INTRODUCTION:Niemann-Pick disease type C1 (NPC1, OMIM 257220) is a rare, progressive, and fatal autosomal recessive lysosomal storage disorder caused by pathogenic variants in the NPC1 gene. These variants disrupt intracellular lipid trafficking, leading to the accumulation of cholesterol and glycosphingolipids and resulting in severe, multisystem dysfunction for which no cure currently exists. MATERIALS AND METHODS:To investigate the potential founder effect and shared ancestry of the p.Ala1035Val variant, we analyzed 30 genetically confirmed NPC1 cases, comprising 18 Brazilian (12 of whom were homozygous) and 12 Portuguese participants (3 of whom were homozygous), each carrying at least one p.Ala1035Val allele. Diagnosis was established by clinical evaluation, biochemical assays, and filipin staining, with molecular confirmation by NPC1 genotyping. RESULTS:All analyzed individuals exhibited a conserved haplotype across the SNVs in exons 6 (c.644 A > G, p.His215Arg), 12 (c.1926G > C, p.Ile642Met), 17 (c.2572 A > G, p.Ile858Val), and 18 (c.2793C > T, p.Asn931=), strongly supporting a shared founder effect consistent with an Iberian-associated ancestral background. Among Brazilian (n = 14), visceral involvement occurred in 10/14 (71.4%), predominantly hepatosplenomegaly (6/14, 42.9%), and developmental/cognitive alterations in 10/14 (71.4%), followed by ataxia or gait disturbance in 4/14 (28.6%). Among the Portuguese (n = 3), all presented with visceral involvement, characterized by hepatosplenomegaly (3/3, 100%); one had developmental delay (1/3, 33.3%), and none exhibited ataxia/gait disturbance. Despite the small sample size, clinical patterns appeared similar between the two groups, with differences likely reflecting sampling variability. DISCUSSION:These findings expand the known variant spectrum of NPC1 in Brazilian and Portuguese populations, supporting a possible founder effect resulting from Portuguese colonisation. They also highlight the clinical value of haplotype analysis as a tool for tracing disease origin and improving stratification in medical settings. Furthermore, they emphasise the importance of refining early diagnostic strategies to optimise patient management and improve outcomes in NPC, while highlighting the need for larger, multicenter studies to corroborate this hypothesis and refine its clinical and genetic implications.

ATP6AP2 splicing variants cause syndromic X-linked intellectual disability Hedera type (XPDS; OMIM#300423) and X-linked parkinsonism with spasticity (MRXSH; OMIM#300911). Alternatively, ATP6AP2 missense variants lead to hepatopathy, immunological abnormalities, cutis laxa and only mild intellectual disability with N-/O-glycosylation defects (ATP6AP2-CDG; OMIM#301045). The disparity between neurological and hepatic ATP6AP2-related disease entities is an ongoing puzzle. We aimed to investigate whether patients with an isolated neurological presentation of ATP6AP2-related disease, consistent with XPDS/MRXSH, also have abnormal glycosylation biomarkers, potentially implicating this as part of the pathological mechanism. We identified three males and one female from three families with ATP6AP2 splicing variants and ID/DD, epilepsy, axial hypotonia, axonal neuropathy and microcephaly; the heterozygous female has a milder phenotype. RNA-Seq in patient-derived fibroblasts validated defective splicing, correlated with lowered ATP6AP2 protein levels in fibroblasts alongside glycosylation abnormalities. We describe defective glycosylation alongside ATP6AP2 splicing variants in four patients, including the first female with ATP6AP2-related disease. This connects more closely the phenotypes of XPDS/MRXSH and ATP6AP2-CDG and indicates that abnormal glycosylation markers may be a consistent feature of splicing variants, and potentially part of the pathological mechanism underlying ATP6AP2-related disease caused by abnormal splicing. We also provide additional evidence that neurodevelopment is uniquely sensitive to the gene dosage of ATP6AP2, linked to the isolated neurological phenotype found in patients with splice variants and the attenuated, but still severe, phenotype of the female in our study. Glycosylation defects can be found in "splicing" forms of ATP6AP2-related diseases, bridging the gap between XPDS, MRXSH and ATP6AP2-CDG.

