Abstract

Dr. Mei Hsu, Dartmouth Medical School, Dartmouth College, US.



The chromatin context or epigenotype of a gene is highly influential of its biological activities. A permissive chromatin environment is more open to transcription whereas a repressive chromatin environment will limit gene expression. It is generally held that epigenetic regulation of gene expression is the mechanism by which stem cell potential is progressively restricted over development. The beta-globin switch is a ontogenic process whereby developmental shifts in sites of erythropoiesis roughly correlates with changed expression of a beta-like globin genes along with differences in cell morphology. The human gamma-to-beta-globin switch is the focus of much research because the clinical manifestations of many beta-hemoglobinopathic diseases can be alleviated with a full or partial reversal of the switch. The mammalian beta-globin switch was used as a model to study how alterations in the DNA methylation, acH3 and 2meK4H3 patterns relate to developmentally regulated changes of gene expression. The CpG methylation pattern associated with the murine beta-like globin switch was assessed via bisulfite genomic sequencing. It was found that the embryonic epsilon-y and beta-H1 globin genes are contained in a large (~20 kb) hypomethylated multi-gene domain specific to primitive red cells. Chromatin immunoprecipitation assays were used to determine the acH3 and 2meK4H3 epigenotype across the beta-globin gene cluster in human embryonic stem cell derived primitive-like erythroid cells and primary definitive erythroblasts from human fetal liver and adult bone marrow. The data indicate a positive relationship between acH3 and 2meK4H3 enrichment and the transcription of the beta-globin genes and that the presence of these marks reflect only their transcription state, not their levels of transcription. The epigenotype of the G- and A-gamma globin genes under conditions of maximum expression never occurs in isolation but is instead, always shared by either an embryonic or adult beta-globin gene. And the acH3 and 2meK4H3 distribution pattern of primary human adult bone marrow erythroid cells incorporates the dominant adult delta- and beta-globin genes, the 5.6 kb of intergenic sequence between them and 5’ and 3’ flanking sequence. This region contains sites of initiation for multiple biological processes including an origin of replication, a cryptic promoter for intergenic transcription and a meiotic recombination hotspot. The extended domain of histones with ‘permissive’ modifications likely reflects aspects of these processes.

Host: Dr. Ernesto Guccione