Human Genome Project may help find treatment for muscular dystrophy
One of this country’s most eminent geneticists is using results coming from the human genome project to help combat muscular and nervous system disorders. She hopes that the knowledge gained will enable drugs to be developed that will compensate for defects and effectively treat these devastating diseases.
Professor Kay Davies is head of the Department of Human Anatomy and Human Genetics at the University of Oxford. One of the disorders her group is studying is Duchenne muscular dystrophy (DMD), one of 20 different types of muscular dystrophy. DMD, which affects boys only, is caused by a lack of a protein called dystrophin, which joins the inside of muscle fibers to the outside. The result of the defect is that the body’s skeletal muscles gradually waste away and affected individuals eventually lose their ability to walk.
The dystrophin gene is located on the X chromosome, which is present in duplicate in women and as a single copy in men. This means that boys who inherit the disease will do so from a mother who carries one copy of the defective gene.
“Because it is one of the largest of all known genes, using gene therapy to introduce it into all affected muscle cells, including those in the heart, is likely to be problematic.”
Professor Kay Davies Head of the Department of Human Anatomy and Human Genetics at the University of Oxford
Even if this were feasible, there is also the possibility that patients would mount an immune response and reject the protein. Her team therefore needed to find innovative ways of compensating for its absence.
In 1989, they made a key discovery by identifying a relative of dystrophin, called utrophin, whose gene was much smaller than that of its big sister, but had similar functions.
The locations of utrophin and dystrophin are similar early in embryonic development – both are present at the junctions between muscles and nerves and on the membranes of muscles. Later in development, utrophin is located only to the nerve-muscle junctions. Davies and her team thought that if utrophin could be persuaded to relocate to the muscle membrane then it may substitute for the missing dystrophin in DMD patients.
“There are several examples of how the absence of one gene can be compensated for by the presence of another. This is the rational behind the replacing the missing dystrophin gene with utrophin in DMD.”
Professor Kay Davies
Her group are presently identifying genetic sequences in the utrophin gene that may control the location of the protein.
“Every day we look at the freely available sequence information coming from the human genome project. We are now piecing together these sequences to reproduce the entire regulatory region of the utrophin gene.”
Professor Kay Davies
Armed with this knowledge Her team are pursuing two possible treatment options.
The first involves gene therapy. Here, the group plans to introduce the gene into viruses that will infect the muscle cell and carry the utrophin gene with it, so the virus will produce utrophin protein that will travel to the muscle membrane and the junctions between nerve and muscle. The problem here is to find a safe virus that will deliver the gene to all muscle cells.
The other approach the team is actively pursuing aims to identify drugs that increase utrophin’s production. If a muscle cell produces extra utrophin, some will very likely be forced to move to the muscle membrane. They screen for small chemical compounds by linking the regulatory region of the utrophin gene to another gene whose product can be detected easily in cells, for example by fluorescence. They then screen thousands of chemicals in cells and look for an increase in fluorescence.
Davies and her team have already started the screening test, and as more sequence becomes available, they will learn more about the regulatory regions and be able to narrow down their search for drugs.
“This screen may take two years to complete, and the drug would then need to be processed through the usual clinical trials for efficacy and safety.”
Professor Kay Davies
One of the most powerful uses of the HGP is that it can help to rapidly identify new genes that are related to known ones.
“We found the utrophin gene by chance but if we deliberately set out to look for a relative of dystrophin it may have taken five years or more before the advent of the HGP. It now could take a single day if the sequences are already there.”
Professor Kay Davies
Tracking down equivalent genes in other organisms – known as orthologues – is greatly aided by the human genome project Says Davies. The HGP is being used to identify orthologues of important genes in the nematode worm Ceanorhbditis elegans and the fruit fly Drosophila melanogaster, for example.
“If organism has a known defect when a specific gene is mutated, it can help define the gene’s function in humans.”
Professor Kay Davies
Her group is using similar principles to identify genes involved in a related disorder called spinal muscular atrophy (SMA). This disease, of which there are several types, is caused by mutations in a gene known as SMN, which causes loss of motor neurons in the spinal cord. This motor neuron loss prevents some nerve impulses from being passed to muscles and leads to weakness and wasting in affected individuals.
Davies’ team have now identified other proteins that interact with SMN and have obtained partial gene sequences corresponding to these proteins. They are presently scanning the data from the HGP in order to piece together the full-length sequences of these genes, including the regions that control their activity.
She hopes that this short cut will save the group the years of effort needed to identify the genes and determine how they are regulated. Once the sequence of the SMA-interacting genes is available, Davies plans to look for mutations in patients with spontaneously arising SMA and in other neuromuscular disorders.
Colleagues at the Hammersmith Hospital in London are working with patients with these disorders and will collaborate with Professor Davies on the project.
“The identification of genes that interact with SMN will be useful for two reasons. First, they may turn out to be involved in other neuromuscular disorders. In addition, they may help to build of a picture of the complete pathway in which SMN is just a player.”
Professor Kay Davies
“Professor Davies and her team are conducting such vital work in understanding the genetics around SMA. We sincerely hope that this work will soon lead to the answers that our families who live with SMA on a daily basis await.”
“My daughter died from SMA Type I fifteen years ago – whilst any potential treatments are too late for her I have seven nieces and nephews who may be carriers and also at risk of having a child with SMA one day. I live in hope that answers are found not only for my own family but for the many, many families who have experienced the pain of watching a beloved baby die.”
Anita Macaulay, Director of the Jennifer Trust for Spinal Muscular Atrophy
More information
1. DMD affects 1 in 3,000 males, and around 1,500 boys with the disorder are living in the UK at any one time.
Muscular Dystrophy Campaign:
7-11 Prescott Place,
London SW4 6BS.
Tel: 020 7 720 8055
Web: www.muscular-dystrophy.org
2. SMA affects around 1 in 10,000 (boys and girls are both affected).
There are several different types of SMA with varying severity; see the website below for more details.
A family affected by SMA have expressed their willingness to talk to the media. Please contact the Wellcome Trust Press Office for information. There are also some personal stories from families affected by SMA on the website below.
Jennifer Trust for Spinal Muscular Atrophy:
Elta House,
Birmingham Road,
Stratford-upon-Avon
Warwickshire,
CV37 0AQ.
Tel: (+44) 01789 267520;
Web: www.jtsma.demon.co.uk
3. Professor Davies is Head of the Department of Human Anatomy and Genetics at Oxford University. She is also Honorary Director of the Medical Research Council Functional Genetics Unit in Oxford.
Professor Davies’ work on DMD is funded by the MRC and the Muscular Dystrophy Group. Her work on SMA is funded by the Jennifer Trust for Spinal Muscular Atrophy.
4. As part of the Joint Infrastructure Fund, the Wellcome Trust have recently funded a new Oxford Center for Gene Functions. This is a collaboration between the Department of Physiology, headed by Professor F. M. Ashcroft, and Kay Davies’ Department of Human Anatomy and Genetics. The Wellcome Trust also funds Professor Davies in a number of other areas of neurological and cardiovascular research.
