Potential Association between Somatic CAG Repeat Instability in HTT Intermediate Alleles and Clinical Phenotype

Research Background

Huntington’s disease (HD) is a neurodegenerative disorder caused by the expansion of CAG trinucleotide repeats (≥36 CAG repeats) in the HTT gene. Intermediate alleles (IAs) with 27-35 CAG repeats are generally not considered a direct cause of HD, but their potential association with neurocognitive symptoms remains controversial. Previous studies have shown that somatic CAG expansion in HTT can affect the onset time of HD. Therefore, the authors hypothesized whether intermediate alleles exhibit somatic instability, and whether somatic CAG expansion might play a role in the HD-like phenotypes observed in some IA carriers.

Paper Source

This article was written by Ainara Ruiz de Sabando, Marc Ciosi, Arkaitz Galbete, Sarah A. Cumming, Spanish HD Collaborative Group, Darren G. Monckton, and Maria A. Ramos-Arroyo. The authors are from institutions including Hospital Universitario de Navarra, Universidad Pública de Navarra, and the University of Glasgow. The paper was published in the European Journal of Human Genetics, Volume 32 (pages 770-778) in 2024, with an online publication date of March 4, 2024.

Research Process

Subjects and Methods

The study included blood DNA samples from 164 HD patients and 191 IA carriers, as well as brain DNA samples from a symptomatic gene carrier with 33 CAG repeats. Researchers quantified HTT somatic CAG expansion using the MiSeq sequencing system and conducted genotype-phenotype analysis. Additionally, detailed clinical and family data analysis was performed on four symptomatic IA carriers, with post-mortem brain tissue analysis conducted on one of them.

Sample Collection and DNA Sequencing Techniques

All DNA samples were re-analyzed at the University Hospital of Navarra, and HTT CAG length was determined by capillary electrophoresis. For 355 carriers of ≥27 CAG alleles, repeat region DNA was sequenced using the MiSeq system to quantify variant numbers based on inherited CAG repeat numbers. To avoid interference from PCR slippage products of long alleles, samples were selected for somatic expansion studies according to specific criteria.

Main Results

Somatic Expansion in Blood DNA

The study found that alleles across all CAG ranges exhibited CAG length-dependent somatic expansion, particularly in longer alleles. Moreover, the proportion of somatic expansion in blood DNA for both IAs and expanded alleles increased with age. Data showed that the CAG expansion proportion for all IAs increased by 0.004 for each additional CAG and by 0.0001 per year.

Somatic CAG Expansion in Symptomatic IA Carriers

Symptomatic IA carriers exhibited motor (85%), cognitive (27%), and/or behavioral (29%) symptoms, with a later average age of onset (58.7±18.6 years). However, symptom presentation was not associated with CAG repeat numbers. This is consistent with previous observations of IA carriers in the literature. MiSeq sequencing detected +1 and +2 CAG somatic expansions in different brain regions of a 33 CAG carrier, with the highest expansion proportion in the striatum (10.3%) and the lowest in the cerebellum (4.8%).

Research Conclusions

The study demonstrates that HTT intermediate alleles exhibit somatic instability but found no direct association with HD-like phenotypes. However, the researchers note that some IAs close to the HD pathological threshold and with susceptible genetic backgrounds may exhibit neurocognitive symptoms. The study emphasizes the continuity of somatic expansion in the HTT locus, rather than just clinically defined categories (normal, intermediate, reduced penetrance, full penetrance).

Research Highlights

A major highlight of this study is the first verification of somatic instability in HTT intermediate alleles in both blood and brain tissue, revealing the effects of CAG length and age on somatic expansion through large sample sizes and high-precision sequencing technology. Furthermore, the detailed brain tissue analysis of a symptomatic 33 CAG carrier adds to the study’s persuasiveness.

Research Value

This study has significant scientific value in understanding the HD disease course at the genetic level. By exploring somatic instability in HTT intermediate alleles, the research provides new perspectives for more accurately understanding and predicting HD risk. Simultaneously, the study helps explain potential neurocognitive symptoms in individuals near the HTT allele pathological threshold, further advancing research on HD and related diseases.

Through this report, we can understand the importance of this research and its potential impact on HD and other neurocognitive disorders. Future research will require more long-term follow-up data and large-scale clinical neuropathological analyses to further confirm and expand these findings.