Delayed Embryonic to Adult Globin Switching in HMGB2 Knock Out Mice

Hypersensitive site 2 (HS2) of the locus control region (LCR) is required for the optimal regulation of the beta globin gene cluster. Screening a gt11 cord blood cDNA library with the tandem NFE2 repeat of HS2 as recognition site probe, we isolated 14 cDNA clones of HMGB2, a chromatin non histone protein. Binding to the HS2 region was confirmed in vivo by ChIP assay. Transactivation analysis in K562 cells showed mild repression of a luciferase reporter driven by HS2 and the -promoter. The DNA bending capacity and the increased expression of HMGB2 during erythroid differentiation are properties well suited to facilitate LCR looping toward the ?-globin genes, the mechanism thought to mediate globin gene activation. Hence, HMGB2 binding to HS2 may be relevant for the regulation of the ?-globin gene cluster. To assess the function of HMGB2 as a possible regulator of the globin genes we analyzed the hematological phenotype of the HMGB2 knock out mice during erythroid differentiation. In peripheral blood from E18 fetus or adult mice no hematological differences were found among normal, heterozygous or homozygous HMGB2 mice. However, at earlier stages of development, at E12, ?-minor and major were slightly reduced in HMGB2-/- mice, whereas embryonic y and ?H1 globin genes were overexpressed 4-5 times compared to normal littermate mice. These results support a repressing activity of HMGB2 on embryonic globin genes and a possible role in the switching of hemoglobins during early stages of differentiation. The lack of any hematological phenotype in the latter stages of development may be explained by a temporal restriction in the expression of HMGB2 or by the vicarious activity of other HMGB proteins during definitive erythropoiesis