Human genetics: the molecular challenge

The 1986 Cold Spring Harbor Symposium was on the subject of human genetics; it was the first symposium at Cold Spring Harbor on this topic since 1964. In the opening remarks for the conference, Walter F. Bodmer first summarized the progress in this field since 1964. He then described what is presently known about the functional complexity of the human genome and discussed the case for a definitive characterization and sequencing of the human genome. The following is an abridged and slightly adapted version of this talk; it is reproduced courtesy of the Cold Spring Harbor Laboratory © 1987.     

Meiotic Recombination in Human Oocytes

Studies of human trisomies indicate a remarkable relationship between abnormal meiotic recombination and subsequent nondisjunction at maternal meiosis I or II. Specifically, failure to recombine or recombination events located either too near to or too far from the centromere have been linked to the origin of human trisomies. It should be possible to identify these abnormal crossover configurations by using immunofluorescence methodology to directly examine the meiotic recombination process in the human female. Accordingly, we initiated studies of crossover-associated proteins (e.g., MLH1) in human fetal oocytes to analyze their number and distribution on nondisjunction-prone human chromosomes and, more generally, to characterize genome-wide levels of recombination in the human female. Our analyses indicate that the number of MLH1 foci is lower than predicted from genetic linkage analysis, but its localization pattern conforms to that expected for a crossover-associated protein. In studies of individual chromosomes, our observations provide evidence for the presence of “vulnerable” crossover configurations in the fetal oocyte, consistent with the idea that these are subsequently translated into nondisjunctional events in the adult oocyte.     

Catechol-O-methyltransferase biochemical genetics: Human lymphocyte enzyme

Human erythrocyte (RBC) catechol-O-methyltransferase (COMT) is under genetic control. Experiments were performed to determine whether COMT in the human lymphocyte is regulated in parallel with RBC COMT. Supernatants of lymphocyte homogenates contained COMT activity. However, they also contained a potent COMT inhibitor, the effect of which could be negated by dilution. Lymphocyte COMT activity was maximal at a reaction pH of 7.7 and at a MgCl₂ concentration of 0.67mm. The apparent K _m value for 3,4-dihydroxybenzoic acid, the catechol substrate for the reaction, was 1.2×10^?5 m and that for S-adenosyl-l-methionine, the methyl donor, was 2.3×10^?6 m. An average of 48.3±3.3% (mean ± SEM) of the enzyme activity in crude lymphocyte homogenates from 3 subjects was removed by centrifugation at 100,000 g for 1 hr and was presumed to be membrane associated. The average COMT activity in lymphocytes isolated from blood of 23 randomly selected adult subjects was 14.0±1.2 units/10⁶ cells (mean ± SEM) or 913±69 units/mg protein. There was a significant correlation of relative RBC with relative lymphocyte COMT activity in these 23 subjects. The correlation coefficient was 0.733 (P<0.001) when lymphocyte enzyme activity was expressed per milligram of protein and 0.649 (P<0.001) when lymphocyte activity was expressed per 10⁶ cells. These results are compatible with the conclusion that the genetic polymorphism which regulates RBC COMT activity may also regulate the level of human lymphocyte COMT activity.