Heterocellular hereditary persistence of fetal hemoglobin (HPFH). Molecular mechanisms of abnormal γ-gene expression in association with β thalassemia and linkage relationship with the β-globin gene cluster

We report a study of four families of Italian origin in which heterocellular HPFH is inherited linked to β thalassemia over two or three generations. The HPFH + β thalassemia carriers showed thalassemic blood pictures and elevated HbF and F-cell number without increase in the HbF/F-cell content. Association of this gene complex with a second β thalassemia trait gives rise to a mild clinical picture characterized by 9–12 g/dl of mainly HbF in peripheral blood and no transfusion requirement. In two families, independent segregation of the HPFH or β-thal trait was observed, and in one case the study of the DNA polymorphisms within the γδβ gene cluster indicated that the HPFH mutation lies outside that DNA region. In one family the coexistence of a polymorphic variant of the Aγ chain (the AγT chain) allowed us to demonstrate that the increased γ chain synthesis caused by the heterocellular HPFH determinant is directed by both chromosomes.