hypotonia manifests as puffiness, perhaps ptosis of eyelids, or a thick upper eyelid. In the mid-part of the face it manifests as a thick helix and thick alae nasi. In the lower part of the face it manifests as an open mouth and prominent lips. Examples were given of syndromes where this is very prominent such as ATRX, Chudley-Lowry and Coffin-Lowry, Juberg-Marsidi, Carpenter, Holmes-Gang, Finemann-Smith-Myers syndromes, whereas it is not present in X-linked lissencephaly. Sabine Dassay discussed the clinical characteristics in her families with FG syndrome. Previously genetic heterogeneity along the X chromosome had been indicated, but she discussed whether the actual family suffered from a different syndrome, based on clinical dissimilarity. She had performed X inactivation studies in 10 families, and in 5 of them found random X inactivation. Finally, Connie Schrander-Stumpel reported a family with complicated spastic paraplegia and evidence for the disease not mapping to the L1-CAM region, but rather pointing to a possible locus on Xp21.1–Xq21.3 and Xq22.1–Xq26.3. Candidate genes, such as the PLP gene and Synapsin 1 gene, were investigated and no mutations were found. The responsible gene remained to be identified. The initial two presentations in this session dealt with clinical delineation of the mitochondria problems associated with the Mohr-Tranebjaerg syndrome (MTS). Three subsequent talks presented data on two new XLMR syndromes and the analysis of a family with ATR16. The final presentation was an update of the XLMR syndromes. MTS comprises deafness, dystonia and MR. The gene for MTS had previously been cloned [Jin et al., 1996] and was named DDP for deafness/dystonia peptide. Recently, Koehler et al. [1999] provided evidence that DDP, also known as TIM8p, mediates the import of transporters from the cytoplasm into the mitochondrial inner membrane, showing MTS to be a mitochondrial disorder. In view of these findings, two groups presented their analysis, clinically and at the mitochondrial level, of several patients with MTS. Ben Hamel (Nijmegen, Holland) described 2 families with 3 males affected with MTS. In one family, a missense mutation (C233G) in DDP gave rise to deafness, mild MR and dystonia. Results of muscle biopsy and metabolic screening were normal, however, as were MRI and ophthalmologic studies. The second family had a young male with deafness and mild MR associated with severe behavior problems. His uncle was also deaf, had eye movement problems, decreased vision after age 30, cerebellar ataxia, and hyperreflexia without dystonia or MR. This family had a deletion that removed exon 2 of DDP as well as other genomic material beyond DDP. A muscle biopsy revealed an increased number of fat vacuoles that might be associated with decreased mitochondrial ATP production. Metabolic tests demonstrated elevated alanine and results by an oral glucose tolerance test were normal. This family is of some interest due to the variable presentation in the nephew and uncle not all of which could be explained by the progressive nature of the disease in the uncle because the latter did not have MR. Lisbeth Tranebjaerg presented the clinical and laboratory findings in a set of 8 patients with MTS. Seven of the patients had truncating mutations of DDP whereas the eighth, also studied by Hamel's group, had a missense mutation (C233G) that disrupts the ZnF motif in the gene. Of interest was the lack of any apparent genotype/phenotype correlation among these patients. Only one of four patients studied exhibited ragged red fibers on muscle biopsy whereas 4/4 had increased numbers and subsarcolemmal aggregates of mitochondria. Furthermore, there was only a slight reduction in ATP production in the muscle without any complex specificity. One other interesting finding was noted: the absence of TIM13p, that is related to TIM8p/DDP, in the MTS patients analyzed. The significance of this is not understood at present. Elke Holinski-Feder presented a 5 generation family in which the affected individuals had moderate MR and minor anomalies. Linkage analysis showed this family to actually link to 16p13.3 rather than the X-chromosome. Subsequent analysis demonstrated a subtelomeric deletion of 16p in the affected and a balanced translocation t(3q;16p) in the carriers. Thus, the condition in this family fits into the ATR16 syndrome (MR, mild ∝-thalassemia, and non-specific minor anomalies). No mention was made of any family members having trisomy 3q. The fourth paper was to have been presented by David Cabezas, but difficulties with US Immigration authorities prevented him from attending the meeting. The abstract dealt with the localization between DXS424(Xq23) and DXS1047 (Xq25) of a new XLMR syndrome delineated by the presence of short stature, small testes, muscle wasting and