PMM2-CDG (phosphomannomutase 2-deficiency) is the most prevalent N-glycosylation disorder and results from impairments of PMM2 activity. This disease presents a large variety of pathogenic variants, which cause a wide phenotypical spectrum. This diversity, together with the low number of affected patients, raises the challenge of determining genotype–phenotype correlations in PMM2-CDG. This type of correlation could be highly significant in determining disease progression, prognosis, severity and in developing genome-personalized therapies. Structural analyses offer a valuable approach for assessing the pathogenic mechanisms within the PMM2 protein structure at a molecular level. Such an approach can reveal novel insights into the consequences of missense variants and their relationship with patients'phenotype. In this comprehensive review, we evaluate at a structural level 41 missense mutations in PMM2-CDG, examining their phenotypical characteristics and clinical severity, protein properties and interference at the enzymatic level. This work broadens the understanding of the intricate relationships between genotype and clinical manifestations of PMM2-CDG.

We report on the largest single dataset of patients with PMM2-CDG enrolled in an ongoing international, multicenter natural history study collecting genetic, clinical, and biological information to evaluate similarities with previous studies, report on novel findings, and, additionally, examine potential genotype/phenotype correlations. A total of 137 participants had complete genotype information, representing 60 unique variants, of which the most common were found to be p.Arg141His in 58.4% (n = 80) of participants, followed by p.Pro113Leu (21.2%, n = 29), and p.Phe119Leu (12.4%, n = 17), consistent with previous studies. Interestingly, six new variants were reported, comprised of five missense variants (p.Pro20Leu, p.Tyr64Ser, p.Phe68Cys, p.Tyr76His, and p.Arg238His) and one frameshift (c.696del p.Ala233Argfs∗100). Patient phenotypes were characterized via the Nijmegen Progression CDG Rating Scale (NPCRS), together with biochemical parameters, the most consistently dysregulated of which were coagulation factors, specifically antithrombin (below normal in 79.5%, 93 of 117), in addition to Factor XI and protein C activity. Patient genotypes were classified based upon the predicted pathogenetic mechanism of disease-associated mutations, of which most were found in the catalysis/activation, folding, or dimerization regions of the PMM2 enzyme. Two different approaches were used to uncover genotype/phenotype relationships. The first characterized genotype only by the predicted pathogenic mechanisms and uncovered associated changes in biochemical parameters, not apparent using only NPCRS, involving catalysis/activation, dimerization, folding, and no protein variants. The second approach characterized genotype by the predicted pathogenic mechanism and/or individual variants when paired with a subset of severe nonfunctioning variants and uncovered correlations with both NPCRS and biochemical parameters, demonstrating that p.Cys241Ser was associated with milder disease, while p.Val231Met, dimerization, and folding variants with more severe disease. Although determining comprehensive genotype/phenotype relationships has previously proven challenging for PMM2-CDG, the larger sample size, plus inclusion of biochemical parameters in the current study, has provided new insights into the interplay of genetics with disease. Trial Registration: NCT03173300.

The authors report the natural history of three patients with late-diagnosed Classic Galactosemia (CG) (at 16, 19 and 28 years). This was due to a combination of factors: absence of neonatal screening, absence of some typical acute neonatal symptoms, and negative galactosemia screening. This report underlines the value of neonatal screening and the importance of further diagnostic testing in case of late-onset manifestations.

