Hereditary adult-onset ataxias are a heterogeneous group of phenotypically overlapping conditions, often caused by pathogenic expansions of short tandem repeats. Currently, 18 repeat disorders with a core phenotype of adult-onset ataxia are known. Diagnosis typically relies on sequential PCR-based methods, which are labour-intensive and lack precision. Long-read sequencing (LRS) has the potential to overcome these limitations and is currently implemented and validated in clinical genetics. Using clinical nanopore Cas9-targeted sequencing (Clin-CATS) for parallel in-depth repeat analysis, we evaluated a diagnostic cohort of 513 adult-onset ataxia patients, determining frequencies of all known repeat-associated ataxias except Spinocerebellar ataxia 4 (SCA4), as well as the carrier frequencies for autosomal-recessive disorders, RFC1 spectrum disorder and Friedreich's ataxia (FRDA). Additionally, phenotypes of patients with established genetic diagnoses were characterized, especially those of patients living with RFC1 spectrum disorder and SCA27B. Repeat-associated ataxias were confirmed in 33.3% of cases, including rare ataxias, such as SCA10, SCA36 and SCA37, alongside as the most prevalent conditions SCA27B and RFC1 spectrum disorder. Potentially pathogenic expansions in FGF14 were identified in an additional 4.7% of patients. Testing of another 347 patients for ZFHX3 expansions linked to SCA4 did not identify any cases. Dual diagnoses were frequent, occurring in 6.4% of patients with repeat-associated ataxia. We confirmed a high RFC1 spectrum disorder carrier frequency (7.2%) and reclassified certain FXN expansions as likely non-pathogenic, resulting in a lower than estimated carrier frequency for FRDA of 0.8%. We also identified novel repeat configurations in several loci and illustrated the high heterogeneity of repeat expansions in RFC1, highlighting it as a potential source of false results when using PCR-based methods. This study underscores the diagnostic advantages of LRS for comprehensive repeat analysis and recommends its adoption as a standard in clinical genetics, replacing Southern blot and PCR-based approaches. Furthermore, based on our findings in a large patient cohort a re-evaluation of existing phenotype-genotype correlations is recommended as well as evaluating additional parameters alongside the repeat length to improve diagnostic precision of repeat analysis.

Most patients with a rare movement disorder (MD) do not receive a molecular diagnosis, and the underlying genetic variants and mediating genes remain elusive. Here, we evaluate the diagnostic accuracy of conventional and next-generation sequencing-based genetic testing strategies in a cohort of 2,811 individuals with ataxia, spastic paraplegia and dystonia. Exome sequencing establishes genetic diagnoses in 19.3% of cases, and specificity of phenotypic features and age at testing are positive predictors. Genome analysis 'beyond the exome' increases the diagnostic yield by 7.5%, mostly due to the improved detection of structural variants and repeat expansions. Unsolved cases are included in the Solve-RD cohort and subjected to gene-burden analysis, providing evidence for loss-of-function variants in X-chromosomal CD99L2 causing spastic ataxia. Cellular studies show that the transmembrane protein CD99L2 occurs mainly in a ubiquitinated form and serves as an activating interactor of the calcium-dependent protease CAPN1. Ablation of cytoplasmic or extracellular domains of CD99L2 leads to its intracellular mislocalization and abrogation of its interplay with CAPN1. Transcriptome analysis in CD99L2 patient-derived fibroblasts reveals synaptic function-specific disturbances. Impaired CAPN1 activation and dysregulation of downstream neuronal pathways constitute the likely molecular cause for neurodegeneration.