tremor. Frank Kooy described the linkage analysis in an XLMR family in which the affected males had exhibited mild MR, choreoathetosis, episodic and spastic hypertonia and arachnodactyly. This new XLMR syndrome mapped to Xp11, flanked by DXS1201 (Xp11.3) and DXS1190 (Xp11.2). Suggested potential candidate genes included GPR34, a G-protein receptor and KCND1, an ion channel gene. The last presentation of the session was an XLMR syndrome update by Ben Hamel. At this time (August 23, 1999), 124 specific XLMR were identified. His summation of new XLMR syndromes, refined localizations, and isolation of new genes is presented elsewhere in this report [Hamel et al., 2000]. Since the localizations of MRX1 and MRX2 in 1988, the number of mapped but overlapping loci for nonspecific X-linked mental retardation (MRX) has increased to 75 worldwide (August 1999). These are genetically heterogeneous families. They segregate genes for cognitive impairment in men (and women where inactivation is skewed). There are no syndromal characteristics. A two-point or multipoint lod score of at least +2 is mandatory to demonstrate linkage (preferably with exclusion from the remainder of the X chromosome). Localizations are the intervals between the closest flanking recombination points in each family. These large MRX families remain an extremely rare but invaluable resource for mutation screening leading to the identification of genes for familial MRX. The present indications are that large numbers of MRX genes await discovery and that most will account for very few of the families that map across their locations. This means that true MRX genes from among the candidate genes tested may remain unidentified. This can be due to chance, where families with that defective gene are absent from the sample of families tested. Testing may be required using significantly more than 75 MRX families to exclude a candidate gene as an MRX gene. Smaller families large enough to infer sex linkage are now valued for the purpose of identification of genes responsible for familial MRX. This is especially true when supported by suggestive linkage to, and exclusion from, specific regions of the X chromosome. The families ascertained so far have been skewed heavily toward the severely or moderately retarded range of cognitive dysfunction. FMR2 is the only gene identified to date that is consistently responsible for milder mental impairment. It is also responsible for a relatively large proportion of the families that have nonspecific XLMR. Ascertainment of such families would have been difficult without the FRAXE marker given the relatively mild and variable presence of measurable mental impairment. The location of other genes for similar mild dysfunction of cognitive ability will remain elusive due to the difficulties in both the ascertainment of these families and in making the distinction between affected and normal men. Locating autosomal genes for nonspecific mental retardation remains as a future challenge, but is an approachable goal using homozygosity mapping in consanguineous families with affected sibships. John Mulley reported on a woman with a balanced translocation t(X;12)(q24;q15) and bipolar affective disorder, seizures and MR. A gene interrupted by the translocation breakpoint had been identified: glutamate receptor subunit 3 (GR1A3) that is a neurotransmitter and involved in neuronal plasticity. No mutations, however, have been found so far in one family with an X-linked bipolar affective disorder nor in MRX27. Additional families with non-specific XLMR will be analyzed. The Heidelberg group presented results of the study of 14 Xp22.3 deletion patients (4 with MR, 10 without MR). A gene VCX-A (Variably Charged, X chromosome) was isolated, belonging to a gene family that contains at least 4 paralogs on Xp22.3: VCX-A, B, B1, and C. All deletion-patients without MR had either VCX-A or B or both, whereas those with MR lacked both VCX-A and B, suggesting that one intact copy is enough for normal intelligence. In 5 linked families, however, no mutations were found. Helger Yntema reported on Xq21 deletions, associated with a contiguous gene syndrome consisting of deafness (DFN3, gene POU3F4), choroideremia (CHM, gene REP-1) and MRX. A novel gene was isolated, that was called RSK4, based on its homology to the known RSKs (e.g., RSK2 in Coffin-Lowry syndrome and MRX). No mutations have been found so far in a large cohort of familial MR, compatible with X-linked inheritance. Lin Jun presented a male patient with a syndromic form of MR and a pericentric inversion inv(X)(p21.1;q22). Molecular analysis of the Xq breakpoint (Xp breakpoint contained no MR gene) demonstrated a novel gene: TLP1, homologous to TAP gene, a nuclear transport gene. The function of TLP1 is unknown. It is expressed in fetal brain. Results of