Many roads lead to the human genome
Development of new biochemical tools has always been an essential prerequisite for progress in human genome research. One of the best examples may be the work of Dr. Jean Weissenbach, who is the scientific director of the privately funded genome research institute, Genethon, near Paris. Since 1991, when the french molecular biologist started his project at Genethon, he established a collection of 5,300 pieces of DNA (microsatellite markers), of which he constructed the first extensive genetic map of the human genome. "Good genome maps highly accelerate the search for disease genes", stressed Dr. Weissenbach at a symposium on "Complex Genetic Diseases" at the MAX DELBRÜCK CENTER FOR MOLECULAR MEDICINE (MDC) BERLIN-BUCH on Thursday, 28 September 1995.
To study the human genome, molecular biologists use a whole set of different approaches. At Genethon, which was established four years ago by the French Muscular Dystrophy Association (AFM) and the Centre d'Etude du Polymorphisme Humaine (CEPH) at Paris, scientists use a technique which is highly informative for genetic differences between individuals. The method is based on the use of microsatellite markers. Microsatellites are sequences consisting of either two, three or four elements (nucleotides) of the genetic material DNA, repeated several times and lined up in a row. Such microsatellite stretches are spread all over the genome. They are valuable tools for studying inheritance because the number of repeats within those stretches varies from one individual to another. Thus, they show which family members have certain sequence patterns in common.
Microsatellite markers require hard work to construct because the length of the 100 to 300 nucleotides long pieces should differ between individuals as much as possible. Moreover, every marker has to be unique within the genome. Its the two ends of a microsatellite marker that determine its specifity, each of them spanning a mean length of 16 to 20 nucleotides. To discover a unique 20 unit sequence within the three billion nucleotides of the human genome is similar to searching for a needle in a haystack. However, with the support of special robots and computers, Dr. Weissenbach and his colleagues at Genethon succeeded in constructing a genetic map consisting of 5,300 microsatellite markers. Thus, the time needed to search for a specific disease gene was reduced from years to months.
To locate a gene, scientists also need DNA pieces of bigger sizes. For this reason, molecular biologists store big fragments of human DNA in yeast cells, more than a million nucleotides long, called Yeast artificial chromosomes (YACs). By defining the position of thousands of different YACs in the genome, scientists at the French CEPH and Genethon constructed a physical map. To date, this map covers more than 75 percent of the genome. Thus, nearly every gene, of which the position is roughly known by studies with a genetic map, can be related to a specific genome fragment.
Using YACs, biologists at the CEPH and Genethon constructed the first continuous map of a chromosome - the chromosome number 21. The other maps of human chromosomes currently being developed by scientists in the USA, Great Britain and Japan contain gaps, i.e. regions to which a YAC could not be related yet. There is one exemption, David Page of the Whitehead Institute in Cambridge, Massachusetts, succeeded in mapping the male Y-chromosome a few years ago.
During the Berlin symposium, Dr. Weissenbach also stressed that the search for genes involved in complex genetic diseases could be greatly accelerated with an additional technique. Its ,development is closely linked with the name of the American scientist, J. Craig Venter, head of the "The Institute of Genome Research" (TIGR), a research institute financed by industry. Venter's method takes only a small part of the genome into account. It is the part which cells of different organs produce copies of for the production of proteins. If these copies are biochemically retranslated into DNA, they form so called ESTs (Expressed sequence tags). By reference to their ESTs, genes recognizable by ESTs can be pulled out from the neighbourhood of certain linkage markers.
"The massive numbers of ESTs constructed all over the world calls for cl large international effort to mapping those new genes", Weissenbach stressed. The various techniques could complement each other, if all mapping projects were coordinated. A first step in that direction, Weissenbach added, has been the recent organization of a consortium of several European and American laboratories to map large numbers of ESTs.
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