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."
Read the
article in full here.