GENETIC CONSIDERATIONS IN NONDISJUNCTION

William J. Young, Ph.D.

Department of Anatomy University of Vermont College of Medicine

Burlington, Vt .

Among the chief objectives of geneticists is the identification of those com- ponents of variation that can be ascribed to the action of single genetic loci, as well as the subsequent characterization of the biochemical steps through which the primary gene action determines the segregating phenotype. Implicit in such an analysis is knowledge of the specific familial distribution of the character under observation. This objective is perhaps the more taxing in human biology; for so refractory an object of genetic study, however, we may wish to determine whether there are model systems in other organisms that are better suited for refined genetic and biochemical analysis. The existence of these model systems offers us confidence that careful scrutiny of the human phenotype may yield significant genetic insight. Present knowledge of the genetic control of meiotic events in man, most especially in the human female, persuades us that it is appropriate, indeed necessary, to continue to ask for direction from experi- mental organisms.

The vigorous efforts to partition the etiology of the meiotic disturbances leading to the aneuploid gametes in parents of Down’s syndrome patients are well illustrated by the several sessions of this conference; they include an array of exogenous (radiation, viruses) and endogenous (maternal age, auto-anti- bodies, specific genetic effects) agencies. That the attempts to ascribe meiotic error to the action of specific gene loci has been disappointing is indicated by Penrose’s suggestion that in perhaps ten percent of cases a genetic locus may be implicated, and in no instance has a genetic risk for a particular kindred been established.

Following the lead of Penrose,* a number of studies have been conducted in an effort to determine whether the existence of a gene predisposing to nondis- junction may be revealed by the occurrence of increased consanguinity in fami- lies of affected individuals.3-6 (See Reference 7 for review and discussion.) The guiding assumptions in these studies have been that the putative gene is rare in the population, acts only in the homozygous state, and hence would be most effectively identified by analysis of grandparental consanguinity, especially that on the maternal side, due of course to the well-known association of increased maternal age and Down’s syndrome incidence. These studies have analyzed a variety of populations, including those with relatively high inbreeding coeffi- cients as well as less highly selected populations. The broad conclusion to be drawn 1, is that there appears to be no significant increase in consanguinity in the critical components of the kindreds of affected individuals as compared with appropriate control populations. This suggests that if the postulated genes are indeed segregating in the human population they must exist at relatively high frequencies, and hence are not readily identified by the analytical methods hitherto employed.

It has also been suggested 137-r’ that genes with a dominant mode of inheri-

39 1

392 Annals New York Academy of Sciences

tance may be segregating in certain kindreds, leading to the appearance of several different aneuploidies in the same kindred. The multiple aneuploidy has in- volved, in addition to that for chromosome 21, the X chromosomes as well as chromosomes in the D and E groups. The frequent association of trisomy 21 and XXY is especially striking,8 although there is to date no convincing evidence for the existence of a gene responsible for the observations and the mechanism remains unknown. Other explanations for these associations, based on the dis- tributive pairing hypothesis of Grell 1 0 ~ 1 1 have been advanced,lZ but additional studies in mice,13 corn,l+ and man ITr have not provided support for the existence of this mechanism in organisms other than Drosophila. Thus, there is no persua- sive evidence in man linking specific genetic elements, distorted segregation, and morphological modification of the meiotic process. The general reason for this ignorance is known to students of the genetics and cytogenetics of human beings and need not be explored further here.

There is perhaps a more specific reason, however, which reflects upon our current inability to engage in the genetic dissection of the heterogeneous pheno- type we describe as nondisjunction. The recognition of heterogeneity at the phenotypic level, and its genetic identification, has had significant consequences in human biology (see Reference 16 for review). This highly fruitful analysis may well be extended to gametogenesis in man. It is evident to all students of the reduction divisions that aneuploid gametes may derive from alterations at a number of sites and times during meiosis. There may, for example, be failure of conjunction of homologs (asynapsis), premature disjunction (desynapsis) premature terminalization, and failure of chiasma formation, as well as classical nondisjunction of the conjoined homologs (first division) or chromatids (second division).l Hence, the search for a gene conditioning the phenomena we lump under the notion “nondisjunction” is unrealistic. If we would understand how gene changes may lead to the meiotic alterations whose results we observe, we should perhaps turn to the models developed from the analysis of meiotic mu- tants in other species.

mutants affecting one or another aspect of the meiotic process have been known for some 20, z1 and have been identified in a variety of plant and animal species.”. 2~ ? 3 Of the mutants affecting meiosis in Drosophila (see reviews by Sandler and coworkers l 7 and King 2 4 ) , several have been analyzed in sufficient detai1 to provide evidence for the proposition that the gamete-generating process may be genetically dissected into specifically controlled steps. The mutants known as c3G, can‘z, sbdlOS, and mei-S332a are illustrative, and each will be briefly described.

