2 CASES OF PARTIAL TRISOMY 8P AND PARTIAL MONOSOMY 21Q IN A FAMILY WITH A RECIPROCAL TRANSLOCATION (8-21)(P21.1-Q22.3)

We report on two mentally retarded adults with an unbalanced karyotype resulting from a familial balanced translocation between chromosomes 8 and 21, t(8;21) (p21.1;q22.3). This translocation has not been repor ted before. Both patients had partial trisomy 8p and partial monosomy 21q. Fluorescence in situ hybridisation (FISH) was used to determine t he chromosomal breakpoints more precisely. The first patient showed mi ld mental retardation and facial dysmorphism, slightly resembling the earlier described trisomy Sp phenotype. He did not resemble his affect ed niece, who was more severely retarded, had serious epilepsy, but la cked the facial dysmorphism. Comparing the data of both patients with published reports of trisomy Sp, marked differences were found between patients with an inversion duplication (inv dup) 8p, patients with pa rtial trisomy 8p caused by an unbalanced translocation, and our patien ts. Inv dup(8p) causes a recognisable phenotype, whereas the phenotype of trisomy 8p resulting from a translocation is much more variable, p robably because of the accompanying monosomies. However, even the same abnormal karyotype can cause different phenotypes, as our patients sh ow. Counselling carriers of the balanced translocation in this family, a 20-25% recurrence risk for unbalanced offspring and a 25% risk for miscarriages seem appropriate.