INCIDENCE AND SIGNIFICANCE OF CHROMOSOME MOSAICISM INVOLVING AN AUTOSOMAL STRUCTURAL ABNORMALITY DIAGNOSED PRENATALLY THROUGH AMNIOCENTESIS- A COLLABORATIVE STUDY

Among 179 663 prenatal diagnosis cases collected from ten institutions and two publications, 555 (0.3 per cent) were diagnosed as having chr omosome mosaicism. Of these, 57 (10.3 per cent) were mosaic for an aut osomal structural abnormality, 28 (5 per cent) for a sex chromosome st ructural abnormality, and 85 (15.3 per cent) were mosaic for a marker chromosome. Ninety-five cases of prenatally diagnosed mosaicism with a structural abnormality in an autosome and a normal cell line, and wit h a known phenotypic outcome, were collected for karyotype-phenotype c orrelations through our collaboration (40 cases), a prior survey (26 c ases), and published reports (29 cases). They included 13 balanced rec iprocal translocations, one unbalanced reciprocal translocation, four balanced Robertsonian translocations, four unbalanced Robertsonian tra nslocations, four inversions, 17 deletions, three ring chromosomes, 19 i(20q), seven +i(12p), six other isochromosomes, and 17 partial triso mies resulting from a duplication or other rearrangement. All cases mo saic for a balanced structural rearrangement resulted in a normal phen otype. All cases of 46/46,i(20q) resulted in normal liveborns. Five of seven cases with 46/47,+i(12p) had an abnormal phenotype compatible w ith Killian-Pallister syndrome, The overall risk for an abnormal outco me for a mosaic case with an unbalanced structural abnormality, exclud ing 46/46,(20q) and 46/47,+i(12p), is 40.4 per cent. In the same categ ory, the study also suggested a correlation between the percentage of abnormal cells and an abnormal phenotype. For mosaicism involving a te rminal deletion, the possibility of a familial fragile site should be considered.