c3G21 is a third chromosome mutant which exhibits several meiotic pheno- types Z s p 2 6 : females homozygous for the mutant have a normal body phenotype. Intrachromosomal recombination in their oocytes is reduced to an extreme degree, while the incidence of first meiotic division nondisjunction (sensu strictu) is very high. No comparable effect in the male is observed; nondisjunc- tion in heterozygous females (c3G/ + ) is not elevated, although, curiously, crossing over is.?; Electron microscopic analysis of the developing oocyte by Meyer 28 and by Smith and King 2rr has revealed the failure of formation of synaptonemal complexes in the homozygous females, These structures, not observed in the Drosophila male (see Moses 2 R for review), have been correlated with meiotic recombination as well as with regular first-division disjunction. King has suggested 26 that the normal allele ( c3C+) is required for the synthesis, presumably prior to li the initiation of synapsis, of some presently unidentified

Spontaneous and induced

Young: Genetic Considerations in Nondisjunction 393

structural components of the synaptinemal complex (see References 30, 32 for other evidence on the existence and presumed roles of DNA and protein syn- thesis during meiotic prophasc). This mutant, therefore, evidently acts at a specific site and time during gametogenesis and is also sex specific.

Claret-nondisjunctional ( cand) is also a third chromosome mutant of Drosophila melanogaster, which is probably homologous with the claret ( c n ) mutant of Drosophila simulans discovered by Sturtevant in 1929 l8 and analyzed cytologically by Wald 33 in 1936. The meiotic phenotype of females homozygous for these mutants includes a very high frequency of first-division nondisjunction for all chromosomes although, in distinction to c3G, recombination appears to be unaffected. Multiple aneuploidy is not uncommon. Wald 33 suggested that the defect in Drosophila simuluns lay in spindle abnormalities in the first divi- sion. Davis s4 analyzed c a n d more intensively and concluded that nondisjunction is restricted to the first division, is apparently unrelated to prior crossing over and, in agreement with Wald, suggests that the defect is in the division mecha- nism, presumably in some spindle component, and thus is clearly a different type of lesion than that responsible for the c3G phenotype.

Stubbloid ln5 is a third chromosome mutant which is deficient for a region including the c3G locus. The homozygote is lethal; heterozygous females, in addition to reduced crossing over 2T, have markedly increased nondisjunction. In comparison to c3G, however, the frequency of error is reduced. Analysis of the developing oocyte 24 indicates that synaptonemal complex synthesis is not inhibited but is terminated precociously. This mutant, then, is an important temporal variant of c3G.

have shown that a recently discovered second chromosome mutant, rnei-S332a, produces second-division nondisjunction in both males and females. This partial dominant mutant is especially significant in that it strongly suggests that the second division has a common control mechanism in the two sexes in Drosophila.

have added another parameter to our under- standing of meiosis by examining natural fly populations for the existence of occult meiotic mutants. Using the sophisticated genetic technology available to drosophilists they discovered 13 different mutants, with a wide variety of meiotic phenotypes, which discriminate several control points through meiosis. The success of this population analysis, which recovered a remarkably large number of loci influencing meiotic behavior ( 1 1 , specific for female meiosis, were found in 11 8 complements examined), clearly indicates that the ten percent of Down’s syndrome patients attributed to specific genetic elements may well be an under- estimation, and a consequence of the blurring of relatively common genetic effects by the social and biological structure of human populations.

Sandler and colleagues

Sandler and his coworkers

REFERENCES

1. PENROSE, L. S. & G. F. SMITH. 1966. Down’s Anomaly: 160-171. Little, Brown

2. PENROSE, L. S. 1961. Mongolism. Brit. Med. Bull. 17: 184-189. 3. HECHT, F., .I. S. BRYANT, D. GRUBER & P. L. TOWNES 1964. The nonrandomness

of chromosomal abnormalities. Association of trisomy 18 and Down’s syn- drome. New Eng. J . Med. 271: 1081-1086.

4. JUBERG, R. C. L. M. DAVIS. 1966. Etiology of nondisjunction: lack of evi- dence for genetic control. Genetics 54: 342 (Abstr.).

& Co. Boston, Mass.

394 Annals New York Academy of Sciences

5. KWITTEROVICH, P. O., H. E. CROSS & V. A. MCKUSICK. 1966. Mongolism in an inbred population. Bull. Hopkins Hosp. 119: 268-275.

6. WELCH, P. 1969. Ph.D. Dissertation. The Johns Hopkins University. Balti- more, Md.

7. FORSSMAN, H. & H. 0. AKESSON. 1967. Consanguineous marriages and mon- golism. In Mongolism. Ciba Foundation Study Group No. 25. G. E. W. Wolstenholme & R. Porter, Eds.: 23-34. Little, Brown. Boston, Mass.

