H4 acetylation, XIST RNA and replication timing are coincident and define x;autosome boundaries in two abnormal X chromosomes

The inactive X (Xi) differs from its active homologue (Xa) in a number of ways, including increased methylation of CpG islands, replication late in S phase, underacetylation of histone H4 and association with XIST RNA. Global changes in DNA methylation occur relatively late in development, but the other properties all change during or shortly after the establishment of Xi and may play a role in the mechanism by which an inactive chromatin conformation spreads across most of the chromosome. In the present report, we use two human X;autosome translocation chromosomes to study the spreading of inactive X chromatin across X;autosome boundaries. In one of these chromosomes, t(X;6), Xp distal to p11.2 is replaced by 6p21.1-6pter and, in the other, ins(X;16), a small fragment derived from 16p13 is inserted into the distal third of Xq. In lymphoid cells from patients carrying these translocations in an unbalanced form, Xi was shown by HUMARA assay to be derived exclusively [t(X:6)] or predominantly [ins (X;16)] from the derived X chromosome. We used a combination of immunolabelling and RNA/DNA fluorescence in situ hybridization to define the distribution of XIST RNA, deacetylated H4 and late-replicating DNA across the two derived X chromosomes in inactive form. Within the limits of the cytogenetic techniques employed, the results show complete coincidence of these three parameters, with all three being excluded from the autosomal component of the derived X chromosome.      

Functional study of lipids of PNH red cell membranes: susceptibility of liposomes to reactive lysis

Lipids extracted with chloroform-methanol from red blood cell membranes of 7 PNH and 13 control subjects were used for the preparation of liposomes, which were then examined with the reactive lysis test. PNH liposomes lysed to a higher extent than control liposomes as indicated by the higher dilution of the limiting complement reagent that was necessary to lyse 50% of the PNH liposomes. A similar finding was also observed with liposomes made of lipids from AET-treated red cells. The enhanced reactive lysis can be attributed to the polar lipid fraction, as indicated by the increased lysis of hybrid liposomes prepared from this polar lipids extracted from PNH erythrocyte membrane and lipids extracted from normal erythrocyte membrane. The increased susceptibility to reactive lysis does not seem to be specific of PNH liposomes, since it was also observed with liposomes prepared from lipids of red cells from beta-thalassemia major and autoimmune hemolytic disease.