Gene required for sperm stem cells linked to male infertility, UCSD study suggests

Even in this day and age, when a couple is having trouble conceiving a child, it’s often the woman who is initially suspected of having infertility problems and is likely the first to seek out the advice of doctor. But according to Miles Wilkinson, professor of reproductive medicine at UC San Diego School of Medicine, infertility issues can just as likely be due to problems with sperm production in the male. In fact, about 4 million men of reproductive age in the U.S. are confronted with infertility challenges and in 30% percent of those cases, the cause of the infertility is not well understood.

Through some scientific detective work, Wilkinson and his research team have zeroed in on a gene called RhoX10 that plays an essential role in the development of adult stem cells which give rise to sperm production. The study results, funded in part by CIRM and published yesterday in Cell Reports, may help provide a path toward new treatment options for male infertility.

sperm

The process of sperm formation (spermatogenesis). Image: Wikipedia

To get at a cellular and molecular understanding of infertility, the team focused on the function of spermatogonial stem cells (SSCs). Through a multistep process within the male reproductive system, SSCs form into mature sperm cells capable of fertilizing an egg.  The SSC itself forms from primordial germ cells. The key genetic switches that help these germ cells give rise to SSCs was not well understood.  Earlier studies had shown that a group of adjacent genes, called the Rhox cluster, on the X-chromosome are expressed in the testes, suggesting a role in sperm production.

Using genetic engineering techniques, Wilkinson’s team bred mice lacking the 33 genes of the Rhox cluster. The resulting male mice showed a reduction in the number SSCs leading to low sperm number. Through a process of elimination, the team found that deleting just one of those genes, Rhox10, produced nearly the same flaw.

Rhox10 bus_HW, 9-26-16.jpg.jpg

Rhox 10: a genetic “bus” that “drives” sperm stem cells toward sperm production. Illustration by Hye-Won Song.

Further analysis, indicated Rhox10 was key to driving the development of germ cells into SSCs. So when the gene is deleted, not enough SSCs develop in the testis, leading to low sperm counts.  The researchers also found the Rhox10 is a master regulator of genes that control the germ cells’ movement from one part of the testis to another that, due to a different chemical environment, helps the germ cells transform into SSCs.

In addition to this mouse data, the team also co-authored a recent Human Molecular Genetics report with scientists at the University of Münster in Germany that connects Rhox genes and human male infertility. In the study, three RHOX genes were sequenced in 250 men with extremely low sperm counts and revealed six genetic mutations. Together, these results present solid evidence that mutations in Rhox are the culprit in at least some forms of male infertility. First author Hye-Won Song discussed this point in a university press release:

“Spermatogonial stem cells allow men — even in their 70s — to generate sperm and father children. Our finding that Rhox10 is critical for spermatogonial stem cells, coupled with the finding that human RHOX genes are mutated in infertile men, suggests that mutations in these genes cause human male infertility.”

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