The prevalence of HD in the West is estimated at five to ten people per 100,000 people. However, in Asia, the prevalence of HD is approximately one tenth of the HD prevalence in Western population. The prevalence is 0.4 per 100,000 people in Hong Kong and 0.65 per 100,000 in Japan. In India, there are case reports from Punjab and northern India and until 1990, there were 69 case reports of HD in China. In Southeast Asia, the epidemiology data for HD consists mainly of case reports with HD in Singapore being reported from a Chinese family and second-generation Indian families. There is no accurate data obtained from Thailand, Indonesia and Philippines.
The reason for this wide variation in HD prevalence in different ethnic communities is unclear. Some people advocate a European origin for the mutation, suggesting that migration and inter-racial mixture of genes led to a lower prevalence in non-European countries. As mentioned in the HD in Hong Kong section, it has been proposed that the prevalence of HD in Hong Kong maybe attributed to a European origin. This assumption derives from the fact that the ancestors of the families with HD can be connected to coastal provinces with histories of strong colonial presence. Hence, it has been hypothesized that HD in other Chinese related countries may also have originated from a common Caucasian ancestry by ways of migration.
This section of Global HD aims to highlight regions within Asia in which the HD populations have been studied, and where history and significance within the HD context have been demonstrated.
Malaysia is a multicultural country with three main ethnic populations. The indigenous Malaysians, Chinese and Indians make up approximately 62 percent, 29 percent and 8 percent of the population respectively. The prevalence of HD is 0.0024 per 10,000 people, based on seven reported cases.
To test the hypothesis that HD in Asia has a Caucasian origin, a HD registry was established in Malaysia in 1995 at the University of Malaya Medical Centre, Kuala Lumpur. Within eighteen months, the registry had identified seven unrelated patients with HD. From these individuals, there were four Chinese, one Malaysian and two Indians. Despite the previous hypothesis set forth that HD in Asian countries originated from Europe; only one Chinese patient, out of selected seven patients, had possible Irish ancestry. Moreover, there are several social and cultural reasons that the hypothesis set forth is likely to be false. The remaining three Chinese participants did not have any relatives with Caucasian features. Culturally, Chinese women were forbidden to have close contact with foreigners. Women did not travel unless they were accompanying their husbands. Prostitution as a profession for women was deeply despised and extramarital sexual contact was strongly forbidden. Hence, it was highly unlikely that female ancestors of these patients would have sexual contact with asymptomatic Caucasian HD men and therefore result in the mixing of genes. In addition, other participants also did not have European origin, since one Malaysian patient was a local while the two Indian patients originated from Tamil Nadu and Punjab in India respectively.
There is also evidence against the European migration hypothesis from the current genetic literature. Although the nature of the abnormal trinucleotide repeats of the HD gene in Chinese patients is similar to that in Caucasian HD patients, a majority of the tested Chinese patients have a small number of excess CAG repeats. Additional genetic evidence show that there are seven CCG repeats adjacent to IT15, or the Huntington Gene, in Asians but ten CCG repeats in Caucasians. The distribution of the CCG repeat adjacent to IT15 is of seven repeats in Asians and 10 repeats in Caucasians. The distribution of the CCG repeats differs among populations of low prevalence and the west. Therefore, mutation of the IT15 gene rather than the European migration hypothesis is likely to be the explanation for the variation in Malaysian prevalence of HD.
The interim results of the Malaysian HD registry have shown that HD exists in all major racial groups in Malaysia, with all cases exhibiting an excess of CCG repeats. The theory that Caucasian migration led to genetic mixing and a resulting low HD prevalence in Malaysia is less plausible according to the data collected by the registry; more likely, cases of HD in Malaysia arose due to a separate mutation of the HD gene. Nevertheless, because only seven HD patients were identified, these conclusions should be considered with caution to avoid errors in assumption and generalization.
The Global HD research and articles received partial support from the Bingham Fund for Innovation in the Program in Human Biology.
Resources in Malaysia^
University of Malaya Medical Centre in Kuala Lumpur
In 1995, a nationwide HD registry was established at the University of Malaya Medical Centre, Kuala Lumpur. The Medical Centre is one of the major public tertiary referral hospitals. It keeps reports of clinical and genetic studies of the patients seen since the setting up of the Registry.
For more information regarding the association, please visit: http://www.ummc.edu.my/view/index.php