Exome sequencing reveals a novel MRE11 mutation in a patient with progressive myoclonic ataxia.

Progressive myoclonic ataxia (PMA) is a clinical syndrome defined as progressive ataxia and myoclonus and infrequent seizures in the absence of progressive dementia. Due to the extremely heterogeneous nature of PMA, a large proportion of PMA cases remain molecularly undiagnosed. The aim of this study was to clarify the molecular etiology of PMA. The patient was a 52-year-old female from consanguineous parents. She developed a jerking neck movement at age 9, which gradually expanded to her entire body. On physical examination at age 47, she exhibited generalized, spontaneous myoclonus that occurred continuously. She also presented with mild limb and truncal ataxia. An electroencephalogram revealed no abnormalities. A brain MRI displayed no atrophy of the cerebellum. Electrophysiological studies suggested myoclonus of a subcortical origin. For further evaluation, we performed exome sequencing, and we identified a novel homozygous missense mutation in the MRE11 gene (NM_005590:c.140C>T:p.A47V). Subsequently, we analyzed the expression of MRE11 and related proteins (RAD50 and NBS1) via Western blot, and they were markedly decreased compared to a healthy control. Mutations in the MRE11 gene have been known to cause an ataxia-telangiectasia-like (ATLD) disorder. Accumulating evidence has indicated that its wide phenotypic variations in ATLD correspond to genotypic differences. Interestingly, our case exhibited a relatively mild decrease in NBS1 compared to previously reported cases of a homozygous missense mutation, which may account for the milder phenotype in this patient. Moreover, together with a recently reported case of an MRE11 mutation, it is suggested that MRE11 mutations can present as PMA.