Niemann-Pick type C (NPC) is a lysosomal storage disorder (LSD) caused by pathogenic variants in either the NPC1 or NPC2 genes, which encode proteins involved in the lysosomal export of unesterified cholesterol. In patients of Western European descent, the p.Ile1061Thr variant in NPC1 is especially prevalent. However, mounting evidence has positioned p.Ala1035Val as the most common variant in Portugal and the second most prevalent variant worldwide. By analyzing 10 Portuguese NPC patients homozygous for p.Ala1035Val, we found an SNP in cis on position 858 (p.Ile858Val), which we hypothesize could have a disease-modifying effect. To address this query, we created variant-specific in vitro models of NPC by stably transducing NPC1-/- ARPE-19 cells with constructs encoding different fluorescently-tagged variants of NPC1, which we used, alongside patient-derived skin fibroblasts, to investigate lysosomal positioning and the trafficking routes elicited by p.Ile1061Thr and p.Ala1035Val (with and without the p.Ile858Val SNP in cis). Our results corroborate the previously described decrease in p.Ile1061Thr-NPC1 trafficking to the lysosome and suggest a similar, if not worse, scenario for the p.Ala1035Val variant, especially when in cis with p.Ile858Val. This is the first reported functional study addressing the impact of the p.Ala1035Val variant at the cellular level, paving the way for novel therapeutic options.

While the identification and diagnosis of congenital disorders of glycosylation (CDG) have rapidly progressed, the available treatment options are still quite limited. Mostly, we are only able to manage the disease symptoms rather than to address the underlying cause. However, recent years have brought about remarkable advances in treatment approaches for some CDG. Innovative therapies, targeting both the root cause and resulting manifestations, have transitioned from the research stage to practical application. The present paper aims to provide a detailed overview of these exciting developments and the rising concepts that are used to treat these ultra-rare diseases.

Congenital disorders of glycosylation (CDG) are a heterogeneous group of genetic defects affecting the synthesis of glycans and their attachment to proteins and lipids to form glycoproteins and glycolipids, respectively. The most common is PMM2-CDG, caused by pathogenic variants in the PMM2 gene. Affected individuals manifest a multisystem phenotype with predominant neurological involvement and, typically, inverted nipples and abnormal fat distribution. Although there is currently no effective treatment for PMM2-CDG, there are several drug repurposing trials under way, such as for celastrol (pre-clinical), acetazolamide (phase 2), and epalrestat (phase 3), as well as research on the possible therapeutic potential of pharmaceutical chaperones, morpholino oligonucleotides, mannose-1-phosphate encapsulated in liposomes, and gene therapy. As yet, no genotype–phenotype correlation has been established for PMM2-CDG. The discovery of genetic modifiers in other glycosylation-associated genes could however help to improve our understanding of the highly variable clinical phenotypic spectrum. The need for genetic models to study the interaction between intragenic and/or intergenic variants increases in parallel with the number of novel genetic variants detected by exome and genome sequencing. Genetic modifiers are variants that either compensate for the deleterious effect of the disease-associated variant or enhance its pathogenicity. The effect of genetic modifiers has generally been studied in common diseases, with only about one quarter of studies focussing on rare diseases. In the context of rare diseases, and specifically for PMM2-CDG, it has been shown that disease severity increases in carriers of both ALG6 c.911T>C/p.(Phe304Ser) and PMM2 c.357C>A/p.(Phe119Leu)/c.422G>A/p.(Arg141His) variants. Other studies have revaled that CDG patients can harbor clinically relevant non-allelic variants at different gene loci. In a recent paper, Vignogna and colleagues used experimental evolution to disentangle the molecular interactions underlying the rescue of phosphomannomutase deficiency in yeast models of the common and well studied PMM2 c.357C>A/p.(Phe119Leu) and c.691G>A/p.(Val231Met) human disease variants by creating homologous versions in yeast (sec53-V238M and sec53-F126L). The yeast mutants exhibited a fitness increase over time (1000 generations) due to the acquisition of spontaneous de novo genetic variants in several compensatory genes. Among these is PGM1, the enzyme that catalyzes the bidirectional glucose1-phosphate (G1P) to glucose-6-phosphate (G6P) reaction. Several PGM1 hypomorphic mutants were found to be able to rescue glycosylation and the growth impairment observed in yeast clones harboring the sec53-V238M and sec53-F126L alleles, although in the latter the compensatory effect was smaller. PGM1 variants were shown to be recessive suppressors of sec53-F126L while being dominant suppressors of sec53-V238M. How do the PGM1 variants ameliorate the deleterious effect of PMM2 variants? The authors proposed that, among other mechanisms, the novel PGM1 variants potentially (a) increase glucose-1,6-bisphosphate synthesis as a dissociated intermediate of the PGM1 phosphoglucomutase reaction or (b) increase glucose-1,6-bisphosphate levels by increasing the flux of the G6P to G1P reaction in relation to the flow in the G1P to G6P direction. In both cases, higher levels of glucose-1,6-bisphosphate, a stabilizer and coactivator of the human PMM2 mutants, could also serve to increase the stability of the yeast PMM2. In conclusion, although these diseases might theoretically be monogenic, in practice the action of modifiers renders them multifactorial. Future studies should now address the potential for rescue of phosphomannomutase deficiency in humans by other genes, as has been accomplished in yeast. While the complexity introduced by the action of genetic modifiers is likely to make it more difficult to understand the pathophysiology of the disease, it may actually be a blessing in disguise if it provides opportunities for therapeutic interventions at other points in the biochemical pathway.

