Classic hereditary persistence of fetal hemoglobin (HPFH) is characterized by a substantial elevation of fetal hemoglobin (HbF) in adult red blood cells. There are no other phenotypic or hematologic manifestations. Expression of the HBG1 and HBG2 genes, which encode the gamma isoforms of HbF, is normally suppressed shortly before birth and replaced by expression of the beta- (HBB; 141900) or delta- (HBD; 142000) chains, which form adult hemoglobin. Adults normally have less than 1% HbF, whereas heterozygotes for HPFH have 5 to 30% HbF. HPFH heterozygotes have essentially normal red cell indices and a rather homogeneous distribution of HbF among red cells, termed 'pancellular.' Homozygotes for HPFH can express HbF in up to 100% of red blood cells (Thein and Craig, 1998).
Delta-beta thalassemia is a hemoglobin disorder characterized by decreased or absent synthesis of the delta- and beta-globin chains with a compensatory increase in expression of fetal gamma-chain synthesis from the affected chromosome. Individuals with delta-beta thalassemia have hypochromic, microcytic anemia and increased HbF, which may mitigate the anemia depending on the level of HbF. Delta-beta thalassemia and some forms of HPFH result from deletions within the beta-globin gene cluster on chromosome 11p15; this has been referred to as 'deletional' HPFH. HPFH can also result from point mutations in the promoter regions of the gamma globulin genes HBG1 and HBG2; this has been referred to as 'non-deletional' HPFH (Ottolenghi et al., 1982; Forget, 1998).
Forget (1998) noted that HPFH and delta-beta thalassemia are not clearly distinct disorders, but rather show partially overlapping features that may defy classification. Higher expression of HbF is often termed 'pancellular,' whereas lower expression of HbF is often termed 'heterocellular,' although these represent a spectrum.
Approximately 10% of the population has HPFH manifest as modest elevations of HbF (1 to 4%) present in a subset of red cells (about 4.5%) termed F cells. This is also sometimes referred to as 'heterocellular' HPFH, and is considered to be a multifactorial trait influenced by multiple genetic loci (Thein and Craig, 1998). [from
OMIM]