Compound heterozygous intermediate MJD alleles cause cerebellar ataxia with sensory neuropathy

Spinocerebellar degeneration (SCD) is a group of disorders characterized by progressive ataxia caused by dysfunction and atrophy of the cerebellum or its projections. Approximately one-third of SCD cases are familial SCD, the majority of which are attributed to CAG triplet repeat expansions including spinocerebellar ataxia (SCA)1, SCA2, Machado-Joseph disease (MJD)/SCA3, SCA6, SCA8, SCA12, SCA17, and dentate-rubro-pallido-luysian atrophy (DRPLA). The triplet repeat number of the alleles representing complete penetrance varies among diseases. Generally, there is a gap between the normal alleles and the complete penetrance alleles. Rarely, intermediate alleles with the repeat numbers between the abnormal and normal ranges are observed, although the implications of these intermediate alleles remain ambiguous.