Earwax, also known as cerumen, is a mixture of dead skin cells and lipid secretion from the apocrine gland in the ear canal. There are two types of earwax: wet and dry. These types may be the result of climate adaptation and are associated with risks for certain diseases and different responses to medications.

Earwax type is determined by a variation in the ABCC11 gene (1). ABCC11 is a member of a large gene family called ATP-binding cassette (ABC) transporters. The ABC transporters are large membrane proteins that transport a variety of compounds including lipids, bile salts, toxic compounds and peptides across the cellular membrane. This makes them essential for the physiological functions of many glands and for drug metabolism. Mutations in ABC transporter genes have been associated with many heritable diseases (2).

One SNP (single nucleotide polymorphism) of the ABCC11 gene called rs17822931 is the cause of the two different earwax types. An individual can have either T or C at this SNP. The T variant makes an unstable protein and affects the function of apocrine glands, leading to dry earwax while the C variant leads to wet earwax.

In late 2011, a population genetics study revealed that the C variant is the original form of the gene. The T variant emerged around 2,000 generations ago in the East Asian populations and has steadily become more common. Today in East Asia, most of the population carries the T variant; compare this to Africa where almost no one carries the T variant.

There is a theory that the A variant began spreading because of the slight selective advantage (0.1%) it gives to its carriers. The author of this theory did another evaluation by studying the allele abundance of a set of gene polymorphisms in relation to geographic location. They found that the earwax polymorphism is the only one in their data set associated with latitude; i.e. the higher the latitude, the more abundant the T allele. This observation suggests that the selective advantage of T genotype may be related to cold climate adaptation (3).

Like the earwax glands, the mammary glands are also a form of the apocrine gland and express the ABCC11 gene. The C variant of ABCC11 associated with wet type earwax is also associated with colostrum secretion from the mammary glands (4). Some research studies have proposed that breast cancer is less common in geographic areas with high frequencies of dry earwax. Furthermore, the C variant reportedly has the ability to export cyclic nucleotides and nucleoside-based anticancer drugs from cells and is associated with drug resistance (2). Whether these links exist or not depends on the findings of ongoing research studies.

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References

1. Yoshiura K, Kinoshita A, Ishida T, et al. (March 2006). “A SNP in the ABCC11 gene is the determinant of human earwax type.” Nature Genetics 38 (3): 324-30 doi:10.1038/ng1733. PMID 16444273.
2. Toyoda, Y., and Ishikawa, T. 2010. Pharmacogenomics of Human ABC Transporter ABCC11 (MRP8): Potential Risk of Breast Cancer and Chemotherapy Failure. Anti-Cancer Agents in Medicinal Chemistry, 10, 617-624.
3. Ohashi, J., I. Naka, and N. Tsuchiya. 2011. The impact of natural selection on an ABCC11 SNP determining earwax type. Molecular Biology and Evolution 28:849-857. doi:10.1093/molbev/msq264
4. Miura et al. (2007) . “strong association between human earwax-type and apocrine colostrum secretion from the mammary gland.” Hum Genet 121(5):631-3.