mutation screening are pending. Vincent Des Portes performed a systematic mutational analysis of Doublecortin in 11 females (10 sporadic and 1 familial) with subcortical laminar heterotopia (SCLH). Doublecortin is the gene involved in X-linked SCLH/lissencephaly syndrome. In 10/11 cases mutations (nonsense, missense, splice site) were found. A geno/phenotype correlation was not found. Doublecortin mutations were also found in 3/16 males with sporadic lissencephaly. As late breaking news, Jamel Chelly presented exciting data on 2 new genes involved in non-specific XLMR. The first gene named IL1 receptor accessory protein-like = IL1RAPL (because of its homology to IL1RACP), was isolated studying two overlapping microdeletions in Xp22.1–p21.3 in patients with non-specific XLMR. A stopmutation was found in only one (out of 150 tested probands) small family with non-specific XLMR. The function of IL1RAPL is still largely unknown, but it might be involved in neurotransmitter release and in the formation of long-term potentiation in the hippocampus and synaptic plasticity. The second gene named Tetraspanin (belonging to the transmembrane 4 superfamily = TM4SF), was detected after cloning the X breakpoint in a female with MR and autistic behavior and a balanced t(X;2)(p11.4;p21.3). Subsequently, in 2 families with non-specific XLMR mutations (missense and nonsense respectively) were found. A major characteristic of the Tetraspanin family is its ability to associate with other surface molecules such as α and β integrins. Its function, however, is also largely unknown, but Tetraspanin could play a role in actin cytoskeleton organization and activation of MAP kinase cascades. X-linked mental retardation (XLMR) is a very heterogeneous condition and comprises specific entities (MRXS) and non-specific mental retardation (MRX). More than 70 clinically different entities have been described in the MRXS subgroup and 20 of the corresponding genes are cloned. MRXS, however, seems to be much less frequent than MRX that accounts for more than 2/3 of all XLMRs. Collaborative linkage studies performed in MRX families have shown more than 15 linkage intervals that may correspond to MRX genes, suggesting great genetic heterogeneity. Up to now, molecular studies performed by 1) FISH physical mapping and positional cloning on X-chromosome rearrangements in mentally retarded individuals, and 2) by testing candidate genes that map in the X-chromosome regions defined by linkage studies, have identified only 7 genes mutated in MRX families. At this moment, a great proportion of the different X-chromosome intervals that were delimited for MRX genes by linkage studies are still not known to contain candidate genes or to correspond to one or more of the X-chromosomal rearrangement breakpoints. Systematic testing of the 7 identified MRX genes has shown mutations in a low percentage of families: as presented by Judith Mallolas for PaK3 and by Jamel Chelly's group for oligophrenin 1. These data suggest that the high frequency of MRX in the different populations studied may result from: 1) some frequent particular mutation(s) in one or more known or still unknown MRX gene(s); 2) non-frequent mutations in numerous, still unknown MRX genes, some of them possibly not involved in X-chromosomal rearrangements; or 3) particular mutations in genes that are known to be involved in other X-linked pathologies and especially in MRX. Allelism seems to be a common phenomenon in X-linked pathologies, and at least three examples have been reported so far: L1CAM mutated in X-linked hydrocephalus, MASA and SPG2; ATP7A mutated in Menkes and occipital horn syndromes; PLP mutated both in SPG1 and in Pelizaeus-Merzbacher syndrome. On the other hand, intrafamilial clinical variability is frequently observed in MRX families with known or unknown genes. Therefore, it is suggested to test known MRXS genes in families with specific XLMR conditions mapping in the same X-chromosomal region, but clinically different from the initially described syndrome e.g., 1) Laurent Villard reported in this session a mutation in the XNP gene, different in localization and nature from that found in the ATRX syndrome, in a family originally published as Smith-Fineman-Myers syndrome, or 2) Sylvie Jacquot described a particular mutation in the RSK2 gene in a family with non-specific mental retardation, without obvious Coffin-Lowry syndrome findings. For the same reason, Hans van Bokhoven suggested studying MRX families by searching mutations in some parts of MRXS genes that are not specifically mutated in the syndromal form. Finally, it can not be excluded that the frequency of mutations observed in the different genes involved in MRX may be different in different populations. Collaborative studies as reported by Gena Nothwang from the European XLMR