Huntington’s disease (HD) is a progressive, autosomal dominant, neurodegenerative disease that affects the brain. It is caused by a genetic mutation in the huntingtin gene located in exon 1 of chromosome 4p16.3. The normal range of CAG repeats in the huntingtin gene is 6 to 26, while people with HD typically have 36 or more CAG repeats. The number of CAG repeats can affect the age of onset and the rate of progression of the disease, with more repeats leading to earlier onset and more severe symptoms.
In our sample of 291 individuals, the number of HTT CAG repeats ranges from 12 to 51, with an average of 21.91 copies (Table 1). Following international recommendations, we derived four subgroups according to the number of HTT CAG repeats: Normal (≤ 26 copies, n = 241, 82.8%), intermediate (27–35 copies, n = 16, 16%), reduced penetrance (36–39 copies, n = 1, 0.3%), and full penetrance (≥ 40 copies, n = 33, 11.3%)(Table 1).
Previous studies have shown a significant correlation between the average CAG repeat length of normal chromosomes and the prevalence of HD. The average wild-type CAG repeat size was significantly larger in populations with a higher prevalence of HD [26][27]. Therefore, the HTT CAG repeat size in a large sample of Colombian subjects may in turn reflect the prevalence of HD in the Colombian population. Table 2 shows the comparison of the number of CAG repeat sizes of normal and intermediate alleles between the current study and other populations. This comparison shows close mean values between the Colombian and other populations, with the European population being the closest.
Table 2
Comparison of the number of HTT CAG repeats for normal and intermediate allele chromosomes between Colombian and other populations.
| Number of CAG repeats | |
Population | Mean | SD | Range | Number of patients | P-value | Reference |
Thais | 16.5 | 1.9 | 8–28* | 449 | | [26] |
European | 18.4 | 3.7 | 8–35 | 479 | < 0.0001 | [28] |
American | 19.7 | 3.2 | 11–34 | 545 | < 0.0001 | [29] |
Finnish | 17.1 | 1.8 | 14–23 | 48 | 0.255 | [28] |
Black | 16.2 | 2.5 | 8–24 | 113 | 0.55 | [28] |
Chinese | 16.4 | 1.5 | 8–20 | 90 | 1 | [28] |
Japanese | 16.6 | 1.3 | 13–23 | 166 | 1 | [28] |
Colombian | 18,2 | 3 | 12–35 | 257 | 0.00001 | This study |
The genotype distribution shows that the most common primary genotype is 17/7 followed by 15/7, 17/10 and 18/7; in the secondary genotype, the most common genotype is 17/10 followed by 17/7, 18/9 and 23/7; the genotype combinations (haplotypes) shows the most common is 17/7_17/10 (n = 19, 12.3%) followed by haplotypes 17/7_17/7 (n = 17, 11%) and 15/7_17/7 (n = 13, 8.4%)(Fig. 2a). Our results are consistent with previous reports that the average CAG tract size in the East Asian general population was 16.9 repeats and 17.8 repeats in Europeans [27]. This study also shows a correlation with HTT haplogroups of the general population (< 27 CAG repeats). The A1 and A2 haplotypes are two of the most common haplotypes associated with the HD mutation. These haplotypes are defined by variations found at three specific markers on the huntingtin gene. A person with the A1 haplotype has a specific set of variations at these three markers, while a person with the A2 haplotype has a different set of variations. There is a diversity of haplogroups found in the general European population, although the CAG expansion is most likely to occur on haplogroup A in this population. Note that haplogroup A and the variants with the highest risk of CAG expansion in the European population (A1 and A2) are absent in the general populations of China and Japan [27][26][30].
