Sherpa genes: How to be fit for heights

5. December 2014
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Tibet sits about 4,000 metres above sea level. Whereas tourists suffer from a lack of oxygen, the locals live there without physical discomfort. Recent genetic analyses indicate that a genetic modification affecting erythropoiesis makes this possible.

Already in 2010 Chinese researchers discovered in the Tibetan gene pool a number of alterations which are conducive to life at extreme altitude. One of these relates to gene EGLN1. This gene contains the blueprint for the enzyme prolyl hydroxylase 2 (PHD2), which under circumstances of normal oxygen supply hydroxylises the transcription factors hypoxia-inducible factors (HIFs), whereby they get broken down. HIFs are especially important in situations of insufficient supply of oxygen to cells, that is to say that PHD2 due to a lack of oxygen no longer reacts with the transcription factors. Consequently, HIFs are not broken down and are able to activate various signalling pathways. In the bone marrow the increased presence of HIFs play out their intermediary role in the formation and development of oxygen-transporting red blood cells, the erythrocytes. The more red blood cells are produced, however, the more viscous will be the blood. At some particular point this leads to an impairment of the oxygen supply to the organs. Particularly affected by these processes are people from lower altitudes who make their way to extreme elevations. Tibetans, however, also when on the high plateau do not have increased erythrocyte concentration. Which mutation is the basis for this, and how processes are influenced within the body, remained nevertheless unknown.

Newly discovered “missense” mutation always in double pack

According to the research team headed by Josef Prchal, the cause of these different levels of red blood cell production under conditions of a lack of oxygen resides in a point mutation in the gene EGLN1. For the purpose of their studies the researchers analysed samples from 26 Tibetans who lived in Virginia and Utah, as well as a total of 121 Asians and Europeans. In the selection of participants, however, the degree of admixture between Tibetans and non-Tibetans was not taken into account, nor was population structure. Prchal and his team discovered that two DNA building blocks of the EGLN1 genes were different in the Tibetan group: at nucleotide 380 the nucleotide cytosine was found instead of the nucleotide guanine. This mutation, given the name c.380G>C, was however an already known one. The research team identified it among all the US Tibetans and in every fifth non-Tibetan.
The second point mutation affected nucleotide 12. The cytosine here was replaced by a guanine (c.12C> G). Around 88 percent of the 91 US and highland Tibetans studied, but only 0.8 percent of the 242 non-Tibetans, were carriers of this gene variant. The nucleotide base switch ensures that another amino acid gets inserted into PHD2. Thus, a chemically slightly modified enzyme emerges, which indirectly impedes overproduction of red blood cells when lack of oxygen occurs (“missense” mutation). Interestingly the point mutation c.12C>G always occurs together with the already known c.380G>C. As an explanation, the researchers state that the nucleotide base at position 380 on the chromosome was the subject of a switch first, this one thus being older. Then, about 8,000 years ago, the second “missense” mutation c.12C>G was created on the same chromosome.

One nucleic unit makes the difference

Next, the researchers studied how the double mutation of EGLN1 affects red blood cell formation and development compared to the unchanged original and the singularly mutation. Since EGLN1 contains the blueprint for PHD2, there emerge accordingly versions with no, with a singular or with a double “missense” mutation on the EGLN1 PHD2 proteins, involving zero, one, or two modified amino acids. It was found that the PHD2 with the two alternate amino acids binds better to oxygen molecules than do the non-modified PHD2 or the PHD2 variants in which only one amino acid base had been changed. This higher affinity is believed to result in a higher hydroxylation rate of the HIFs transcription factors under conditions of oxygen deficiency, whereby the latter are increasingly broken down, because the researchers were able to show in another piece of research that cells with the modified double EGLN1 under conditions of lack of oxygen express fewer HIFs than cells with the original gene.
Finally, the researchers examined the blood of three Tibetans as well as ten non-Tibetans. It was found here that the erythrocyte precursors, which came from donors with the double mutation in the EGLN1 gene, when under lack of oxygen replicated significantly more slowly than those in the cells of the people who were carriers of the original gene. Under an absolute lack of oxygen their growth even stopped completely.

Application in medicine?

As interesting as these findings are, one still has to bear in mind the following: over the 21,000 years since the Tibetans settled on the plateau, the body did not only employ one singular mutation to adapt to these extreme environmental conditions. Rather, a variety of gene mutations developed. Only the interaction involving these allows optimal survival in extremely low oxygen content environment. Nevertheless, the findings delivered by the scientists led by Prchal could open up new possibilities in medicine. Potential measures include the use of knowledge gained here in the treatment of people with cardiovascular diseases that are often accompanied by a lack of oxygen, or of people who are suffering from a severe form of mountain sickness. But that is all a glimpse into the future. Before that happens, a number of other studies have to yet follow.

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