consortium will greatly enhance both 1) the number of X-chromosomal rearrangements available for searching new genes disrupted or inactivated at their breakpoint, and 2) the number of mapped MRX families available to test the corresponding candidate genes or others for different mutations at different sites. Currently data suggest that there may be up to 100 genes involved in nonspecific XLMR (MRX) and perhaps an equal number underlying syndromal mental retardation. The known genes each have a modest contribution to the prevalence of MRX (0.5–1%), that confers major problems to the implementation/inclusion of these genes in a DNA-diagnostic routine. With today's DNA-diagnostic protocols it is feasible and sensible to test only those genes that are more frequently causally involved in mental retardation, such as the FMR1 gene. Consequently, only a small fraction of all mental retardation may be explained molecularly. To increase the number of positive DNA-based diagnoses it will be necessary to develop novel high-throughput mutation analysis protocols, suitable for the identification of unknown mutations in large numbers of genes. Several novel developments that are potentially useful for this were discussed in this session. These new techniques may be useful not only for mutation analysis of known MRX genes, but may be applied also for the identification of additional MRX genes. Microarray technology has a high potential for fulfilling that demand. Several methods are used to produce microarrays of either oligonucleotides or cDNA fragments. Affymetrix Gene Chip technology is well suited for screening major genes that are involved in frequently occurring diseases, e.g., p53 in tumors, but obviously this does not apply to MRX. To date, no microarrays are available that were designed for analysis of MRX gene(s). Until such arrays become available alternative high-throughput approaches have to be used, of which D-HPLC and direct sequencing on 96 capillary machines seem to be the most robust ones. People in the MRX field, however, are not sitting back waiting for new technology to come by. For example, H.H. Ropers and J.L. Mandel both alluded to their initiatives to develop cDNA microarrays for the detection of new mutations in known and yet unknown MRX genes. According to Ropers, such arrays may be used for linkage studies (by using SNPs), deletion screening (by comparative genomic hybridization, CGH), mutation screening and expression profiling. J.L. Mandel also envisages that expression profiling with such arrays may be used for the identification of additional MRX genes. The idea behind this is that many of the genes that will be involved in normal brain development will be expressed in cultured cells as in EBV-transformed lymphoblasts and fibroblasts. A mutation in an MRX gene may cause a significant reduction (e.g., by nonsense-mediated RNA decay) or even the total absence (e.g., due to a microdeletion) of transcription. In the experiment, fluorescent cDNA probes will be prepared from total RNA isolated from these cell lines and hybridized onto cDNA microarrays in which for example all X-chromosomal genes or a selection of brain-specific genes are represented. Qualitative and quantitative determination of the hybridization pattern will thus demonstrate a cell-specific expression profile. By comparing the expression profiles of cells derived from MRX patients with those of control individuals it may be possible to identify genes that are mutated in patients or genes that are active in cellular processes involving an MRX gene. Although little experience is yet available, it seems that the sensitivity of this technique has yet to be improved to be successful. Microarrays are being considered also for mutation detection. H.H. Ropers is working along two lines. The first is a primer-elongation method, in which gene-specific oligonucleotides containing an NH2 group at their 5′-terminus are attached to coated glass slides. For each target sequence wild-type and mutant oligonucleotides are designed that contain at their 3′ end the normal or changed nucleotide, respectively. The oligonucleotides can be elongated by one nucleotide (primer extension) in a reaction that contains denatured PCR products of the desired target gene as a template. This method has been applied successfully in a blind experiment in which patients were screened for a variety of all known mitochondrial mutations. Again, this method seems to be suitable only for mutation detection in genes that are frequently involved in human disease. The second protocol that was presented may have a more general application: the construction of a universal mutation detection chip. This chip will consist of all 4096 possible 6-mer oligonucleotides. To increase the stability of homoduplex formation these oligonucleotides