Baldness, altitude and old age
In humans, not many genetic traits can be attributed to the action of a just a single gene. Those that can are either fairly unimportant, such as whether you have free or attached earlobes, or fairly rare, in particular genetic diseases such as Huntington’s disease that are caused by a single gene not working properly. The traits that most people are interested in, such as intelligence and whether they are prone to developing heart disease or cancer, are due to interactions between large numbers of different genes.
Up to now, it’s obviously been easier for scientists to uncover the few single genes responsible for specific traits. But thanks to the increasing ease with which human genomes can be sequenced and the development of complex statistical analysis techniques, scientists are now beginning to uncover the suites of genes responsible for more complex traits. Most recently, scientists have uncovered some of the genes behind thriving at high altitudes, baldness and living to a ripe old age.
It’s actually subtle differences in these suites of genes between different people that are responsible for more complex traits. These differences take the form of variations in one or several of the DNA bases making up a gene, known as single nucleotide polymorphisms (SNPs). These variations are sufficient to alter the action of the gene without stopping it from working completely. Taken together, variations across numerous genes produce the trait of interest.
Sometime these traits are useful, such as living at high altitudes and to a ripe old age, but sometimes they’re not, as with baldness. In a report in this week’s Nature, an international team of scientists led by Angela Christiano from Columbia University in New York revealed that SNPs in a suite of eight genes seem to be largely responsible for a form of baldness known as alopecia areata.
Now, this is not the common-or-garden baldness that afflicts millions of men, which is caused by changing hormone levels. Rather it is an autoimmune disease in which, for some reason, the immune system begins attacking hair follicles, causing them to fall out.
Often this simply results in small, isolated patches of hairlessness, which can be anywhere on the body but usually occur on the head. Occasionally, however, it can cause the loss of all the hair on the head, known as alopecia totalis, or even the whole body, known as alopecia universalis. Famous British sufferers include the comedian Matt Lucas and the Olympic swimmer Duncan Goodhew.
The eight genes pinpointed by Christiano and her colleagues are made up of three genes expressed in hair follicles and five genes related to the immune system. It appears that one of these hair follicle genes codes for a protein that calls on various immune systems cells to attack the follicles. Interestingly, some of these genes have also been implicated in other autoimmune diseases, including rheumatoid arthritis and diabetes.
Although being able to live at high altitudes is a useful genetic trait, it is only really useful if you do actually live at high altitudes. This is the case for those who live on the Tibetan Plateau, more than 4000m up in the Himalayas. Here, oxygen levels are 40% lower than at sea level, causing most people to tire easily and develop headaches. But ethnic Tibetans don’t suffer from these afflictions.
By comparing the genomes of Tibetans, ethnic Chinese and Europeans, a team of Chinese, US and Danish scientists has attributed this ability to SNPs on more than 30 genes. Perhaps unsurprisingly, around half of these genes are involved in the body’s utilisation of oxygen. One of the most interesting of these SNPs was found near a gene that has already been linked to superior athletic abilities, perhaps indicating that this gene is cranked up in ethnic Tibetans. The team reported their findings in this week’s Science.
A much more generally useful trait is not dying until you’re very old. Also in this week’s Science, a team of US and Italian scientists identified 150 SNPs that are related to living to extreme old age. They did this by analysing the genomes of over 1000 OAPs with an average age of over 100 and comparing them with people whose relatives didn’t live to such an advanced age.
They found that, in a very general sense, the more of these SNPs that someone possessed, the longer they were likely to live. This was the case even if they possessed genes that predisposed them to things such as heart disease. The old age genes seemed to trump the disease genes.
Still, these studies also show that it’s not all genes; environment also has a major role to play. Obviously, the high altitude trait evolved in direct response to the environment and alopecia areata is often triggered by some kind of stressful event. While having all the long-living SNPs in the world won’t help you if you smoke and drink too much. Or if you step in front of a bus.
