The \(ABCC11\) gene is a fascinating piece of human genetics, linking two seemingly unrelated physical characteristics: the consistency of earwax and the presence of underarm body odor. Located on chromosome 16, \(ABCC11\) is a specific type of membrane transporter found in humans, and its function dictates whether a person has wet or dry earwax and a strong or faint body odor.
The \(ABCC11\) Protein’s Molecular Function
The \(ABCC11\) gene encodes a protein belonging to the ATP-binding cassette (ABC) transporter superfamily, specifically as a member of the Multidrug Resistance-associated Protein (MRP) subfamily. This protein acts as an efflux pump, meaning its primary molecular function is to move specific molecules out of a cell, across the cell membrane, using energy derived from the breakdown of adenosine triphosphate (ATP). These transported molecules often include lipophilic substances, conjugated steroids, and cyclic nucleotides. The \(ABCC11\) transporter is expressed in various glandular tissues throughout the body, including the apocrine sweat glands and the ceruminous glands in the ear canal.
A single-nucleotide polymorphism (SNP), specifically a G-to-A change at position 538 (rs17822931), is responsible for the dramatic difference in its function. This substitution results in an amino acid change within the protein, which severely impairs the protein’s ability to transport its substrates out of the cell. The presence of the functional G allele allows for robust transport, while the variant A allele leads to a loss-of-function protein. This molecular distinction forms the basis for the observable physical differences in earwax and body odor.
The Earwax Connection
The type of cerumen, or earwax, a person produces is governed by the \(ABCC11\) gene. Earwax is secreted by modified apocrine sweat glands called ceruminous glands, which are found in the outer ear canal.
The functional G allele, which is dominant, produces the wet earwax phenotype, characterized by a brownish, sticky, and moist consistency. Individuals who have at least one copy of the functional G allele (genotypes GG or GA) will exhibit wet earwax because the functional \(ABCC11\) protein is present and actively transporting lipophilic components into the ear canal secretion. Conversely, those who are homozygous for the variant A allele (genotype AA) lack the fully functional transporter protein. This impaired transport function in the ceruminous glands results in the dry earwax phenotype, which is typically grayish, brittle, and flaky.
Impact on Body Odor Production
The \(ABCC11\) gene variant affects the presence of typical axillary body odor. This odor is not caused by sweat itself, but by the action of skin-surface bacteria on specific, odorless precursor molecules secreted in apocrine sweat. The functional \(ABCC11\) protein is highly expressed in the apocrine sweat glands, and its job is to actively transport these odorless odor precursors, such as S-glutathione conjugates and amino-acid conjugates, onto the skin surface.
The presence of the functional G allele results in the secretion of these odor precursors, leading to a strong characteristic axillary odor once they are metabolized by bacteria. However, in individuals with the loss-of-function AA genotype, the \(ABCC11\) transporter is largely inactive, and it cannot effectively pump these precursor molecules into the sweat. Consequently, the concentration of these precursors in the axillary sweat is significantly reduced or nearly undetectable. This lack of available precursor molecules means that the skin bacteria cannot generate the typical pungent volatile organic compounds, resulting in a significant reduction or complete absence of characteristic axillary body odor. This genetic makeup has been shown to influence which types of bacteria can thrive in the armpit microbiome, as the absence of the S-glutathione conjugate alters the available nutrients.
Geographic Distribution and Evolutionary Significance
The distribution of the \(ABCC11\) gene variants displays a striking geographic pattern across human populations. The allele for dry earwax and reduced body odor (the A allele) is highly prevalent in East Asian populations, where its frequency can be as high as 80% to 95% in countries like China and Korea. In stark contrast, this variant is rare in other global populations, with its frequency dropping to only about 2% among Europeans and a similarly low percentage in African populations.
The high concentration of the A allele in Northeast Asia suggests that the variant arose in this region and subsequently spread. One prominent scientific hypothesis suggests that this genetic change conferred a selective advantage to ancient populations living in cold climates.
The dry earwax phenotype is associated with reduced cerumen secretion, and the decreased apocrine gland activity may have been beneficial for thermoregulation or reducing the risk of infection in cold environments. The evolutionary pressure favoring this allele appears to have been strong enough to drive its frequency to near fixation in certain populations.