will contain a peptide backbone, so-called PNAs. Fluorescent PCR products from exons of one or more genes are hybridized to the PNA-based microarray and analysis of the hybridization pattern should reveal possible mutations. D. Nelson discussed a third high-throughput mutation detection protocol. Rather than being based on high-density arrays, this method uses microspheres to covalently link desired target molecules. The microspheres, a product of Luminex (Austin, TX), are color-coded by the use of two dyes in different combinations, that currently produces 100 different color-coded microspheres. Similar as in the array-based method, primers can be attached onto these microspheres and elongated by one (fluorescent) nucleotide. The combination of a unique microspheres and the fluorescent signal produced by the addition of one of the four possible nucleotides yields a new unique fluorescent signal that can be recognized by laser technology. The reactions are carried in liquid environment with all possible microspheres and DNA targets to be analyzed in one tube. After the reactions, individual particles are separated by flow-cytometry and individually analyzed by laser (20,000 microspheres per second). Because of the relative ease of linking oligonucleotides to the microspheres this method seems to be an attractive alternative for microarray-based methods. The use of more dyes will exponentially increase the number of unique color-coded microspheres, and thus the potential for their use in the mutation analysis of genetically complex disorders such as XLMR. Of course, other considerations such as speed, reliability, reproducibility, sensitivity and costs are crucially important aspects to consider which method will be most attractive for implementation in a regular DNA diagnostic routine. Over the last two years, the repertoire of genes involved in non-specific XLMR has expanded very rapidly. At first sight and because of the molecular heterogeneity of primary defects, it would be difficult to point out common physiopathological mechanisms underlying cognitive impairment and intellectual handicap that are the main manifestations in mental retardation conditions. Despite this high degree of apparent heterogeneity, however, some MRX gene products, such as RhoGAP (oligophrenin1) [Billuart et al., 1998], PAK3 [Allen et al., 1998], and TM4SF2/integrin complex [Zemni et al., 2000], seem to have specific biological functions that regulate shared as well as subgroup-specific signaling pathways involving Rho family GTPases to modulate actin cytoskeleton organization (Fig. 1). This potential common characteristic may suggest that the cellular primary defect associated with loss of function mutations in some MRX genes could be the impaired ability of actin cytoskeleton to drive neurite outgrowth leading therefore to aberrant neuronal morphogenesis, dendritogenesis and connectivity between neuronal cells. This hypothesis is supported by reliable data showing that axonal extension, dendritogenesis and synaptogenesis are driven by actin polymerization within the growth cone, a highly dynamic structure at the tip of the axones and dendrites, consisting of filipodial and lamellipodial protrusions that respond to both positive and negative external guidance cues [Luo et al., 1996; Zipkin et al., 1997; Threadgill et al., 1997; Hall et al., 1998]. The identification of further MRX genes is essential for the better comprehension of the physiopathological mechanisms underlying MR. Putative signal transduction pathways in growth cones and schematic illustration of potential interconnections between MRX genes through their involvement in the regulation of Rho family GTPases activity and actin cytoskeleton organization. Extracellular guidance cues (L) interact with growth cone receptors (R), that in turn activate signaling cascades involving Rho-like GTPases. Activated, GTP-bound, RhoGTpases stimulate filopodia and lamellipodia formation or induce growth cone collapse. Dysfunction of PAK3, RhoGAP, TM4SF2 and FGD1 proteins (bold and underlined proteins) is associated with mental retardation. FGD1, the product of the faciogenital dysplasia or Aarskog-Scott syndrome locus [Pasteris et al., 1994], is a guanine nucleotide exchange factor (GEF) [Zheng et al., 1996]. About 30% of patients affected by this syndrome have nonspecific mental retardation [Fryns., 1992]. N-WASP [Miki et al., 1998], neural protein highly homologous to Wiskott-Aldrich syndrome protein (WASP) [Derry et al., 1994]. The tetraspanin/integrin complexes can activate Rac and Rho GTPases through mechanisms reviewed by Gioncotti [1997]. Dotted arrows indicate oversimplified pathways for which proteins and biochemical partners involved are not fully identified. This session started with two papers with a very similar theme. The first was