Huntington’s disease (HD) is notably rare in Japan, with a prevalence of 1-4 cases per million people – about one-tenth of the prevalence in most European and European-origin populations. Even among those who do not have HD, studies consistently report that the average CAG (Perhaps briefly state around here what CAG repeats are) repeat size in western populations is larger than that found in Japanese populations. Research from Chinese Medical Journal shows that the mean CAG repeat length is 18.4 ± 3.7 in western populations, but only 16.6 ± 1.5 in Japan. It is known that larger CAG counts in the trinucleotide expansion (Perhaps briefly explain the trinucleotide expansion a little more between two commas. Maybe not. That’s a style call.) characteristic of HD diagnosis are associated with the course and severity of illness, such as the progression and magnitude of motor and cognitive decline.
HD populations in Japan experience typical symptom onset. Kageyama et al. reported that, similar to observations in Caucasian populations, adult-onset of HD in Japanese patients presents spasticity (unusual stiffness) and cerebellar ataxia (lack of muscle coordination).
It has been hypothesized that the Huntingtin mutation in Japan has a separate origin from the HD mutation in Europe or Africa. Researchers came to this conclusion after comparing the genetic sequences of the Huntingtin gene (HTT) that there are significant differences between the European, Japanese, and African HTT genes. If one HTT allele were responsible for all the HD cases in the world, there would be strikingly similar HTT alleles in all persons with HD – not the distinct varieties that are actually found. The European, African and Japanese general population chromosomes can be grouped into three different major haplogroups (defined as group of genes within an organism that was inherited together from a single parent). The majority of HD chromosomes in Europe, African and Japanese are found on haplogroup A, B and C respectively. The highest risk HD haplotypes (A1 and A2), are absent from the general and HD populations of Japan, and therefore provide an explanation for why HD prevalence is low in Japan. Such differences, therefore, suggest that the HTT allele originated independently in these three regions.
Japanese Huntington’s Disease Network (JHDN). The main goal of JHDN is to use online activities to provide and share information about the latest HD research and resources. Its more specific aims are: 1) to liaison with other HD societies, such as the World Federation of Neurology and International Huntington Association, to gain access the latest HD research; 2) to translate and distribute documents and publications for Japanese readers; 3) to distribute newsletters to HD families and those who are interested; and 4) to organize HD family meetings for informal talks and discussions. For more information, please visit http://www.jhdn.org/ejhdn.html or e-mail firstname.lastname@example.org
The Movement Disorder Society of Japan (MDSJ). The main goal of MDSJ is to promote clinical and basic studies on movement disorders, including chorea and Huntington’s disease. They host seminars, educational lectures, symposiums, video sessions, forums to discuss controversies and poster presentations about genetic studies, biomarkers, recent advances in drug therapies, and regenerative medicine. For more information, please visit http://mdsj.umin.jp/en/index.html
Xu, M., & Wu, Z.-Y. (2015). Huntington Disease in Asia. Chinese Medical Journal, 128(13), 1815–1819. https://doi.org/10.4103/0366-6999.159359