BACKGROUND:Glycogen storage disease type V (GSDV) is an autosomal recessive metabolic condition caused by pathogenic PYGM variants. This is an underdiagnosed condition as it presents with exercise intolerance in children. We reviewed the GSDV cases of a tertiary hospital center to assess diagnostic timing/accuracy, as well as potential clinical/analytical predictors of such factors. METHODS:We retrospectively reviewed all GSDV cases with follow-up in both Pediatric and Adult Metabolic Diseases consultations. We included 28 cases and assessed their hospital record for clinical information. RESULTS:Over 90% of our cases had late diagnoses, with more than 50% being diagnosed in adulthood despite symptom onset in preschool (very late diagnosis). Diagnostic age was lower in patients exhibiting myoglobinuria. Interestingly, patients with a positive family history of GSDV had similar rates of very late diagnoses, likely since the index case was already detected very late in life. Finally, we observe that the R50* variant is associated with increased myoglobinuria and CK elevation, in a dosage-dependent manner. CONCLUSION:We concluded that GSDV is severely underdiagnosed, and that some clinical and analytical aspects of the condition can be more indicative of this diagnosis. Furthermore, we propose for the first time a genotype-phenotype correlation in GSDV. IMPACT:GSDV is a pediatric-onset metabolic disorder that is mostly diagnosed late in the adult age and commonly misdiagnosed. We observed the first genotype-phenotype correlation in GSDV, regarding the common R50* variant. Awareness of GSDV for pediatricians and the overall medical community is vital.

Niemann–Pick type C (NPC, ORPHA: 646) is a neuro-visceral, psychiatric disease caused predominantly by pathogenic variants in the NPC1 gene or seldom in NPC2. The rarity of the disease, and its wide range of clinical phenotypes and ages of onset, turn the diagnosis into a significant challenge. Other than the detailed clinical history, the typical diagnostic work-up for NPC includes the quantification of pathognomonic metabolites. However, the molecular basis diagnosis is still of utmost importance to fully characterize the disorder. Here, the authors provide an overview of splicing variants in the NPC1 and NPC2 genes and propose a new workflow for NPC diagnosis. Splicing variants cover a significant part of the disease-causing variants in NPC. The authors used cDNA analysis to study the impact of such variants, including the collection of data to classify them as leaky or non-leaky pathogenic variants. However, the presence of naturally occurring spliced transcripts can misdiagnose or mask a pathogenic variant and make the analysis even more difficult. Analysis of the NPC1 cDNA in NPC patients in parallel with controls is vital to assess and detect alternatively spliced forms. Moreover, nonsense-mediated mRNA decay (NMD) analysis plays an essential role in evaluating the naturally occurring transcripts during cDNA analysis and distinguishing them from other pathogenic variants’ associated transcripts.