by Christine de Die-Smulders and her colleagues in Maastricht and Leuven and the second was by Claude Stoll and Yves Alembik from Strasbourg. Between them these groups presented 27 children (24 boys and 3 girls) with the fragile X syndrome where the diagnosis had been missed by the referring clinician. There was no single explanation. In some the diagnosis had just not been considered sometimes because the clinical manifestations were not specific, in some because no family history had been taken, or if taken and found positive, the implications had been disregarded and in some the diagnosis was just incorrect (e.g., the Sotos syndrome, Ehlers-Danlos syndrome, the Prader-Willi syndrome). The presenters of both papers made strong pleas for continued education on the fragile X syndrome to all professionals in medicine, psychology and education with exhortations to have a high index of suspicion about the fragile X syndrome and if in doubt to test appropriately for it. The third paper was by Rebecca O'Connor and her colleagues in Denver, Colorado. These workers studied 12 boys out of 108 with the fragile X syndrome chosen because they had severe language delay defined as failure to use meaningful three word phrases by 5 years of age. No significant differences were found in these 12 compared to the rest of the group in physical or behavioral index scores and CTG repeat number, the presence of partial or full methylation or the presence of less than 15% expression of FMR1 protein in peripheral lymphocytes. It was noted that none of the boys with substantial amounts of FMRP had severe language delay. Elizabeth Hi

Conference Abstract| October 01 2000 SIGNALING TO CHROMATIN: FROM CELL PROLIFERATION TO PHYSIOLOGICAL RESPONSES N. Cermakian; N. Cermakian 1CNRS, Institut de Génétique et de Biologie Moléculaire et Cellulaire, B. P. 163, Illkirch - Strasbourg, France Search for other works by this author on: This Site PubMed Google Scholar C. Crosio; C. Crosio 1CNRS, Institut de Génétique et de Biologie Moléculaire et Cellulaire, B. P. 163, Illkirch - Strasbourg, France Search for other works by this author on: This Site PubMed Google Scholar R. Loury; R. Loury 1CNRS, Institut de Génétique et de Biologie Moléculaire et Cellulaire, B. P. 163, Illkirch - Strasbourg, France Search for other works by this author on: This Site PubMed Google Scholar G. M. Fimia; G. M. Fimia 1CNRS, Institut de Génétique et de Biologie Moléculaire et Cellulaire, B. P. 163, Illkirch - Strasbourg, France Search for other works by this author on: This Site PubMed Google Scholar A. Hanauer; A. Hanauer 1CNRS, Institut de Génétique et de Biologie Moléculaire et Cellulaire, B. P. 163, Illkirch - Strasbourg, France Search for other works by this author on: This Site PubMed Google Scholar C. D. Allis; C. D. Allis °University of Virginia, Charlottesville, USA Search for other works by this author on: This Site PubMed Google Scholar P. Sassone-Corsi P. Sassone-Corsi 1CNRS, Institut de Génétique et de Biologie Moléculaire et Cellulaire, B. P. 163, Illkirch - Strasbourg, France Search for other works by this author on: This Site PubMed Google Scholar Author and article information Publisher: Portland Press Ltd Online ISSN: 1470-8752 Print ISSN: 0300-5127 © 2000 Biochemical Society2000 Biochem Soc Trans (2000) 28 (5): A118. https://doi.org/10.1042/bst028a118c Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Facebook Twitter LinkedIn Email Cite Icon Cite Get Permissions Citation N. Cermakian, C. Crosio, R. Loury, G. M. Fimia, A. Hanauer, C. D. Allis, P. Sassone-Corsi; SIGNALING TO CHROMATIN: FROM CELL PROLIFERATION TO PHYSIOLOGICAL RESPONSES. Biochem Soc Trans 1 October 2000; 28 (5): A118. doi: https://doi.org/10.1042/bst028a118c Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAll JournalsBiochemical Society Transactions Search Advanced Search This content is only available as a PDF. © 2000 Biochemical Society2000 Article PDF first page preview Close Modal You do not currently have access to this content.

American Journal of Medical GeneticsVolume 85, Issue 3 p. 214-215 Letter to the Editor Coffin-Lowry syndrome: Current status Sylvie Jacquot, Sylvie Jacquot Institut de Génétique et de Biologie Moléculaire, et Cellulaire, CNRS/INSERM/ULP, Illkirch, FranceSearch for more papers by this authorKarine Merienne, Karine Merienne Institut de Génétique et de Biologie Moléculaire, et Cellulaire, CNRS/INSERM/ULP, Illkirch, FranceSearch for more papers by this authorElisabeth Trivier, Elisabeth Trivier Institut de Génétique et de Biologie Moléculaire, et Cellulaire, CNRS/INSERM/ULP, Illkirch, FranceSearch for more papers by this authorMaria Zeniou, Maria Zeniou Institut de Génétique et de Biologie Moléculaire, et Cellulaire, CNRS/INSERM/ULP, Illkirch, FranceSearch for more papers by this authorSolange Pannetier, Solange