The mammalian Y chromosome encodes a specialized set of genes that are essential for male viability and fertility. In particular, the sex-determining region Y (SRY) protein is necessary to initiate male sex determination. However, a new study from researchers at the University of Hawai’i and INSERM has shown that live mouse progeny can be generated with assisted reproduction using germ cells from males which do not have any Y chromosome genes. The team state that their findings support the hypothesis that Y chromosome genes can be replaced by overexpression of genes on other chromosomes. The study is published in the journal Science.
Earlier studies from the group successfully obtained offspring from male mice that had only two Y chromosome genes, namely testis determinant Sry and spermatogonial proliferation factor Eif2s3y. These males did not produce sperm and researchers had to use the immature precursor cells, known as spermatids, and a technique called Round Spermatid Injection (ROSI) to achieve fertilization. The same team also demonstrated that only testis determinant factor Sry and the spermatogonial proliferation factor Eif2s3y, were needed for male mice to sire offspring with assisted fertilization. The current study takes a step further to produce males completely devoid of the entire Y chromosome.
The current study generated ‘No Y’ males, and defined the ability of these males to produce gametes and sire offspring. The team first replaced the Y chromosome gene Sry with its homologue and direct target encoded on chromosome 11, Sox9. The lab explain that normally, Sry activates Sox9, and this initiates a cascade of molecular events that ultimately allow an XY fetus to develop into a male. The researchers used transgenic technology to activate Sox9 in the absence of Sry. Next, the group replaced the second essential Y chromosome gene, Eif2s3y, with its X chromosome encoded homologue, Eif2s3x, which was then transgenically overexpressed. Data findings show that under these conditions, Eif2s3x took over the function of Eif2s3y in initiating spermatogenesis.
Finally, the team replaced Sry and Eif2s3y simultaneously, and created XOSox9, Eif2s3x males that had no Y chromosome DNA. Results show that mice lacking all Y chromosome genes developed testes populated with male germ cells. Round spermatids were harvested by the researchers and the ROSI technique once again used to successfully fertilize oocytes. Data findings show that when the developed embryos where transferred to female mouse surrogate mothers, live offspring were born. The lab note that the offspring derived from the ‘No Y’ males were healthy and lived for normal life span, adding that the daughters and grandsons of the ‘No Y’ males were fertile and capable of reproducing without technological intervention. The group stress that in humans, ROSI is currently considered experimental due to concerns regarding the safety of injecting immature germ cells and other technical difficulties.
The team surmise that when it comes to assisted reproduction, they have now shown that in the mouse the Y chromosome contribution is not necessary. They go on to add that their study provides new important insights into Y chromosome gene function and evolution, supporting the existence of functional redundancy between the Y chromosome genes and their homologues. For the future, the researchers state that the development of assisted reproduction technologies (ART) allows bypassing various steps of normal fertilization by using immotile, non-viable, or immature gametes. They go on to conclude that they hope that the success in mouse studies may spark the re-evaluation of human ROSI for its suitability to become an option for overcoming male infertility in the future.