Spinocerebellar ataxia type 2 (SCA2) is an autosomal dominant cerebellar ataxia characterized by a progressive cerebellar syndrome associated to saccadic slowing, peripheral neuropathy, cognitive disorders, and other multisystem features. SCA2 is caused by the abnormal expansion of cytosine-adenine-guanine triplet repeats in the encoding region of the ATXN2 gene and therefore the expression of toxic polyglutamine expansions in the ataxin 2 protein, which cause progressive neuronal death of Purkinje cells in the cerebellum and several pontine, mesencephalic, and thalamic neurons among other cells. Worldwide, SCA2 is the second most frequent type of spinocerebellar ataxia, only surpassed by SCA3. Nevertheless, in Holguin, Cuba, the disease reaches the highest prevalence, resulting from a putative foundational effect. This review discusses the most important advances in the genotypical and phenotypical studies of SCA2, highlighting the comprehensive characterization reached in Cuba through clinical, neuroepidemiological, neurochemical, and neurophysiological evaluation of SCA2 patients and presymptomatic subjects, which has allowed the identification of new disease biomarkers and therapeutical opportunities. These findings provide guidelines, from a Cuban viewpoint, for the clinical management of the disease, its diagnosis, genetic counseling, and therapeutical options through rehabilitative therapy and/or pharmacological options.

The prodromal phase of spinocerebellar ataxias (SCAs) has not been systematically studied. Main findings come from a homogeneous SCA type 2 (SCA2) population living in Cuba. The aim of this study was to characterize extensively the prodromal phase of SCA2 by several approaches. Thirty-seven non-ataxic SCA2 mutation carriers and its age- and sex-matched controls underwent clinical assessments, including standardized neurological exam, structured interviews and clinical scales, and looking for somatic and autonomic features, as well as a neuropsychological battery, antisaccadic recordings, and MRI scans. Main clinical somatic features of non-ataxic mutation carriers were cramps, sensory symptoms, sleep disorders, and hyperreflexia, whereas predominating autonomic symptoms were pollakiuria/nocturia, constipation, and frequent throat clearing. Cognitive impairments included early deficits of executive functions and visual memory, suggesting the involvement of cerebro-cerebellar-cerebral loops and/or reduced cholinergic basal forebrain input to the cortex. Antisaccadic task revealed impaired oculomotor inhibitory control but preserved ability for error correction. Cognitive and antisaccadic deficits were higher as carriers were closer to the estimated onset of ataxia, whereas higher Scale for the Assessment and Rating of Ataxia (SARA) scores were associated most notably to vermis atrophy. The recognition of early features of SCA2 offers novel insights into the prodromal phase and physiopathological base of the disease, allowing the assessment of its progression and the efficacy of treatments, in particular at early phases when therapeutical options should be most effective.

The role of short, large or intermediate normal alleles (ANs) of the ataxin-2 gene in generating expanded alleles (EAs) causing spinocerebellar ataxia type 2 (SCA2) is poorly understood. It has been postulated that SCA2 prevalence is related to the frequency of large ANs. SCA2 shows the highest worldwide prevalence in Cuban population, which is therefore a unique source for studying the relationship between the frequency of large and intermediate alleles and the frequency of SCA2 mutation. Through genetic polymorphism analyses in a comprehensive sample (B3000 chromosomes), we show that the frequency of large ANs in the ataxin-2 gene is the highest worldwide, although short ANs are also frequent. This highly polymorphic population displayed also high variability in the CAG sequence, featured by loss of the anchor CAA interruption(s). In addition, large ANs showed germinal and somatic instability. Our study also includes related genotypic, genealogical and haplotypic data and provides substantial evidence with regard to the role of large and intermediate alleles in the generation of pathological EAs.

Tatton-Brown-Rahman syndrome (TBRS) was recently described in 13 isolated cases with de novo mutations in the DNMT3A gene. This autosomal dominant condition is characterized by tall stature, intellectual disability and a distinctive facial appearance. Here, we report six cases of inherited TBRS caused by novel DNMT3A germline mutations. The affected individuals belong to two sib-ships: four from an Old Order Amish family in America and two from a French Canadian family in Canada. All of them presented with characteristic features of TBRS, including dysmorphic facial features, increased height, intellectual disability, and variable additional features. We performed clinical exome sequencing and identified two mutations in the DNMT3A gene, a c.2312G>A (p.Arg771Gln) missense mutation in the Amish family and a c.2296_2297delAA (p.Lys766Glufs*15) small deletion in the French Canadian family. Parental DNA analysis by Sanger sequencing revealed that the Amish mutation was inherited from the healthy mosaic father. This study reflects the first cases with inherited TBRS and expands the phenotypic spectrum of TBRS.

Having reported the world's highest prevalence of spinocerebellar ataxia type 2 (SCA2), health professionals in Cuba developed a program for the predictive testing of this condition. Between February 2001 and December 2011, a total of 1050 individuals requested their inclusion in the presymptomatic testing (PST) program. Their medical records were retrospectively analyzed in the present descriptive study. A total of 768 participants completed the protocol, 204 withdrew and 78 were excluded. The PST uptake was 24.91%. Females predominated and 70.96% had negative test results. Their main motivations were risk assessment in their descendants, physical and psychological preparation to cope with the disease and planning for the future. The profile of Cuban participants in the predictive testing program is similar to the one reported for other programs all over the world, nevertheless the genetic counseling practice at the community level is a distinctive aspect, which is valuable in providing at-risk individuals with wide and proper knowledge before their testing inclusion request. The SCA2 predictive testing program has high uptake rates and is renowned in our population. Future research is needed to assess the long-term psychological impact in the participants, their partners and relatives.