The behavior of sex chromosomes in two human X-autosome translocations: Failure of extensive x-inactivation spreading

Two patients, one adult male and one infant girl, bearing different X-autos ome translocations, were studied with cytogenetical, ultrastructural and ch romosome-painting techniques. The adult male, is a carrier of a reciprocal, balanced translocation involving the X and #2 chromosomes: 46,Y,t(X;2) (q1 3;p21). This man showed infertility with spermatogenesis arrest at the sper matocyte stage. Synaptonemal complex analysis at pachytene showed the quadr ivalent structure and the putative breakage points. Sex-chromatin condensat ion did not spread towards the autosomal regions of the quadrivalent. The f emale infant showed diminished body growth and multiple somatic anomalies. She is a 45,Xp-,t(X;21)(p11;p13) carrier, an unbalanced translocation invol ving chromosomes X and #21, which leads to a monosomy of almost all Xp. The translocated #21 is practically complete, and its centromere is the active one in the rearranged product. The analysis of interphase nuclei with the X-centromere probe shows that the Xq region of the rearranged chromosome is the late -replicating and inactive element. However, X-inactivation does n ot spread to the attached #21, as shown by the R-banding pattern. Thus, bot h in the male adult and in the female infant there is a barrier to the spre ading effect of X-chromosome inactivation, which is probably due to differe nt mechanisms.