Is exome-targeted capture sequencing able to detect mutations in genes broadly linked to ataxia, and what is the prevalence of such mutations in a large cohort of undiagnosed patients with various phenotypic presentations?
This cohort study of 319 patients with undiagnosed cerebellar ataxia used a sequencing approach that allowed the identification of genetically relevant variants in known genes in 91 (28.5%). This approach had the highest success rate for patients with spastic ataxia or a cerebellar ataxia with oculomotor apraxia–like phenotype; SPG7, SACS, SETX, SYNE1, and CACNA1A were the most frequently mutated genes.
Mutations were identified in a broad range of genes implicated in ataxia and related neurologic diseases, even in cohorts that underwent previous extensive screening.
Molecular diagnosis is difficult to achieve in disease groups with a highly heterogeneous genetic background, such as cerebellar ataxia (CA). In many patients, candidate gene sequencing or focused resequencing arrays do not allow investigators to reach a genetic conclusion.
To assess the efficacy of exome-targeted capture sequencing to detect mutations in genes broadly linked to CA in a large cohort of undiagnosed patients and to investigate their prevalence.
Design, Setting, and Participants
Three hundred nineteen index patients with CA and without a history of dominant transmission were included in the this cohort study by the Spastic Paraplegia and Ataxia Network. Centralized storage was in the DNA and cell bank of the Brain and Spine Institute, Salpetriere Hospital, Paris, France. Patients were classified into 6 clinical groups, with the largest being those with spastic ataxia (ie, CA with pyramidal signs [n = 100]). Sequencing was performed from January 1, 2014, through December 31, 2016. Detected variants were classified as very probably or definitely causative, possibly causative, or of unknown significance based on genetic evidence and genotype-phenotype considerations.
Main Outcomes and Measures
Identification of variants in genes broadly linked to CA, classified in pathogenicity groups.
The 319 included patients had equal sex distribution (160 female [50.2%] and 159 male patients [49.8%]; mean [SD] age at onset, 27.9 [18.6] years). The age at onset was younger than 25 years for 131 of 298 patients (44.0%) with complete clinical information. Consanguinity was present in 101 of 298 (33.9%). Very probable or definite diagnoses were achieved for 72 patients (22.6%), with an additional 19 (6.0%) harboring possibly pathogenic variants. The most frequently mutated genes were SPG7 (n = 14), SACS (n = 8), SETX (n = 7), SYNE1 (n = 6), and CACNA1A (n = 6). The highest diagnostic rate was obtained for patients with an autosomal recessive CA with oculomotor apraxia–like phenotype (6 of 17 [35.3%]) or spastic ataxia (35 of 100 [35.0%]) and patients with onset before 25 years of age (41 of 131 [31.3%]). Peculiar phenotypes were reported for patients carrying KCND3 or ERCC5 variants.
Conclusions and Relevance
Exome capture followed by targeted analysis allows the molecular diagnosis in patients with highly heterogeneous mendelian disorders, such as CA, without prior assumption of the inheritance mode or causative gene. Being commonly available without specific design need, this procedure allows testing of a broader range of genes, consequently describing less classic phenotype-genotype correlations, and post hoc reanalysis of data as new genes are implicated in the disease.
Coutelier M, Hammer MB, Stevanin G, Monin M, Davoine C, Mochel F, Labauge P, Ewenczyk C, Ding J, Gibbs JR, Hannequin D, Melki J, Toutain A, Laugel V, Forlani S, Charles P, Broussolle E, Thobois S, Afenjar A, Anheim M, Calvas P, Castelnovo G, de Broucker T, Vidailhet M, Moulignier A, Ghnassia RT, Tallaksen C, Mignot C, Goizet C, Le Ber I, Ollagnon-Roman E, Pouget J, Brice A, Singleton A, Durr A, . Efficacy of Exome-Targeted Capture Sequencing to Detect Mutations in Known Cerebellar Ataxia Genes. JAMA Neurol. Published online February 26, 2018. doi:10.1001/jamaneurol.2017.5121
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