Characterisation of Huntington disease in the Spanish population: an epidemiological and molecular study of the HTT gene variability and CAG repeat somatic instability
Authors:   , Ruiz de Sabando Ainara 
Published: February 2025

The aim of this doctoral thesis is to study the epidemiology and the genetic background of the rare and autosomal dominant neurodegenerative disorder of Huntington disease (HD) in the Spanish population. We study the patterns of frequency, distribution and magnitude of the disease, the molecular characteristics of the HD gene (HTT) and the somatic instability of the CAG repeat sequence as a potential modifying factor of the clinical phenotype in intermediate alleles (IAs). For the epidemiologic study, HD cases were ascertained through the Rare Disease Registry and the reference Medical Genetics Centre of Navarra; incidence and mortality rates were estimated for a 27-year period (1991-2017) and prevalence was calculated for December 31st, 2017. HTT diversity was examined by SNP genotyping and deep sequencing analysis of exon one, in two large nationwide cohorts: a population-based control sample (n=520), and 471 families with symptomatic individuals referred for HD genetic testing, which included 331 index HD cases and 140 index IA carriers (27-35 CAG repeats). Clinical and family history data were obtained when available and IA carriers were further classified as symptomatic or population control subjects. Somatic instability of the HTT-CAG repeat sequence was studied in blood DNA samples of individuals with ≥27 CAG repeats. We further analysed the potential association of somatic expansions with neurocognitive signs in IA carriers (53 population controls and 82 symptomatic subjects, including brain tissues of a symptomatic 33 CAG allele carrier). The HD prevalence in Navarra, in December 2017, was 4.94 (95% CI: 3.23, 6.65) per 100,000, with an annual 6.1% increase from 1991 to 1999. Incidence and mortality trends remained stable since 1995-96, with mean annual rates per 100,000 of 0.36 (95% CI: 0.27, 0.47) and 0.23 (95% CI: 0.16, 0.32), respectively. The mean age at onset and death of HD patients was 49.6 ± 18.8 and 66.6 ± 17.1 years, respectively, with an average duration of disease of 16.7 ± 8.1 years. The HTT molecular study revealed that Spanish normal alleles (<27 CAGs) are enriched in C haplotypes (40.1%), while A1 (39.8%) and A2 (31.6%) prevail among intermediate and expanded chromosomes, respectively. Alleles of ≥50 CAG repeats are primarily associated with haplotypes A2 (38.9%) and C (32%), which are also present in 50% and 21.4%, respectively, of HD families with large intergenerational expansions. The deletion of CAACAG in HTT exon one is associated with haplotype C and appears to correlate with larger intergenerational expansions and early onset of symptoms. The phenotype of IA carriers was characterised by motor (84%), cognitive (29%) and/or behavioural (28%) signs, with a late age at onset (58.7 ± 18.6 years) that was not inversely correlated with the CAG length. IAs present somatic expansions that are CAG- and agedependent, with 0.4% and 0.01% of DNA molecules expanding by CAG and year, respectively. Somatic instability was not statistically different in symptomatic vs population control IA carriers (p=0.066). Somatic expansions of +1 and +2 CAGs were detected in brain tissues of the individual with 33 CAGs, with the highest ratio in the putamen (10.3% of the DNA molecules) and the lowest in the cerebellum (4.8%). In conclusion, HD epidemiological estimates observed in our population are within the Southern European countries range, and they did not experience true temporary variations over the last decades of post-molecular testing. Spanish HD haplotypes are characterised by a high genetic diversity, potentially admixed with other non-Caucasian populations, with a higher representation of A2 and C haplotypes than most European populations. Differences in haplotype distributions across the CAG length range support differential germline expansion dynamics, with A2 and C showing the largest intergenerational expansions, possibly under the effect of specific cis-elements such as the CAACAG deletion. Somatic CAG expansions follow a gradual CAG correlation, ranging from expanded to intermediate alleles, in an age and tissue-dependent manner, but they do not seem to be a major triggering factor of an associated neurocognitive phenotype in IA carriers. However, it might be posible that some HTT IAs, especially those close to the pathological threshold with a predisposing background, may manifest with an HD-like phenotype.