In the current issue of Development, Professor Lovell-Badge, Professor Peter Koopman of the University of Queensland in Brisbane and Professor Andrew Sinclair of the Royal Children's Hospital in Melbourne write about this landmark research. They reflect on what the discovery meant at the time, as well as progress since then and remaining challenges:
On Thursday 9 May 1991, the world awoke to front-page news of a breakthrough in biological research. From Washington to Wollongong, newspapers, radio and TV were abuzz with the story of a transgenic mouse in London called Randy. Why was this mouse so special? The mouse in question was a chromosomal female (XX) made male by the presence of a transgene containing the Y chromosome gene Sry. This sex-reversal provided clear experimental proof that Sry was the elusive mammalian sex-determining gene.
Three decades ago, Professor Lovell-Badge's mouse developmental genetics group was working with Peter Goodfellow's human molecular genetics group at, respectively, the Medical Research Institute's National Institute for Medical Research and the ICRF (which became the CRUK Laboratory in Lincoln's Inn Fields), now both part of the Crick. In a race with a US group led by David Page, the scientists were looking for a gene called the testis-determining factor (TDF) - essentially the gene on the Y chromosome that starts the development of testis in an embryo. Once TDF has done its job, the testis produces all the hormones needed to make a male.
The US group thought they'd found TDF in 1987 but were proved wrong by later evidence. Then, in 1990 the London group identified a 35 kb region of Y chromosome DNA from four masculinised XX patients with testicular tissue. They cut this region into small fragments and tested each on Southern blots - identifying one human Y-fragment that showed male specific bands in mouse, human and other mammal genomes. They tentatively called this fragment sex-determining region on the Y chromosome, or SRY (Sry in mice). Subsequent research supported the finding but it wasn't until Randy the mouse was born that it was definitively confirmed.
The scientists injected a 14 kb DNA fragment containing only the mouse Sry gene. After some initial disappointment, transgenic XX embryos with testes developed that were indistinguishable from control XY embryos. In an experiment where the mice were allowed to develop through to birth, one of the transgenic XX mice appeared like a normal male and was allowed (and very keen!) to mate with females - the nickname Randy stuck. Further investigation showed that Randy had a normal male reproductive tract but no sperm, as predicted for an XX male, proving that Sry was sufficient on its on to initiate male development.
Over the next years Sry proved to be very difficult to work with, not least because it is expressed in only a small number of cells in an inaccessible foetal tissue for a short time. Standard techniques for discovering upstream and downstream genes didn't work. After its discovery, further progress in understanding SRY was slow. But scientists persevered, and over the years have gained a basic understanding of the SRY protein's cellular and molecular functions. It plays a role in activating SOX9, a gene that plays a key role in male sexual development and that Professor Lovell-Badge and many others have gone on to study in detail. The discovery of SRY also led to a search for essential sex-determining genes in non-mammalian vertebrates and insights into the evolution of SRY. However there are still many unanswered questions.
Professor Lovell-Badge says: "In writing this, it was interesting to reflect on how much we have learned over the last 25 years and on how much we still need to know. It was also scary to think how rapidly the years have gone by and that students in the lab now were not even born when Randy was!
"It is also peculiar how things come around; our discovery of Sry depended on a close collaboration between labs in the Institutes at Mill Hill and Lincoln's Inn Fields, and now the institutes are one within the Crick."