Allele-specific expression of ATXN3 in blood samples of Machado-Joseph disease expansion carriers.

Although the CAG repeat expansion in the ATXN3 gene was identified over 30 years ago as the cause of Machado-Joseph disease (MJD), the disorder remains untreatable. Notably, MJD is the most prevalent hereditary spinocerebellar ataxia worldwide and is particularly frequent in the Azores Islands (Portugal). This results from two independent founder effects, with two major ancestral lineages - "Joseph" and "Machado" - segregating in Azorean families. Although MJD pathogenesis is mainly driven by the (CAG)n expansion, regulatory cis-elements may modulate ATXN3 expression, thereby influencing phenotypic variation. Here, we investigated allele-specific expression (ASE) of the ATXN3 gene and examined whether distinct ATXN3 lineages modulate the differential expression of non-expanded and expanded alleles, as well as its association with age at onset. We quantified ATXN3 ASE in 38 cDNA samples from the blood of Azorean MJD expansion carriers. Notably, all 28 carriers of the Joseph lineage demonstrated higher relative expression of the expanded allele, whereas eight of the 10 Machado lineage carriers exhibited the opposite trend (Mann-Whitney U test, p < 0.0001). By incorporating genetic and clinical data from an additional 76 Azorean MJD patients we found that, based on the expanded allele size alone, Joseph carriers would be expected to develop symptoms later than Machado carriers. Both lineages, however, report similar ages at onset. suggesting a counterbalance effect of ATXN3 ASE: higher expression of the expanded allele in Joseph carriers may shift individuals toward earlier onset, while higher expression of the non-expanded allele in Machado carriers, may contribute to delayed onset. Our findings show that ATXN3 exhibits haplotype-dependent allelic imbalance. This imbalance may be tissue-specific, underscoring the need for future studies using brain samples. Furthermore, it will be important to determine whether the observed association with age at onset is driven primarily by increased levels of the expanded protein, leading to enhanced protein toxicity, or by toxic effects at the RNA level.