Tech UPTechnologyIs it a boy or is it a girl?...

Is it a boy or is it a girl? Chromosomes don't determine everything

It is one of the most basic biology lessons: we know that in the human species there are two types of sex chromosomes: X and Y. The XY combination is what determines that a baby is a boy and the XX combination results in a girl. However, the processes that take place during embryonic development and that lead to the establishment of the sex of the baby are very complex, and sometimes it happens that an embryo with XY genes does not develop testes , and vice versa.

A team of researchers from the University of Melbourne has studied these alterations and has found the answer in what are called DNA amplifiers in genetics: they are short regions of DNA that regulate the activity of certain genes. As explained by Andrew Sinclair, one of the participants in the work, 90% of the human genome is made of what has been called ‘junk DNA’ or dark matter, which does not contain genes but does contain important regulators capable of increasing or decreasing the activity of these.

“The Y chromosome contains a critical gene, called SRY, which works together with another gene called SOX9 to initiate the growth of the testes in the embryo. For their correct development, very high levels of this SOX9 gene are needed ”, explains Brittany Croft, another of the authors. “However, if SOX9 activity is disrupted and the levels are low, there will be no testicle, and the baby will have abnormal sexual development.”

What can cause those SOX9 levels to drop? This is where DNA amplifiers come into play. The work, published in the journal Nature Communications , describes three amplifiers or enhancers that, acting together, ensure that the SOX9 gene is activated at high levels in XY embryos to ensure normal development of the testes. “It is also important to note that we have identified XX patients who should have ovaries and be female, but having additional copies of these amplifiers and high levels of SOX9 ended up developing testes,” explains Croft. “In turn, we identified XY patients who had lost the enhancers, therefore had low levels of SOX9 and ovaries rather than testes,” he adds.

In these cases, the alteration in sexual development is caused by an increase or decrease of the amplifiers with respect to their normal levels. “The results are significant, since we have always focused on the genes to diagnose these patients, but here we have shown that we have to look further”, concludes the researcher.

Not so junk DNA

According to Sinclair, in the entire human genome we have around a million DNA amplifiers that control the activity of about 22,000 genes. “They are found in DNA, but outside of genes, which is why they have always been disparagingly called junk DNA or dark matter.” But more and more research is focused on them, as their importance is essential for the proper functioning of the body. “The key to diagnosing many disorders can be found in these enhancers that hide in dark matter, that unknown region of our DNA.”

Reference: Croft et al. 2018. Human sex reversal is caused by duplication or deletion of core enhancers upstream of SOX9. Nature Communications 9 (1) DOI: 10.1038 / s41467-018-07784-9

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