Somatic Mosaicism of the HTT Cag Repeat in Intermediate Allele Carriers with Neurocognitive Symptoms Compatible with Huntington Disease

We aimed to determine the possible role of CAG somatic expansions on the clinical expression of intermediate allele (IA) carriers of the HTT gene, responsible for Huntington disease (HD). We performed exon one sequencing analysis of the HTT gene on peripheral blood DNA in a Spanish cohort of asymptomatic IA carriers (n=55), symptomatic IA carriers (n=86) and HD subjects (n=124). Additionally, we investigated different brain regions of an individual carrying an HTT allele with 33 CAGs, with neurocognitive symptoms. Linear regression models were used to analyse the association between CAG length and age with somatic mosaicism. Symptomatic IA carriers presented with motor (80%), cognitive (20%) and/or behavioural (22%) signs, and an average age of onset of 58.7 years±18.6. Somatic mosaicism is CAG- and age-dependent in alleles of CAG≥27 CAGs, with b=0.04 (95% CI: 0.035-0.046), and b=0.001 (95% CI: 0.001-0.002), respectively, for all IAs. There was no statistical difference between HTT somatic mosaicism in symptomatic vs asymptomatic IA carriers (p=0.066). Somatic expansions of +1 and +2 CAGs were detected in the brain of the individual with 33 CAGs, with the highest expansion ratio observed in the putamen, where up to 10% of the DNA molecules underwent somatic expansion. In conclusion, somatic CAG expansions observed in blood cannot explain, overall, the neurocognitive signs of IA carriers. However, somatic instability occurs in IAs, which changes with CAG number and age; therefore, the presence of cells in the brain that express up to +2 CAGs may be important when considering the phenotypes of those alleles close to the pathological threshold

Somatic CAG repeat instability in intermediate alleles of the HTT gene and its potential association with a clinical phenotype.

Huntington disease (HD) is a neurodegenerative disorder caused by ≥36 CAGs in the HTT gene. Intermediate alleles (IAs) (27-35 CAGs) are not considered HD-causing, but their potential association with neurocognitive symptoms remains controversial. As HTT somatic CAG expansion influences HD onset, we hypothesised that IAs are somatically unstable, and that somatic CAG expansion may drive phenotypic presentation in some IA carriers. We quantified HTT somatic CAG expansions by MiSeq sequencing in the blood DNA of 164 HD subjects and 191 IA (symptomatic and control) carriers, and in the brain DNA of a symptomatic 33 CAG carrier. We also performed genotype-phenotype analysis. The phenotype of symptomatic IA carriers was characterised by motor (85%), cognitive (27%) and/or behavioural (29%) signs, with a late (58.7 ± 18.6 years), but not CAG-dependent, age at onset. IAs displayed somatic expansion that were CAG and age-dependent in blood DNA, with 0.4% and 0.01% of DNA molecules expanding by CAG and year, respectively. Somatic expansions of +1 and +2 CAGs were detected in the brain of the individual with 33 CAGs, with the highest expansion frequency in the putamen (10.3%) and the lowest in the cerebellum (4.8%). Somatic expansion in blood DNA was not different in symptomatic vs. control IA carriers. In conclusion, we show that HTT IAs are somatically unstable, but we found no association with HD-like phenotypes. It is plausible, however, that some IAs, close to the HD pathological threshold and with a predisposing genetic background, could manifest with neurocognitive symptoms

Spanish HTT gene study reveals haplotype and allelic diversity with possible implications for germline expansion dynamics in Huntington disease

We aimed to determine the genetic diversity and molecular characteristics of the Huntington disease (HD) gene (HTT) in Spain. We performed an extended haplotype and exon one deep sequencing analysis of the HTT gene in a nationwide cohort of population-based controls (n = 520) and families with symptomatic individuals referred for HD genetic testing. This group included 331 HD cases and 140 carriers of intermediate alleles. Clinical and family history data were obtained when available. Spanish normal alleles are enriched in C haplotypes (40.1%), while A1 (39.8%) and A2 (31.6%) prevail among intermediate and expanded alleles, respectively. Alleles ≥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. Non-canonical variants of exon one sequence are less frequent, but much more diverse, in alleles of ≥27 CAG repeats. The deletion of CAACAG, one of the six rare variants not observed among smaller normal alleles, is associated with haplotype C and appears to correlate with larger intergenerational expansions and early onset of symptoms. 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. This haplotype-dependent germline instability may be driven by specific cis-elements, such as the CAACAG deletion

Olfactory characterization and training in older adults: protocol study

The aim of this article is to present the research protocol for a prospective cohort study that will assess the olfactory function and the effect of an intervention based on olfactory training in healthy very old adults (>= 75 years old). A convenience sample of 180 older people (50% female) will be recruited in three different environments: hospitalized control group (CH) with stable acute illness (n = 60); ambulatory control group (CA) of community-based living (n = 60); and an experimental odor training group (EOT) from nursing homes (n = 60). The odor training (OT) intervention will last 12 weeks. All the volunteers will be assessed at baseline; CA and EOT groups will also be assessed after 12 weeks. The primary end point will be change in olfactory capacity from baseline to 12 weeks period of intervention or control. The intervention effects will be assessed with the overall score achieved in Sniffin Sticks Test (SST) - Threshold, Discrimination, and Identification (TDI) extended version. Secondary end points will be changes in cognitive tasks, quality of life, mood, immune status, and functional capacity. All these measurements will be complemented with an immune fitness characterization and a deep proteome profiling of the olfactory epithelium (OE) cultured ex vivo. The current study will provide additional evidence to support the implementation of olfactory precision medicine and the development of immunomodulatory nasal therapies based on non-invasive procedures. The proposed intervention will also intend to increase the knowledge about the olfactory function in very elderly people, improve function and quality of life, and promote the recovery of the health