By Sharon Kingman

Special To BioWorld Today

A gene which probably accounts for some cases of human infertility may allow the development of new contraceptive methods, researchers in Scotland said. They have shown that, in mice, the gene's protein product is essential for germ cells to survive in both males and females.

Howard Cooke, head of chromosome biology at the Medical Research Council's human genetics unit at the Western General Hospital, in Edinburgh, said mutations in the gene will probably be found to be responsible for some cases of infertility in women, and that the gene may play a role in male infertility. Its discovery also could lead to new contraceptives.

Cooke told BioWorld Today: "As far as we can detect, this gene is only expressed in germ cells. This is highly speculative, but if you were able to design a drug that interfered with the function of this gene's product, this might provide a route into contraceptives other than those provided by the usual hormonal approaches."

Cooke, together with colleagues from the same unit and from the Medical Research Council's reproductive biology unit, also in Edinburgh, reported the team's results in Nature, dated Sept. 4, 1997, in a letter titled "The mouse Dazla gene encodes a cytoplasmic protein essential for gametogenesis."

Several years ago, Cooke's group began studying the genomes of men who were infertile and who produced either no sperm or very few — conditions called azoospermia and oligozoospermia, respectively. Some of these men had large chunks of their Y chromosomes missing, while others had more subtle Y chromosome abnormalities, detectable only at the molecular level. Cooke and his colleagues went on to identify one of the gene families missing from some of these men as RBM, while a competing group found that a related gene family, called Daz, was missing from the Y chromosome of others.

Gene Function Studies Done In Mice

The Edinburgh researchers went on to investigate the functions of these genes in mice. They found that mice did not have a Daz gene on their Y chromosomes, although — like other mammals — they did have an ancestor of the Daz gene on one of their other chromosomes, which Cooke and his colleagues called Dazla. Cooke's group showed that the only animals which had the Daz gene on their Y chromosomes were the Old World monkeys, anthropoid apes such as chimpanzees, and humans.

Cooke said: "We wanted to know exactly what the Daz gene was doing. It was not clear that it was essential for spermatogenesis because its loss did not always result in complete infertility. It was difficult to study this in humans because in humans Daz is represented by a gene family rather than by a single gene. So we decided to work with the mouse which has only a single copy of Dazla."

The group made transgenic mice in which they had disrupted the Dazla gene, and studied animals with two copies of the disrupted gene (homozygotes). These animals were infertile. Cooke said: "The homozygotes turned out to be perfectly normal in all respects except that they don't have any germ cells surviving by the time they are adults. This was true for both male and female animals."

Anatomical and histological analysis of homozygous females showed that their ovaries were tiny by comparison with those of normal controls, and that there were no follicles or ova. The testes of homozygous males were about one third the size of the testes of normal controls, and no sperm were found in the epididymes of homozygous males. Histology showed that there was "an almost complete absence of germ cells" in the testes of these animals, the researchers wrote in their letter to Nature.

Normally, germ cells proliferate in the fetus and then survive until puberty and throughout reproductive life. The group therefore went on to examine whether normal levels of germ cells were present in fetal homozygous mice, and how long these survived. They found that ovaries of homozygous mice appeared relatively normal in 15-day fetuses but that by 19 days' gestation there were lower than normal numbers of oocytes and many of those that were present were degenerating. A similar picture was present in male fetuses.

Human Implications Still Unclear

Cooke said: "These results suggest that the Dazla gene product is necessary for the germ cells to survive beyond embryonic development. The germ cells are there in the embryos but they die before the mice reach puberty." Other experiments have shown that the protein is present in the cytoplasm of germ cells, which, Cooke said, suggests that it may be involved in controlling the translation of RNA into protein.

He added: "Because women do not have a Y chromosome, it is possible to extrapolate the findings from female mice to women rather directly. You would expect women who had two mutated copies of the Dazla gene to be infertile.

"It is less easy to extrapolate from male mice to men because of the Daz genes on the Y chromosome," Cooke said. "We don't yet know if the Daz genes are functionally equivalent to the Dazla genes and we are working on that at the moment."

To find out, the group plans to put in the normal human Daz gene in place of the mutated Dazla genes in the mice to determine if this maneuver "rescues" the animals' ability to produce sperm and eggs.

The group also wants to find out what the effect in humans would be of having one mutated copy of the Dazla gene, as some of the work in mice suggested that males heterozygous for the faulty gene produce high levels of abnormal sperm.

Some laboratories are currently screening people attending infertility clinics to find out whether they have mutations in their Dazla genes. Cooke said the results so far suggest that Dazla is not a commonly mutated gene in this population, but adds that it is likely to be one of many genetic components of infertility in human females.

It may also become important to be able to diagnose the genetic causes of infertility, he said, particularly where techniques such as intracytoplasmic sperm injection are used. This assisted conception technique is used to help couples where the male partner has a very low sperm count. It involves taking a single sperm and injecting it into the egg in order to bring about fertilization.

But, Cooke pointed out, if the cause of the infertility is genetic, this method may result in the couple having a child who will in turn grow up to be infertile. *