Friday, April 8, 2011

Don't put anything smaller than your elbow in

the genetics of ear wax

Recently, an exciting genetic discovery was made in
the field of ear wax. It appears that a change in a single
nucleotide of your DNA can determine whether
your ear wax is wet or dry. This marks the first time
that a single-nucleotide polymorphism (SNP) has
been found to determine a visible genetic trait.
Known medically as cerumen, ear wax is found in the
ears of humans and many other mammals. Secreted
by apocrine glands in the ear canal, the exact function
of ear wax is not clear. Proposed benefits range from
keeping the ear canal clean and lubricated, to acting
as an insect trap.
In contrast, many have experienced the disadvantages
of ear wax. In those who are hard of hearing,
ear wax can damage their hearing aids. Ear wax is
also a cause of hearing loss—a build up of wax can
block the ear canal, hindering the passage of sound
waves to the ear drum.
But it has long been noted that there are 2 types of
ear wax, "wet" and "dry". Wet ear wax common in
Caucasians and African-Americans, it tends to be
honey-to-brown in color and sticky in nature. In contrast,
dry ear wax is common in East Asians and is
gray in color and more brittle and flakey. And now we
know the underlying genetic basis.
A major clue came in 2002 when scientists reported
the case of a Japanese woman who had a rare
genetic disorder that caused her arms and legs to
twist uncontrollably (paroxysmal kinesigenic
choreoathetosis). Uncommon in the Japanese, shealso happened to have wet ear wax, as did several of
her relatives who also had choreoathetosis. This suggested
that the inheritance of the rare neurological
disorder and the inheritance of the dry ear wax type
were linked.
Linkage anaylsis of these 2 traits in affected family
members pointed to the pericentromeric region of
chromosome 16 as the site that contained both the
"choreoathetosis locus" and the "ear wax locus". To
isolate the specific genetic change that determined
ear wax type, scientists compared the sequence of
this region in Japanese people with dry ear wax with
the sequence of Japanese people with wet ear wax.
They found 3 SNPs which were associated with ear
wax type, spread over a 5-gene region of DNA.
But only 1 of the SNPs, found in the ABCC11 gene
, resulted in a nonsynonymous change in the
protein product. The SNP, found in the ABCC11
gene, is a G538A substitution (in the reference
sequence rs17822931) and results in a G180R substitution
in the transporter protein it encodes. Individuals
who inherit at least 1 copy of guanine at position
538 (GG homozygotes and GA heterozygotes) have
wet ear wax, a dominant trait. Individuals with dry ear
wax , a recessive trait, are AA homozygotes.
It is possible that the ABCC11 protein transports
some of the lipids and granules found in wet ear wax,
and a change of just one of its amino acids results in
the production of dry ear wax that lacks some of these
Around the world, the "dry" A allele is more common
in Asia, being most common in the North and East of
Asia (100% in Northern Han Chinese and Koreans,
less high in Mongolians, other areas of China, and
Japan). Having apocrine glands that secrete dry ear
wax may also be linked to decreased sweating from
apocrine glands under the arms, and a decrease in
bodily secretions in colder climates could be a survival
The A allele is also found in Native Americans, possibly
reflecting the migration of their Ancestors from
Siberia to North America.
In contrast, the "wet" G allele is more common in Sub-
Saharan Africans and Caucasians.
Regardless of whether the ear wax is wet or dry, the
ear has the same built-in conveyer-belt mechanism
for ridding itself of cerumen—it uses epithelial cells
that migrate away from the ear drum and up the ear
canal to transport ear wax, and any debris it contains,
out of the ear. These cells migrate at speeds of about
0.05 mm/day (similar to the rate of nail growth) .
Many problems with ear wax result from people using
cotton tips in an attempt to remove the wax. But what
they actually do is push the wax further into the ears,
against the tide of migrating cells, causing it to impact
and accumulate. So, to quote the words said by many
Ear, Nose, and Throat (ENT) surgeons, "Do not put
anything smaller than your elbow in your ear!".
Let nature take its course, or see a doctor instead.

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