01 September 2011

There's no point in such a slender figure...

Lack of weight is a consequence of a "genetic overdose"Kirill Stasevich, Compulenta 

Doubling of the chromosome section carrying 28 genes leads to severe thinness. The owners of this chromosomal aberration have a body mass index below 18.5. At the same time, such a genetic deviation is often accompanied by insufficient brain development.

Multiple chromosomal disorders; dark colored areas with various chromosomal "breakdowns".
Photo by Arturo Londono, ISM

Everyone knows that DNA with genes is packed into chromosomes and that each chromosome is present in two copies, and both have the same set of genes defining the same traits. For example, on both chromosomes there is a gene responsible for eye color, but on one of them it gives a brown color, and on the other — green. Which gene will gain an advantage, such a sign will appear in the body.

So ideally we have two copies of each gene. But the real state of affairs is very different from the ideal picture. And the reason for this is a lot of chromosomal rearrangements: while there is a hole in the place of a gene on one chromosome, several identical copies of this gene may be present in the paired chromosome. That is, say, there is no brown eye color option at all, but there are four copies of the green option. Such losses (deletions) and doubling (duplications) do not always lead to serious consequences, but they can cause noticeable deviations in the development of the organism.

An international group of scientists published an article in the journal Nature (Mirror extreme BMI phenotypes associated with gene dosage at the chromosome 16p11.2 locus), which reports that doubling a certain section of the 16th chromosome leads to extreme thinness.

The researchers analyzed the DNA of more than 95 thousand people. Those who had this section of the chromosome doubled had a body mass index below 18.5. Children with such a chromosomal feature gained weight much worse. But you should not envy their slimness: a fourth of the owners of chromosomal rearrangement also received microcephaly in the kit — a feature of individual development in which the head and brain are abnormally small. Microcephaly is often associated with various neurological disorders, including mental disability, and the life expectancy of microcephals is less than usual.

A section of the chromosome doubled in very thin people includes 28 genes, so it is not surprising that the effect of such a chromosomal rearrangement extends to a number of traits, not just weight. In addition, the lack of weight caused by the doubling of genes, with a high degree of probability, can simply be dangerous to health.

Men with such duplication are more susceptible to the resulting lack of weight than women: the former are 23 times more likely to get an extremely slim figure, the latter — five times. At the same time, as it was shown earlier, the loss of some of these 28 genes increases the risk of absolutely exceptional obesity - with a body mass index of about 40. The researchers say this is the first time that gene doubling leads to the opposite effect. What exactly each of this set does is still unclear; it is only known that deletions and duplications of some of these genes are associated with the development of schizophrenia and autism.

A practical conclusion that can be drawn from all this: a diet can be completely useless against obesity, and a child can be thin not only because of the parents' oversight, but simply because of genetic disorders. Although for his parents, this can hardly be a consolation...

Prepared based on the materials of Medical Xpress: 'Gene overdose' causes extreme thinness.

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