New discovery about how a baby's sex is determined

Monday, 17 December, 2018

New discovery about how a baby's sex is determined

Australian medical researchers have made a new discovery about how a baby’s sex is determined. It turns out the process is not just about the X-Y chromosomes, but involves a ‘regulator’ that increases or decreases the activity of genes which decide if we become male or female.

The study was led by Murdoch Children’s Research Institute, with PhD student Brittany Croft serving as first author. She explained, “The sex of a baby is determined by its chromosome make-up at conception. An embryo with two X chromosomes will become a girl, while an embryo with an X-Y combination results in a boy.

“The Y chromosome carries a critical gene, called SRY, which acts on another gene called SOX9 to start the development of testes in the embryo. High levels of the SOX9 gene are needed for normal testis development.

“However, if there is some disruption to SOX9 activity and only low levels are present, a testis will not develop, resulting in a baby with a disorder of sex development.”

The study’s lead author, Professor Andrew Sinclair, noted that 90% of human DNA is made up of so-called ‘junk DNA’, which contains no genes but does carry important regulators that increase or decrease gene activity. “These regulatory segments of DNA are called enhancers,” he said.

“These enhancers lie on the DNA but outside genes, in regions previously referred to as junk DNA or dark matter,” Prof Sinclair continued. Across the human genome there are about one million enhancers controlling about 22,000 genes, he said.

Prof Sinclair said the new study sought to understand how the SOX9 gene was regulated by enhancers and whether disruption of the enhancers would result in disorders of sex development. The results were published in the journal Nature Communications.

“We discovered three enhancers that together ensure the SOX9 gene is turned on to a high level in an XY embryo, leading to normal testis and male development,” Prof Sinclair said.

“Importantly, we identified XX patients who would normally have ovaries and be female but carried extra copies of these enhancers and instead developed testes. In addition, we found XY patients who had lost these SOX9 enhancers and developed ovaries instead of testes.”

Croft said human sex reversal such as seen in these cases is caused by gain or loss of these vital enhancers that regulate the SOX9 gene; consequently, these three enhancers are required for normal testes and male development.

“In the past researchers have only looked at genes to diagnose these patients, but we have shown you need to look outside the genes to the enhancers,” Croft said.

Prof Sinclair added, “The key to diagnosing many disorders may be found in these enhancers, which hide in the poorly understood dark matter of our DNA.”

Image credit: © Goddard

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