Juan de Acosta is a corregimiento in the Atlántico department of the Northern Caribbean coast of Colombia, with a founding origin of Basque. Historically, several corregimientos in the Atlántico department have different ancestral origins with an ethnic composition based on migratory flows over the years. This event would confirm that the founding mutation in this area occurred in Western Europe and spread to other regions through migration. Furthermore, the CCG7 allele is the predominant allele in Western Europe and could generate variations in the number of CAG repeats through independent mutational events. This finding is consistent with the population studied[31]. In 2020, a study of the CAG intermediate HTT alleles in the general population of Rio de Janeiro, Brazil, compared with a sample of families affected by HD, showed that CCG7 was the most frequent allele [32]. On the other hand, the haplotypic analysis of CAG and CCG was repeated in 21 Brazilian families with HD. In total, 40 different haplotypes were identified. Further analysis showed that CCG10 was linked to a normal CAG allele in 19 haplotypes and to expanded alleles in two haplotypes. In addition, CCG7 was linked to expanded CAG repeats in 40 haplotypes (95.24%) and CCG10 was linked to expanded CAG repeats in only two haplotypes (4.76%). Therefore, the CCG7 allele was the most common allele on HD chromosomes in this Brazilian sample, which is consistent with the results obtained in this Brazilian sample [33], which is consistent with the results obtained in our Caribbean sample.
In 2015, researchers analyzed the CCG repeat polymorphism located near the CAG repeat and identified HD chromosome haplotypes. Surprisingly, the results revealed a strong linkage disequilibrium between the CAG repeat expansion and the CCG10 allele on Japanese HD chromosomes, which differs from what has been reported in Western populations in the past [34]. These repeats suggest that HD mutations in Asian populations may originate from different ancestral lineages and therefore be associated with high (CCG7 and CCG10) or low (CCG6 and CCG11) prevalence of HD. For example, in the Caucasian population, the CCG11 allele is less prevalent in individuals with HD. Conversely, populations of Western European descent, which have a higher prevalence of HD, have a higher frequency of the CCG7 allele. In contrast, in populations such as black African, Japanese, Chinese, and Finnish, where HD is less common, the most common CCG alleles are CCG11 and CCG6 [35]. On the other hand, the frequency and distribution of the HD mutation in Caribbean populations may vary depending on factors such as ancestry, migration patterns, and population history. Indeed, some Caribbean populations, such as those in Jamaica and Trinidad and Tobago, have been reported to have a higher frequency of HD than other populations of African descent [16][30].
The CAG repeat is the genetic mutation responsible for HD, and the number of CAG repeats in the HTT gene is used to determine a person’s risk of developing the disease. However, PCR amplification can sometimes cause small errors or “slippage” in the number of CAG repeats counted, leading to inaccurate results. Slippage can also lead to false negative or false positive results in HD genetic testing, particularly in cases where the CAG repeat length is close to the diagnostic threshold for the disease. In this study, we found that secondary allele slippage is associated with HD allele type and does not differ by gender (Fig. 4b). However, slippage tends to decrease with age regardless of HD diagnosis in our sample (Fig. 4c). We also found that slippage tends to increase with the number of CAG repeats in HD affected individuals (Fig. 4d).
The presence of mosaicism in HD can pose challenges for genetic testing and counselling because standard testing methods may miss the mutation if it is present in a small proportion of cells. This can lead to false negative results and an inability to accurately estimate the risk of developing HD or passing it on to offspring. In some cases, mosaicism in HD may result in a less severe form of the disease or a delayed onset of symptoms because the number of cells carrying the mutation may be lower [1][36] [37]. In other cases, however, the severity and onset of symptoms may be more unpredictable, as the proportion of cells carrying the mutation can vary widely between individuals. Therefore, if mosaicism in HD is suspected, more sensitive testing methods such as repeat primed PCR or Southern blot analysis may be required to detect the mutation. On the other hand, genetic counselling should also consider the potential impact of mosaicism on disease onset and progression [38][36]. Here we found that mosaicism tends to increase with age in HD affected individuals, but not in HD unaffected individuals. Furthermore, mosaicism in the primary and secondary alleles is associated with HD allele type and gender. It is noteworthy that in reviewing similar research on HD, we did not find any previous studies reporting information on mosaicism and slippage in a pre-symptomatic population at risk of developing HD. Future studies may benefit from considering our findings.