Pannetier Institut de Génétique et de Biologie Moléculaire, et Cellulaire, CNRS/INSERM/ULP, Illkirch, FranceSearch for more papers by this authorAndré Hanauer, Corresponding Author André Hanauer Institut de Génétique et de Biologie Moléculaire, et Cellulaire, CNRS/INSERM/ULP, Illkirch, FranceI.G.B.M.C., B.P. 163, F-67404 Illkirch Cedex, FranceSearch for more papers by this author Sylvie Jacquot, Sylvie Jacquot Institut de Génétique et de Biologie Moléculaire, et Cellulaire, CNRS/INSERM/ULP, Illkirch, FranceSearch for more papers by this authorKarine Merienne, Karine Merienne Institut de Génétique et de Biologie Moléculaire, et Cellulaire, CNRS/INSERM/ULP, Illkirch, FranceSearch for more papers by this authorElisabeth Trivier, Elisabeth Trivier Institut de Génétique et de Biologie Moléculaire, et Cellulaire, CNRS/INSERM/ULP, Illkirch, FranceSearch for more papers by this authorMaria Zeniou, Maria Zeniou Institut de Génétique et de Biologie Moléculaire, et Cellulaire, CNRS/INSERM/ULP, Illkirch, FranceSearch for more papers by this authorSolange Pannetier, Solange Pannetier Institut de Génétique et de Biologie Moléculaire, et Cellulaire, CNRS/INSERM/ULP, Illkirch, FranceSearch for more papers by this authorAndré Hanauer, Corresponding Author André Hanauer Institut de Génétique et de Biologie Moléculaire, et Cellulaire, CNRS/INSERM/ULP, Illkirch, FranceI.G.B.M.C., B.P. 163, F-67404 Illkirch Cedex, FranceSearch for more papers by this author First published: 21 June 1999 https://doi.org/10.1002/(SICI)1096-8628(19990730)85:3<214::AID-AJMG5>3.0.CO;2-1Citations: 10AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article.Citing Literature Volume85, Issue3Special Issue: X‐Linked Mental Retardation, Part III30 July 1999Pages 214-215 RelatedInformation

An unreported missense mutation of the ribosomal S6 kinase 2 (RSK2) gene has been identified in two male sibs with a mild form of Coffin-Lowry syndrome (CLS) inherited from their healthy mother. They exhibit transient severe hypotonia, macrocephaly, delay in closure of the fontanelles, normal gait, and mild mental retardation, associated in the first sib with transient autistic behaviour. Some dysmorphic features of CLS (in particular forearm fullness and tapering fingers) and many atypical findings (some of which were reminiscent of FG syndrome) were observed as well. The moderate phenotypic expression of this mutation extends the CLS phenotype to include less severe mental retardation and minor, hitherto unreported signs. The missense mutation identified may be less deleterious than those previously described. As this mutation occurs in a protein domain with no predicted function, it could be responsible for a conformational change affecting the protein catalytic function, since a non-polar amino acid is replaced by a charged residue.

During the immediate-early response of mammalian cells to mitogens, histone H3 is rapidly and transiently phosphorylated by one or more unidentified kinases. Rsk-2, a member of the pp90(rsk) family of kinases implicated in growth control, was required for epidermal growth factor (EGF)-stimulated phosphorylation of H3. RSK-2 mutations in humans are Linked to Coffin-Lowry syndrome (CLS). Fibroblasts derived from a CLS patient failed to exhibit EGF-stimulated phosphorylation of H3, although H3 was phosphorylated during mitosis. Introduction of the wild-type RSK-2 gene restored EGF-stimulated phosphorylation of H3 in CLS cells. In addition, disruption of the RSK-2 gene by homologous recombination in murine embryonic stem cells abolished EGF-stimulated phosphorylation of H3. H3 appears to be a direct or indirect target of Rsk-2, suggesting that chromatin remodeling might contribute to mitogen-activated protein kinase-regulated gene expression.

We report on the evaluation of a strategy for screening for XNP/ATR-X mutations in males with mental retardation and associated dysmorphology. Because nearly half of the mutations in this gene reported to date fall into a short 300 bp region of the transcript, we decided to focus in this region and to extend the mutation analysis to cases with a negative family history. This study includes 21 mentally retarded male patients selected because they had severe mental retardation and a typical facial appearance. The presence of haemoglobin H or urogenital abnormalities was not considered critical for inclusion in this study. We have identified six mutations which represents a mutation detection rate of 28%. This figure is high enough for us to propose this strategy as a valid first level of screening in a selected subset of males with mental retardation. This approach is simple, does not require RNA preparation, does not involve time consuming mutation detection methods, and can thus be applied to a large number of patients at a low cost in any given laboratory.

