Saturday, October 14, 2006

Hormones, size and long life

Nate Sutter and his colleagues at the Human Genome Research Institute has found a correlation between a variant of the insulin like growth factor-1 (igf-1) gene and dog size. The research team started out by studying the genes of tiny Portuguese water dogs and found that they carried the variant gene. They then compared the sequence of this gene in 350 dogs from very large (such as Great Danes and St. Bernards) to very small (Yorkshire terriers and pomeranians).
They found that almost all of the 18 small breeds carried the identical variant of the gene as small Portuguese water dogs. But almost none of the 15 giant breeds carried this gene variant. That suggested that the gene plays a major role in controlling dog body size, Sutter said on 11 October at the annual meeting of the American Society of Human Genetics in New Orleans, Louisiana.
Igf-1 appears to regulate the amount of growth factor made, with smaller breeds making less growth factor than larger breeds. In theory, introduction of the "larger" variant could create large chihuahuas. (For more background information see the canine genetics primer.)

Igf-1 regulation of growth is not unique to dog; "knocking out" the Igf-1 gene in mice results in dwarf "mini mice" (abstract). Other studies of genetically-manupulated mice has helped reveal another effect of low Igf-1: longer life spans (articles: Holzenberger (2004), Shimokawa et al (2002)). In contrast, high levels of Igf-1 are associated with increased risk of cancer (Chan et al. (1998) abstract). There is evidence that Igf-1 plays a similar role in humans.

What is the significance of all this to science fiction? We've all read or seen stories that use a "mutation" to create characters with new shapes or abilities. In some stories the "mutants" have amazing non-human abilities (e.g., X-men and the new TV series Heroes). I'll have a lot more to say about "mutants" and mutation in a later post, but for this one I'd like to point out that human physiology is a complex and carefully balanced system. Any mutation is going to potentially have wide-ranging and unexpected effects: longer lifespan but smaller size; larger size, but more likely to get cancer. Realistic science fiction takes such issues into account.

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