Individuals with ultrarare disorders pose a structural challenge for healthcare systems since expert clinical knowledge is required to establish diagnoses. In TRANSLATE NAMSE, a 3-year prospective study, we evaluated a novel diagnostic concept based on multidisciplinary expertise in Germany. Here we present the systematic investigation of the phenotypic and molecular genetic data of 1,577 patients who had undergone exome sequencing and were partially analyzed with next-generation phenotyping approaches. Molecular genetic diagnoses were established in 32% of the patients totaling 370 distinct molecular genetic causes, most with prevalence below 1:50,000. During the diagnostic process, 34 novel and 23 candidate genotype–phenotype associations were identified, mainly in individuals with neurodevelopmental disorders. Sequencing data of the subcohort that consented to computer-assisted analysis of their facial images with GestaltMatcher could be prioritized more efficiently compared with approaches based solely on clinical features and molecular scores. Our study demonstrates the synergy of using next-generation sequencing and phenotyping for diagnosing ultrarare diseases in routine healthcare and discovering novel etiologies by multidisciplinary teams.

BACKGROUND:Short-read genome sequencing (GS) is among the most comprehensive genetic testing methods available, capable of detecting single-nucleotide variants, copy-number variants, mitochondrial variants, repeat expansions, and structural variants in a single assay. Despite its technical advantages, the full clinical utility of GS in real-world diagnostic settings remains to be fully established. METHODS:This study systematically compared singleton GS (sGS), trio GS (tGS), and exome sequencing-based standard-of-care (SoC) genetic testing in 416 patients with rare diseases in a blinded, prospective study. Three independent teams with divergent baseline expertise evaluated the diagnostic yield of GS as a unifying first-tier test and directly compared its variant detection capabilities, learning curve, and clinical feasibility. The SoC team had extensive prior experience in exome-based diagnostics, while the sGS and tGS teams were newly trained in GS interpretation. Diagnostic yield was assessed through both prospective and retrospective analyses. RESULTS:In our prospective analysis, tGS achieved the highest diagnostic yield for likely pathogenic/pathogenic variants at 36.1% in the newly trained team, surpassing the experienced SoC team at 35.1% and the newly trained sGS team at 28.8%. To evaluate which variants could technically be identified and account for differences in team experience, we conducted a retrospective analysis, achieving diagnostic yields of 36.7% for SoC, 39.1% for sGS, and 40.0% for tGS. The superior yield of GS was attributed to its ability to detect deep intronic, non-coding, and small copy-number variants missed by SoC. Notably, tGS identified three de novo variants classified as likely pathogenic based on recent GeneMatcher collaborations and newly published gene-disease association studies. CONCLUSIONS:Our findings demonstrate that GS, particularly tGS, outperforms SoC in diagnosing rare diseases, with sGS providing a more cost-effective alternative. These results suggest that GS should be considered a first-tier genetic test, offering an efficient, single-step approach to reduce the diagnostic odyssey for patients with rare diseases. The trio approach proved especially valuable for less experienced teams, as inheritance data facilitated variant interpretation and maintained high diagnostic yield, while experienced teams achieved comparable results with singleton analysis alone.

SYNE2 mutations have been associated with skeletal and cardiac muscle diseases, including Emery-Dreifuss muscular dystrophy (EDMD). Here, we present a 70-year-old male patient with muscle pain and elevated serum creatine kinase levels in whom whole-exome sequencing revealed a novel heterozygous SYNE2 splice site mutation (NM_182914.3:c.15306+2T>G). This mutation is likely to result in the loss of the donor splice site in intron 82. While a diagnostic muscle biopsy showed unspecific myopathological findings, immunofluorescence analyses of skeletal muscle and dermal cells derived from the patient showed nuclear shape alterations when compared to control cells. In addition, a significantly reduced nesprin-2 giant protein localisation to the nuclear envelope was observed in patient-derived dermal fibroblasts. Our findings imply that the novel heterozygous SYNE2 mutation results in a monoallelic splicing defect of nesprin-2, thereby leading to a rare cause of myalgia and hyperCKemia.

