Spinocerebellar ataxia type 17-digenic TBP/STUB1 disease: neuropathologic features of an autopsied patient.

ABSTRACT

Spinocerebellar ataxia (SCA) type 17-digenic TBP/STUB1 disease (SCA17-DI) has been recently segregated from SCA17, caused by digenic inheritance of two gene mutations - intermediate polyglutamine-encoding CAG/CAA repeat expansions (polyQ) in TBP (TBP41 - 49) and STUB1 heterozygosity - the former being associated with SCA17, and the latter with SCA48 and SCAR16 (autosomal recessive). In SCA17, most patients carry intermediate TBP41 - 49 alleles but show incomplete penetrance, and the missing heritability can be explained by a new entity whereby TBP41 - 49 requires the STUB1 variant to be symptomatic. The STUB1 gene encodes the chaperone-associated E3 ubiquitin ligase (CHIP) involved in ubiquitin-mediated proteasomal control of protein homeostasis. However, reports of the neuropathology are limited and role of STUB1 mutations in SCA17-DI remain unknown. Here we report the clinicopathologic features of identical twin siblings, one of whom was autopsied and was found to carry an intermediate allele (41 and 38 CAG/CAA repeats) in TBP and a heterozygous missense mutation in STUB1 (p.P243L). These patients developed autosomal recessive Huntington's disease-like symptoms. Brain MRI showed diffuse atrophy of the cerebellum and T2WI revealed hyperintense lesions in the basal ganglia and periventricular deep white matter. The brain histopathology of the patient shared features characteristic of SCA17, such as degeneration of the cerebellar cortex and caudate nucleus, and presence of 1C2-positive neurons. Here we show that mutant CHIP fails to generate the polyubiquitin chain due to disrupted folding of the entire U box domain, thereby affecting the E3 activity of CHIP. When encountering patients with cerebellar ataxia, especially those with Huntington's disease-like symptoms, genetic testing for STUB1 as well as TBP should be conducted for diagnosis of SCA17-DI, even in cases of sporadic or autosomal recessive inheritance.