New landmarks on human chromosomes.

Diagnosis and prenatal determination of the thousands of genetic diseases require a detailed map of the human chromosomes. The number of one type of known landmark on this map has just doubled, according to scientists at Collaborative Research, Inc., of Lexington, Mass. At the recent Human Gene Mapping Workshop in Helsinki, Finland, they reported 500 newly discovered markers, 29 of which they expect to be exceptionally useful for mapping human genes.

Each landmark is a pattern of DNA pieces resulting from the cutting of the chromosomes by specific enzymes called restriction enzymes. The 500 new markers represent regions of the chromosomes where different people show different patterns. These regions are said to be "polymorphic" and the landmarks are known as RFLPs (restriction fragment length polymorphisms). The most useful markers will be those for which the most patterns occur in different individuals, so that they are most likely to distinguish between chromosomes, for instance one inherited from the father and one inherited from the mother. The cut DNA pieces are identified by their binding to short segments of DNA called probes (SN: 8/18/84, p. 104).

"Twenty-nine of these 500 [probes] are among the most polymorphic human markers known and will significantly facilitate tracing the inheritance of certain chromosomes in families," says Thomas O. Oesterling of Collaborative Research. Previously, only one such "highly informative" probe had been identified.

Collaborative Research recently filed a patent application for the first commercial use of RFLPs. A set of highly informative probes is described for paternity testing and for determining after bone marrow transplantations which cells are derived from the patient and which from the donor.

"The test," Oesterling says, "will give doctors a method that can tell with 99 percent accuracy whether bone marrow transplantations have been successful."