LONDON - Estimates of the number of genes in the human genome vary from 40,000 to 120,000. Mutations in each of these genes could, in theory, produce disease. Yet there are only about 2,500 mouse strains worldwide that model human diseases.

The discrepancy between these numbers highlights the importance of developing new mutant mouse strains, says Steve Brown, director of the Medical Research Council's Mammalian Genetics Unit at Harwell, UK. He and his colleagues report in the August Nature Genetics on the value of a systematic mutagenesis program in mice in generating new models of this kind.

In a letter titled, "A systematic, genome-wide, phenotype-driven mutagenesis programme for gene function studies in the mouse," Brown, together with colleagues at SmithKline Beecham Pharmaceuticals in Harlow, UK, and collaborators from various centers in the UK and France, describe how they generated large numbers of mutants for further study.

Brown told BioWorld International, "We are now entering a golden era of mouse genetics. We now have the techniques that allow us to move down from interesting new disease phenotypes to the mutated gene and its function. People are looking at both new disease models as well as many of the interesting mutations that have been archived decades ago."

The approach complements that of altering the function of a gene once it is thought to be involved in a particular disease, to find out the effect. Brown said, "The traditional route for studying gene function in mice has been by introducing mutations through the knockout route. We have decided to come in from the other end, scanning mutagenized mice to find new disease phenotypes. This is a very powerful approach because it allows us to find out novel gene functions that would be entirely unexpected. We start with an interesting disease model, and we know that we must have affected a gene that is pivotal in the development of that particular disease."

Mutant Mice Speed Mapping

Findings such as these, he predicted, will prove important in developing new treatments and understanding the underlying mechanisms of various diseases. "As a result we will be able to develop new approaches to disease management, and to develop new drugs," he added.

The group reports the use of the chemical mutagen ethylnitrosourea (ENU) to generate mutant mice.

A similar study, also using ENU, is reported in the same issue of Nature Genetics by researchers running the German Human Genome Project. Martin Hrabe de Angelis, of the Institute of Experimental Genetics in Nuremberg, Germany, together with international collaborators, describe their results in a letter titled, "Genome-wide, large-scale production of mutant mice by ENU mutagenesis."

Brown and his colleagues induced mutations in the sperm cells of male mice by injecting them intraperitoneally with ENU. The progeny of these mice were screened and bred to identify inherited mutations resulting in disease phenotypes.

Out of more than 26,000 progeny obtained, approximately 500 mutants were recovered. The type of screening carried out included tests to evaluate neuromuscular, sensory, motor and neuropsychiatric function, the animals' behavior and blood chemistry.

The phenotypes identified included size, craniofacial abnormalities, defects of eye, tail, skin, hair and ear, as well as renal, neurological and neuromuscular defects. "The models of human diseases we obtained range from osteoporosis, visual impairment, renal function and hearing impairment to abnormal cholesterol function and diabetes," Brown said.

Having identified an abnormal phenotype, it might be expected that finding the mutant gene responsible is like looking for a needle in a haystack. Not so, said Brown. "This is where this project is extremely timely. Because we now have the human genome sequence, and because the mouse genome is also being sequenced, this allows us to map very quickly where the causative mutation is in the mouse genome, and so identify the likely gene that is causing the disease phenotype."

Increase In Mouse Disease Models Sought

The group already has identified the genes involved for some of the mutations and, Brown said, "the process of getting from phenotype to gene is getting faster and faster."

All the mutant mice being generated are being archived as frozen embryos. They are being stored at a network of mutant mouse archives being set up worldwide, one of which is at the Mouse Genome Centre at Harwell.

"This is just the beginning of a much bigger international program to increase the number of mouse disease models that we have available. We have made a good start in adding another 500, but if you think about the number of genes in the human genome, there is a lot to do and a long way to go," Brown concluded.

The German group reported the screening of more than 14,000 mice for a large number of clinically relevant parameters and identified 182 mouse mutants with various phenotypes. The mutants they describe include one with hypophosphatasia and another with high levels of plasma urea. This team also conducted a screen focused on identifying inherited immunological abnormalities, and discovered seven mutant lines with absence of or reduced levels of certain cell populations or cell-surface molecules.

Another nine mutant lines were identified which had undetectable levels of immunoglobulin E, and three mutant lines with high plasma IgE concentrations.

Hrabe de Angelis and colleagues conclude: "Even when chromosomal mapping has reduced the candidate region to a few hundred kilobases or a few candidate genes, one of the major rate-limiting steps is still the actual identification of the individual causative mutations. But once cheap, high-throughput mutation detection technology becomes available, the DNA of offspring from mutagenized mice can be directly analyzed for mutations in specific genes, permitting the convergence of phenotype and gene-driven mutagenesis."