Binding of HMG-I(Y) elicits structural changes in a silencer of the human beta-globin gene

Proteins involved in repression of the human beta-globin gene may be useful in the treatment of sickle cell anemia, in conjunction with therapy to rea ctivate fetal globin genes. If there is a reciprocal elevation of gamma-glo bin expression upon repression, this approach could be useful in additional hemoglobinopathies, We previously showed that repression of the beta-globi n gene appears to be mediated through two DNA sequences, silencers I and II , and identified a protein termed BP1 which binds to both silencer sequence s. In this study, we cloned two cDNAs encoding proteins which bind to an ol igonucleotide in silencer I containing a BP1 binding site. These cDNAs corr espond to HMG-I and HMG-Y, isoforms regarded as architectural proteins. We demonstrate that binding of HMG-I(Y) to this ogligonucleotide causes bendin g/flexure of the DNA, HMG-I(Y) also binds to a second oligonudeotide contai ning a BP1 binding site located in a negative control region upstream of th e delta-globin gene, suggesting a role for HMG-I(Y) in repression of adult globin genes. Expression studies revealed that HMG-I(Y) is ubiquitously exp ressed in human tissues that do not express beta-globin, being present in 4 8 of 50 tissues and six hematopoietic cell lines examined. Furthermore, HMG -I(Y) expression is down-regulated during differentiation of primary erythr oid cells. We present a model in which HMG-I(Y) alters DNA conformation to allow binding of repressor proteins, and in which the relative amount of HM G-I(Y) helps to determine the repressive state of the beta-globin gene. (C) 1999 Wiley-Liss, Inc.