Introduction - Glycogen storage disease type V (GSDV, MIM #232600) is an autosomal recessive metabolic myopathy caused by pathogenic variants in the PYGM gene. The characteristic symptoms of exercise intolerance, myalgia, and cramps, which improve after a few minutes of rest, are frequently unrecognized in affected children. When there is clinical suspicion, the initial approach with a forearm exercise test has diagnostic value by detecting low post-exercise plasma lactate-to-ammonia ratio values. The diagnostic algorithm is followed by genetic testing if the results suggest myophosphorylase deficiency. Methods - This was a retrospective observational study conducted based on reviewing medical records of patients with GSDV in a tertiary hospital. We assessed demographic variables, including the timing of onset and diagnosis, relevant clinical characteristics, and whether genetic testing was performed, including its results. Results/Case Report - Our goal was to review the GSDV cases in our center to assess our cohort's diagnostic timing and clinical and genetic characteristics. We identified 28 patients from 24 families, three with consanguinity. The mean age at the time of the study was 43 years. While most (26/28; 93%) recalled their first symptoms in childhood/adolescence, only 25% (7/28) were diagnosed then. All patients had exercise intolerance and CK elevation, while about half reported the second wind phenomenon. Genetic testing was performed in 22 patients, revealing biallelic PYGM variants (9 homozygous, 13 compound heterozygous) as the most common (p.R50*). Conclusion - GSDV is rare and presents in the pediatric age, with subtle manifestations often underestimated for decades. A late diagnosis may negatively impact the psychosocial development of affected children. It is essential to recognize some unique features that facilitate diagnosis: history of exercise intolerance, the second wind sign, and high resting serum CK levels. Identifying the disease-causing variants in PYGM is currently the gold standard for diagnosis as it is less invasive than performing a muscle biopsy, and may promptly diagnose the condition and avoid wrongful labelling of patients.

INTRODUCTION:The diagnostic approach for adulthood parkinsonism can be challenging when atypical features hamper its classification in one of the two main parkinsonian groups: Parkinson's disease or atypical parkinsonian syndromes (APS). Atypical features are usually associated with non-sporadic neurodegenerative causes.METHODS:Retrospective analysis of patients with a working clinical diagnosis of "atypical" APS and complex parkinsonism. "Atypical" APS were classified according to the diagnostic research criteria and the "4-step diagnostic approach" (Stamelou et al. 2013). When not indicated, the final aetiological diagnosis was prospectively assessed. Brain MRI of progressive supranuclear palsy (PSP) look-alikes was reviewed by a neuroradiologist.RESULTS:Among 18 patients enrolled, ten were assigned to the "atypical" APS and eight to the complex parkinsonism group. In the "atypical" APS group, nine patients had PSP and one had corticobasal degeneration. In the PSP group the median magnetic resonance parkinsonism index was 17.1. A final aetiological diagnosis was established for 11 patients, four from the complex parkinsonism (L-2-hidroxiglutaric aciduria and DiGeorge syndrome) and seven from the "atypical" APS (Perry syndrome, postencephalitic PSP, vascular PSP, and MTP-AT6 mitochondrial disease) group.CONCLUSIONS:In this study, the identification of atypical APS features, as proposed in the "4-step diagnostic approach", successfully guided the investigation of alternative diagnoses. Distinctive non-neurodegenerative etiologies causing "atypical" atypical and complex parkinsonism were uncovered, including acquired (post-encephalitis and vascular) and genetic (MTP-AT6 mitochondrial disease mimicking PSP, described for the first time) ones. In the future, accurate clinical identification and distinction between neurodegenerative and non-neurodegenerative parkinsonism etiologies will allow for refining clinical trials.

