Cx-Men: Rise of the mutants

Connexin 26 mutations are quite common in Caucasians. About 2% of the population carry them. As well as deafness, they can cause severe problems with skin, and in some cases blindness.  So why are they so common? Especially considering most of them are believed to be caused by a single person getting a mutation in the past, and passing it on (the Founder Effect).

Geneticists believe that for these mutations to be so common there must be some advantage to them. And they think they know of at least one. It's to do with the gap junctions; those paths that allow things to pass between cells, and are made from Connexin proteins. Turns out that sometimes it's better to keep the gates closed.

Ever since humans have been around, viruses and bacteria have been looking for new ways to attack us. Every single aspect of our physiology gets probed for weaknesses, or for some way them to get an advantage over our immune system. At least one of them, a diarrhea-causing bacteria called Shigella flexneri, manipulates gap junctions in our digestive tract to spread itself. And these gap junctions are made from Cx26.

It has been shown that people carrying Cx26 mutations are far more resistant to Shigellosis, the disease this bacteria causes. Shigellosis causes 700,000 deaths a year these days - imagine how bad it must have been before modern hygiene practices came along. Perhaps villages would be struck with a plague of Shigellosis , and only the carriers of mutated Cx26 genes would be left standing? A similar resistance has been seen for certain E. Coli infections, which cause similar intestinal problems and are extremely common. 

From a personal perspective, I rarely get any gastrointestinal illnesses. I had always assumed this was due to my parents' proclivity for foraging in the 'reduced to clear' aisles in Tesco's. The regular consumption of near-rancid meat had led to my steely constitution, or so I believed. Perhaps the true reason was my c.35delG GJB2 mutation.

Scientists have also noticed that people carrying Cx26 mutations tend to have thicker skin. It's not sure whether this provides an advantage, but skin is a barrier to infection, so who knows?

The theory that having one copy of seemingly bad genetic mutations can actually work out for you is called 'Heterozygote advantage'. For example, carriers of the mutation that causes Cystic Fibrosis are resistant to the effects of cholera, and carriers of the mutation that causes Sickle-cell anemia are resistant to Malaria. There's an article on it here if you want to know more.