8. HECHT, F., J. E. NIEVAARD, N. DUNCANSON, J. R. MILLER, J. V. HIGGINS, W. J. KIMBERLING, F. A. WALKER, G. S. SMITH, H. C. THULINE & B. TISCHLER. 1969. Double aneuploidy: the frequency of XXY in males with Down’s syn- drome. Amer. J . Hum. Genet. 21: 352-359.

9. SARLES, H. E., A. E. RODIN, P. R. Posusu , G. H. SMITH, J. C. FISH & A. R. REMMERS. 1968. Hereditary nephritis, retinitis pigmentosa, and chromosomal abnormalities. Amer. J. Med. 45: 312-321.

10. GRELL, R. F. 1965. Chromosome pairing, crossing-over, and segregation in Drosophila melanogaster. National Cancer Institute Monograph 18: 215-242.

11. GRELL, R. F. 1967. Pairing at the chromosomal level. J. Cell Physiol. 70: (Supp.

12. GRELL, R. F. & J. J . VALENCIA. 1964. Distributive pairing and aneuploidy in

13. CATTANACH, B. M. 1967. A test of distributive pairing between two specific non-

14. WEBER, D. F. 1969. A test of distributive pairing in Zea mays. Chromosoma 27:

15. CARATZALI, A. & D. T. STEFANESCU. 1968. Conceptions SUT divers mkcanismes supposCs de la mCiose et determination de quelques anomalies chromosomiques. J . Genet. Hum. 16: 107-118.

16. CHILDS, B. & V. M. DER KALOUSTIAN. 1968. Genetic heterogeneity. New Eng. I. Med. 279: 1205-1212, 1267-1274.

17. SANDLER, L., D. L. LINDSLEY, B. NICOLETTI G. TRIPPA. 1968. Mutants affect- ing meiosis in natural populations of Drosophila melanogaster. Genetics 60: 525-558.

18. STURTEVANT, A. H. 1929. The claret mutant type of Drosophila simulans; a study of chromosome elimination and cell lineage. Z. Wiss. Zool. 135: 323-356.

19. LEWIS, E. B. & W. GENCARELLA. 1952. Claret and nondisjunction in Drosophila melanogaster. Genetics 37: 600-60 1 (Abstr.).

20. BEADLE, G. W. 1932. A gene for sticky chromosomes in Zen mays. Z. Indukt. Abstamm. Vererbslehre. 63: 195-217.

21. GOWEN, M. S. & J. W. GOWEN. 1922. Complete linkage in Drosophila melano- gasiei’. Amer. Natural. 56: 286-288.

22. R E ~ s , H. 1961. Genotypic control of chromosome form and behaviour: Botan. Rev. 27: 288-318.

23. THOMAS, H. & T. RAJHATHY. 1966. A gene for desynapsis and aneuploidy in tetraploid Avena. Canad. J. Genet. Cytol. 8: 506-515.

24. KING, R. C. 1970. The meiotic behaviour of the Drosophila oocyte. Int. Rev.

25. GOWEN, J . W. 1933. Meiosis as a genetic character in Drosophila melanogaster. J . Exp. Zool. 65: 83-106.

26. SMITH, P. A. & R. C. KING. 1968. Genetic control of synaptonemal complexes in Drosophila melanogasteu. Genetics 60: 335-35 1.

27. HINTON, C. W. 1966. Enhancement of recombination associated with the c3G mutant of Drosophila melanogaster. Genetics 53: 157-164.

28. MEYER, G. 1964. A possible correlation between the submicroscopic structure of meiotic chromosomes and crossing over. Proc. Third Europ. Conf. Electron Microscopy: 461-462.

29. MOSES, M. J. 1968. Synaptinemal complex. Ann. Rev. Genetics 2: 363-412.

1): 119-145.

man. Science 145: 66-67.

homologous chromosomes in thc mouse. Cytogenics 6: 67-77.

354-3 70.

CytOl. 28: 125-168.

Young: Genetic Considerations in Nondisjunction 395

30. HOTTA, Y., L. G. PARCHMAN & H. STERN. 1968. Protein synthesis during meiosis. Proc. Nat. Acad. Sci. U.S.A. 6 0 575-582.

31. ROTH, T. & M. ITO. 1967. DNA-dependent formation of the synaptinemal com- plex at meiotic prophase. J. Cell Biol. 35: 247-255.

32. STERN, H. & Y. HOTTA. 1969. DNA synthesis in relation to chromosome pairing and chiasma formation. Genetics 61 (Suppl. 1 ) : 27-40.

33. WALD, H. 1936. Cytologic studies on the abnormal development of the eggs of the claret mutant type of Drosophita simulans. Genetics 21: 264-281.

34. DAVIS, D. G. 1969. Chromosome behaviour under the influence of claret non- disjunctional in Drosophita melanogaster. Genetics 61: 517-594.