INTRODUCTION:Fabry disease is an X-linked lysosomal storage disorder caused by pathogenic variants in the GLA gene, leading to decreased/absent α-galactosidase activity. In clinical practice, enzyme activity and substrate/byproduct accumulation play a role in diagnosis and disease-monitoring biomarkers. However, interpreting biomarker levels is not straightforward and can change according to the underlying GLA protein abnormality.OBJECTIVE:Our goals were to understand how disrupting specific protein regions changes biomarker behaviour and to establish specific patterns for individual variants.METHODOLOGY:We analysed data from the Biochemical Genetics Laboratory regarding GLA variants, GLA enzyme activity (in dried blood spots, plasma or white blood cells), plasma LysoGb3 accumulation, and urinary Gb3 excretion. We assessed correlations, trends, and potential predictor models of biomarker behaviour.RESULTS:We assessed 169 hemizygous male and 255 heterozygous female patients. For both groups, substrate accumulation correlates inversely with GLA activity. Variants affecting residues buried within the protein core or the active site were associated with more severe biomarker changes, while those affecting residues that establish disulfide bonds or are glycosylated were similar to other variants. For each non-truncating variant, we also established specific profiles of biomarker behaviour. Finally, we also designed predictor models of biomarker behaviour based on structural variant information. This study provides the groundwork for the impact of GLA protein variation on GLA activity and substrate accumulation.CONCLUSION:This knowledge is of extreme relevance for diagnostic labs and clinicians, as some genetic variants are challenging to interpret regarding pathogenicity. Assessing whether biomarker changes are in the expected range for a specific variant may help diagnostic evaluation. This study also contributes to recognising non-disease-causing variants, considering their overall biochemical impact, and providing a comparative reference for biomarker discovery studies. In the future, the correlation of these findings with disease severity may be of great relevance for diagnosis and monitoring progression.

Glutaric aciduria type 2 is a rare inborn disease of fatty acid metabolism. The clinical manifestation of this disease is heterogeneous and ranges from severe neonatal forms to mild late-onset forms. We present a case of a previously healthy 22-year-old woman with unexplainable hypoglycemia and encephalopathic hyperammonemia. Acylcarnitine profile and organic acids analysis were compatible with glutaric aciduria type 2. On suspicion of this disease, the patient started supplements with carnitine and riboflavin, along with hemodialysis with a complete recovery. The genetic test confirmed the diagnosis. Glutaric aciduria type 2 has no cure and the metabolic decompensation can be a severe event, but treatable and preventable, if this pathology gets recognized.

Recently, a disorder caused by the heterozygous de novo c.1267C>T (p.R423*) substitution in SLC37A4 has been described. This causes mislocalization of the glucose-6-phosphate transporter to the Golgi leading to a congenital disorder of glycosylation type II (SLC37A4-CDG). Only one patient has been reported showing liver disease that improved with age and mild dysmorphism. Here we report the second patient with a type II CDG caused by the same heterozygous de novo c.1267C>T (p.R423*) mutation thereby confirming the pathogenicity of this variant and expanding the clinical picture with type 1 diabetes, severe scoliosis, and membranoproliferative glomerulonephritis. Additional clinical and biochemical data provide further insight into the mechanism and prognosis of SLC37A4-CDG.

SLC35A2 encodes the X-linked transporter that carries uridine diphosphate (UDP)-galactose from the cytosol to the lumen of the Golgi apparatus and the endoplasmic reticulum. Pathogenic variants have been associated to a congenital disorder of glycosylation (CDG) with epileptic encephalopathy as a predominant feature. Among the sixty five patients described so far, a strong gender bias is observed as only seven patients are males. This work is a review and reports a SLC35A2-CDG in a male without epilepsy and with growth deficiency associated with decreased serum IGF1, minor neurological involvement, minor facial dysmorphism, and camptodactyly of fingers and toes. Sequence analysis revealed a hemizygosity for a novel de novo variant: c.233A > G (p.Lys78Arg) in SLC35A2. Further analysis of SLC35A2 sequence by comparing both orthologous and paralogous positions, revealed that not only the variant found in this study, but also most of the reported mutated positions are conserved in SLC35A2 orthologous, and many even in the paralogous SLC35A1 and SLC35A3. This is strong evidence that replacements at these positions will have a critical pathological effect and may also explain the gender bias observed among SLC35A2-CDG patients.

