A haemoglobin switching activity modulates hereditary persistence of fetal haemoglobin

Abstract
During human development there is a switch from fetal to adult haemoglobin formation, reflecting the differential expression of fetal (Gγ and Aγ) and adult (β and δ) globin genes. Mutations that inhibit this switch produce variants of the syndrome of hereditary persistence of fetal haemoglobin (HPFH). Adult heterozygotes for these mutants produce 15–30% fetal haemoglobin (HbF) in their red cells1. The general assumption is that the mutations result in a permanent switching on of γ-globin genes. Here, however, we show that fetal globin expression can be turned off in cultures of HPFH cells by an uncharacterized factor in fetal sheep serum. This is the first demonstration that mutations affecting the developmental expression of globin genes can be modulated by exogenous factors. The findings raise the possibility that the phenotype of HPFH is not simply the direct result of mutations in or around globin genes but the consequence of the mutations on the interaction of globin genes with trans-acting regulatory factors.