Human earwax consists of wet and dry types. Dry earwax is frequent in East Asians, whereas wet earwax is common in other populations. Here we show that a SNP, 538G --> A (rs17822931), in the ABCC11 gene is responsible for determination of earwax type. The AA genotype corresponds to dry earwax, and GA and GG to wet type. A 27-bp deletion in ABCC11 exon 29 was also found in a few individuals of Asian ancestry. A functional assay demonstrated that cells with allele A show a lower excretory activity for cGMP than those with allele G. The allele A frequency shows a north-south and east-west downward geographical gradient; worldwide, it is highest in Chinese and Koreans, and a common dry-type haplotype is retained among various ethnic populations. These suggest that the allele A arose in northeast Asia and thereafter spread through the world. The 538G --> A SNP is the first example of DNA polymorphism determining a visible genetic trait.
Glucose-6-phosphate dehydrogenase (G6PD) deficiency is the most common enzymopathy in humans. The prevalence of G6PD deficiency and its molecular basis were studied in Phuket islanders, Southern Thailand. A total of 345 volunteers (123 males and 222 females) were recruited in this study. Infection with Plasmodium falciparum or Plasmodium vivax was not detected in any of these subjects by polymerase chain reaction (PCR)-based diagnosis. G6PD-deficient individuals were identified with the WST-8/1-methoxy PMS method. The molecular basis of G6PD deficiency was investigated by PCR-direct sequencing procedures or PCR-restriction enzyme fragment length polymorphism assays. The numbers of individuals showing severe and mild G6PD deficiency were 14 and 21, respectively. A high prevalence of G6PD deficiency was observed in subjects with Moken (15.4%) or Thai (15.5%) ethnic background. G6PD Mahidol (487G>A) (n=14), G6PD Viangchan (871G>A) (n=11), G6PD Gaohe (95A>G) (n=2), G6PD Kaiping (1388G>A) (n=1), and G6PD Kerala-Kalyan (949G>A) (n=1) were identified. The results suggest that several groups of people of the Asian Continent, such as Burmese, Laotian or Cambodian, Thai and Chinese, participated in the establishment of the ethnic identity of the current ethnic groups of Phuket Island.
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