Activation by growth factors of the Ras-dependent signaling cascade results in the induction of p90 ribosomal S6 kinases (p90(rsk)). These are translocated into the nucleus upon phosphorylation by mitogen-activated protein kinases, with which p90(rsk) are physically associated in the cytoplasm. In humans there are three isoforms of the p90(rsk) family, Rsk-1, Rsk-2, and Rsk-3, which are products of distinct genes. Although these isoforms are structurally very similar, little is known about their functional specificity. Recently, mutations in the Rsk-2 gene have been associated with the Coffin-Lowry syndrome (CLS). We have studied a fibroblast cell line established from a CLS patient that bears a nonfunctional Rsk-2. Here we document that in CLS fibroblasts there is a drastic attenuation in the induced Ser-133 phosphorylation of transcription factor CREB (cAMP response element-binding protein) in response to epidermal growth factor stimulation. The effect is specific, since response to serum, cAMP, and UV light is unaltered. Furthermore, epidermal growth factor-induced expression of c-fos is severely impaired in CLS fibroblasts despite normal phosphorylation of serum response factor and Elk-1. Finally, coexpression of Rsk-2 in transfected cells results in the activation of the c-fos promoter via the cAMP-responsive element. Thus, we establish a link in the transduction of a specific growth factor signal to changes in gene expression via the phosphorylation of CREB by Rsk-2.

X-linked retinoschisis (XLRS) is the most common cause of juvenile macular degeneration in males, resulting in vision loss early in life. The gene involved in XLRS was identified recently. It encodes a protein with a disoidin domain, suggested to be involved in cell-cell interactions. We have screened the gene for mutations in 234 familial and sporadic retinoschisis cases and identified 82 different mutations in 214 (91%). Thirty one mutations were found more than once, i.e. 2-10 times, with the exception of the 214G --> A mutation which was found in 34 apparently unrelated cases. The origin of the patients, the linkage data and the site of the mutations (mainly CG dinucleotides) indicate that most recurrent mutations had independent origins and thus suggest the existence of a significant new mutation rate in XLRS1. The mutations identified cover the entire spectrum, from small intra-genic deletions (7%), to nonsense (6%), missense (75%), small frameshifting insertions/deletions (6%) and splice site mutations (6%). Since, regardless of the mutation type, no females with a typical RS phenotype were identified, RS seems to be caused by loss-of-function mutations only. Mutations occurred non-randomly, with hotspots at several CG dinucleotides and a Cg stretch. Exons 1-3 contained few, mainly translation-truncating mutations, arguing against an important functional role for this segment of the protein. Exons 4-6, encoding the discoidin domain, contained most, mainly missense mutations. An alignment of 32 discoidin domain proteins was constructed to reveal the consensus sequence and to deduce the functional importance of the missense mutations identified. The mutation analysis revealed a high preponderance of mutations involving or creating cysteine residues, pointing to sites important for the tertiary folding and/or protein function, and highlights several amino acids which may be involved in XLRS1-specific protein-protein interactions. Despite the enormous mutation heterogeneity, patients have relatively uniform clinical manifestations although with great intra-familial variation in age at onset and progression.

Coffin-Lowry syndrome (CLS) is a syndromal form of X linked mental retardation, in which some associated facial, hand, and skeletal abnormalities are diagnostic features. Accurate diagnosis, critical for genetic counselling, is often difficult, especially in early childhood. We have recently shown that Coffin-Lowry syndrome is caused by mutations in the gene encoding RSK2, a growth factor regulated protein kinase. RSK2 mutations are very heterogeneous and most of them lead to premature termination of translation or to loss of phosphotransferase activity or both. In the present study, we have evaluated immunoblot and RSK2 kinase assays as a rapid and simple diagnostic test for CLS, using cultured lymphoblastoid or fibroblast cell lines. Western blot analysis failed to detect RSK2 in six patients, suggesting the presence of truncated proteins in these patients. This conclusion was confirmed in four patients, in whom the causative mutations, all leading to premature termination of translation, were identified. Of four patients showing a normal amount of RSK2 protein on western blot and tested for RSK2 phosphotransferase activity, one had a dramatically impaired activity. Analysis of the RSK2 cDNA sequence in this patient showed a mutation of a putative phosphorylation site that would be critical for RSK2 activity. Preliminary results show that, at least, the western blot protocol can be successfully applied to lymphocyte protein extracts prepared directly from blood samples. These assays promise to become important diagnostic tools for CLS, particularly with regard to very young patients with no family history of the condition.