Clinical and Molecular Findings of Intermediate Allele Carriers in the <b><i>HTT</i></b> Gene from the Mexican Mestizo Population

Abstract

<b><i>Introduction:</i></b> There are reports of different clinical statuses in carriers of intermediate alleles (IAs) of CAG trinucleotide repeats in the <i>HTT</i> gene, from individuals affected by a clinical picture indistinguishable from Huntington’s disease (HD) to those without manifestations. Therefore, the possible clinical significance of these alleles has been widely debated. <b><i>Objectives:</i></b> The aim of this study was to describe general and clinical features and discard HD phenocopies by molecular assessment in a case series of IA carriers on the <i>HTT</i> gene of a laboratory sample from a neurological center in Mexico. <b><i>Methods:</i></b> We selected individuals who had previously been tested for the <i>HTT</i> gene expansion, which resulted in IAs. Clinical information was obtained from medical records, and molecular analysis of the <i>JPH3</i>, <i>PRNP</i>, and <i>TBP</i> genes was performed only in IA carriers with clinical manifestations. In addition, two patients with IA and acanthocytes were evaluated by whole-exome sequencing. The scientific and ethical committees of the National Institute of Neurology and Neurosurgery Manuel Velasco Suárez (NINNMVS) approved this study. <b><i>Results:</i></b> From 1994 to 2019, the Genetics Department of the NINNMVS confirmed 34 individuals with IAs, 15 of whom belonged to 11 families with HD (IA-HD) and 19 of whom had no family history of HD (IA-non-HD). We found a high proportion of manifestations of the HD phenotypic spectrum in the IA-non-HD subgroup. In addition, among the 20 samples of IA carriers with manifestations molecularly evaluated, we identified two unrelated subjects with CAG/CTG repeat expansions on the <i>JPH3</i> gene, confirming HD-like 2 (HDL2), and one patient with the homozygous pathogenic c.3232G&#x3e;T variant (p.Glu1078Ter) in the <i>VPS13A</i> gene, demonstrating choreoacanthocytosis. <b><i>Discussion/Conclusion:</i></b> Our results show the most extensive series of subjects with IAs and clinical manifestations. In addition, we identify three HD phenocopies, two HDL2 cases, and one choreoacanthocytosis case. Therefore, we emphasize evaluating other HD phenocopies in IA carriers with clinical manifestations whose family background is not associated with HD.