AbstractPurposeShort-read genome sequencing (GS) is a comprehensive genetic testing method capable of detecting multiple variant types. Despite its technical advantages, systemic comparisons of singleton GS (sGS), trio GS (tGS), and exome sequencing-based standard-of-care (SoC) in real-world diagnostics remain limited.MethodsWe systematically compared sGS, tGS, and SoC genetic testing in 448 patients with rare diseases in a blinded, prospective study. Three independent teams evaluated the diagnostic yield, variant detection capabilities, and clinical feasibility of GS as a first-tier test. Diagnostic yield was assessed through both prospective and retrospective analyses.ResultsIn prospective analyses, tGS achieved the highest diagnostic yield for likely pathogenic/pathogenic variants (36.8%) in a newly trained team, outperforming the experienced SoC team (36.0%) and the sGS team (30.4%). Retrospective analyses, accounting for technical variant detection and team experience differences, reported diagnostic yields of 38.6% for SoC, 41.3% for sGS, and 42.2% for tGS. GS excelled in identifying deep intronic, non-coding, and small copy-number variants missed by SoC. Notably, tGS additionally identified three de novo variants classified as likely pathogenic based on recent GeneMatcher collaborations and newly published gene-disease association studies.ConclusionGS, particularly tGS, demonstrated superior diagnostic performance, supporting its use as a first-tier genetic test. sGS offers a cost-effective alternative, enabling faster, more efficient diagnoses for rare disease patients.

Despite linkage to chromosome 16q in 1996, the mutation causing spinocerebellar ataxia type 4 (SCA4), a late-onset sensory and cerebellar ataxia, remained unknown. Here, using long-read single-strand whole-genome sequencing (LR-GS), we identified a heterozygous GGC-repeat expansion in a large Utah pedigree encoding polyglycine (polyG) in zinc finger homeobox protein 3 (ZFHX3), also known as AT-binding transcription factor 1 (ATBF1). We queried 6,495 genome sequencing datasets and identified the repeat expansion in seven additional pedigrees. Ultrarare DNA variants near the repeat expansion indicate a common distant founder event in Sweden. Intranuclear ZFHX3-p62-ubiquitin aggregates were abundant in SCA4 basis pontis neurons. In fibroblasts and induced pluripotent stem cells, the GGC expansion led to increased ZFHX3 protein levels and abnormal autophagy, which were normalized with small interfering RNA-mediated ZFHX3 knockdown in both cell types. Improving autophagy points to a therapeutic avenue for this novel polyG disease. The coding GGC-repeat expansion in an extremely G+C-rich region was not detectable by short-read whole-exome sequencing, which demonstrates the power of LR-GS for variant discovery.

Recently, an exonic GGC repeat expansion (RE) was identified by long-read genome sequencing in the ZFHX3 gen, causing spinocerebellar ataxia type 4 (SCA4), a dominant form of ataxia with sensory neuropathy. However, the analysis of larger cohorts of patients remained demanding, resulting in a challenge to diagnose patients and leaving the question of anticipation in SCA4 unanswered. We aimed to develop a GGC repeat test for clinical SCA4 screening and to apply this test to screen two large German SCA pedigrees and samples of unrelated patients collected over the last 25 years. We modulated a commercial GGC-RE kit (Bio-Techne AmplideX® Asuragen® PCR/CE FMR1 Reagents) with ZFHX3-specific primers and adapted PCR conditions. The test was applied to patients and 50 healthy controls to determine the exact repeat number. Clinical data were revised and correlated with the expanded allele sizes and an exploratory analysis of structural MRI was performed. Repeat size, determined by our protocol for (GGC)n RE analysis shows a strong inverse correlation between repeat length and age at onset and anticipation in subsequent generations. The phenotype also appears to be more strongly expressed in carriers of longer RE. Clinical red flags were slowed saccades, sensory neuropathy and autonomic dysfunction. Our protocol enables cost-effective and robust screening for the causative SCA4 RE within ZFHX3. Furthermore, detailed clinical data of our patients gives a more precise view on SCA4, which seems to be more common among patients with ataxia than expected.