PMM2-CDG is the most common congenital disorder of glycosylation (CDG) accounting for almost 65% of known CDG cases affecting N-glycosylation. Abnormalities in N-glycosylation could have a negative impact on many endocrine axes. There is very little known on the effect of impaired N-glycosylation on the hypothalamic-pituitary-adrenal axis function and whether CDG patients are at risk of secondary adrenal insufficiency and decreased adrenal cortisol production. Cortisol and ACTH concentrations were simultaneously measured between 7:44 am to 1 pm in forty-three subjects (20 female, median age 12.8 years, range 0.1 to 48.6 years) participating in an ongoing international, multi-center Natural History study for PMM2-CDG (ClinicalTrials.gov Identifier: NCT03173300). Of the 43 subjects, 11 (25.6%) had cortisol below 5 μg/dl and low to normal ACTH levels, suggestive of secondary adrenal insufficiency. Two of the 11 subjects have confirmed central adrenal insufficiency and are on hydrocortisone replacement and/or stress dosing during illness; 3 had normal and 1 had subnormal cortisol response to ACTH low-dose stimulation test but has not yet been started on therapy; the remaining 5 have upcoming stimulation testing planned. Our findings suggest that patients with PMM2-CDG may be at risk for adrenal insufficiency. Monitoring of morning cortisol and ACTH levels should be part of the standard care in patients with PMM2-CDG.

PMM2-CDG is a rare disease, causing hypoglycosylation of multiple proteins, hence preventing full functionality. So far, no direct genotype–phenotype correlations have been identified. We carried out a retrospective cohort study on 26 PMM2-CDG patients. We collected the identified genotype, as well as continuous variables indicating the disease severity (based on Nijmegen Pediatric CDG Rating Score or NPCRS) and dichotomous variables reflecting the patients’ phenotype. The phenotypic effects of patients’ genotype were studied using non-parametric and Chi-Square tests. Seventeen different pathogenic variants have been studied. Variants with zero enzyme activity had no significant impact on the Nijmegen score. Pathogenic variants involving the stabilization/folding domain have a significantly lower total NPCRS (p = 0.017): presence of the p.Cys241Ser mutation had a significantly lower subscore 1,3 and NPCRS (p = 0.04) and thus result in a less severe phenotype. On the other hand, variants involving the dimerization domain, p.Pro113Leu and p.Phe119Leu, resulted in a significantly higher NPCRS score (p = 0.002), which indicates a worse clinical course. These concepts give a better insight in the phenotypic prognosis of PMM2-CDG, according to their molecular base.

Phosphoglucomutase 1 (PGM1) deficiency is a rare genetic disorder that affects glycogen metabolism, glycolysis, and protein glycosylation. Previously known as GSD XIV, it was recently reclassified as a congenital disorder of glycosylation, PGM1-CDG. PGM1-CDG usually manifests as a multisystem disease. Most patients present as infants with cleft palate, liver function abnormalities and hypoglycemia, but some patients present in adulthood with isolated muscle involvement. Some patients develop life-threatening cardiomyopathy. Unlike most other CDG, PGM1-CDG has an effective treatment option,d-galactose, which has been shown to improve many of the patients' symptoms. Therefore, early diagnosis and initiation of treatment for PGM1-CDG patients are crucial decisions. In this article, our group of international experts suggests diagnostic, follow-up, and management guidelines for PGM1-CDG. These guidelines are based on the best available evidence-based data and experts' opinions aiming to provide a practical resource for health care providers to facilitate successful diagnosis and optimal management of PGM1-CDG patients.