Compound heterozygosity for the Cretan type of non-deletional hereditary persistence of fetal hemoglobin and beta-thalassemia or Hb Sabine confirms the functional role of the Agamma -158 C>T mutation in gamma-globin gene transcription

Identical mutations in the paralogous human γ-globin genes leading to hemoglobin variants and nondeletional hereditary persistence of fetal hemoglobin

The human fetal globin genes are highly similar at the DNA sequence level, resulting in a single amino acid difference between the (G)γ- and (A)γ-globin chains. A large proportion of hemoglobin (Hb) variants of the (G)γ- and (A)γ-globin chains result from an identical mutation in the HBG2 and HBG1 genes, respectively, while the same is true for a fraction of mutations leading to nondeletional hereditary persistence of fetal Hb (HPFH). In particular, 11 different Hb variants result from identical mutations on either one of the two human γ-globin paralogous genes, while seven other promoter substitutions result either in nondeletional HPFH or are benign polymorphisms. In the former case, the percentage of the Hb variants due to an HBG2 gene mutation was significantly higher than the percentage of Hb variants due to the same HBG1 gene mutation, following the (G)γ/(A)γ-globin chain ratio seen in wild-type individuals. These γ-globin chain variants have most likely occurred via recurrent mutations, gene conversion events or both and, contrary to the situation observed in the human α-globin genes, these mutations lead to distinct variant Hb molecules.

The Hellenic type of nondeletional hereditary persistence of fetal hemoglobin results from a novel mutation (g.-109G>T) in the HBG2 gene promoter

Nondeletional hereditary persistence of fetal hemoglobin (nd-HPFH), a rare hereditary condition resulting in elevated levels of fetal hemoglobin (Hb F) in adults, is associated with promoter mutations in the human fetal globin (HBG1 and HBG2) genes. In this paper, we report a novel type of nd-HPFH due to a HBG2 gene promoter mutation (HBG2:g.-109G>T). This mutation, located at the 3′ end of the HBG2 distal CCAAT box, was initially identified in an adult female subject of Central Greek origin and results in elevated Hb F levels (4.1%) and significantly increased Gγ-globin chain production (79.2%). Family studies and DNA analysis revealed that the HBG2:g.-109G>T mutation is also found in the family members in compound heterozygosity with the HBG2:g.-158C>T single nucleotide polymorphism or the silent HBB:g.-101C>T β-thalassemia mutation, resulting in the latter case in significantly elevated Hb F levels (14.3%). Electrophoretic mobility shift analysis revealed that the HBG2:g.-109G>T mutation abolishes a transcription factor binding site, consistent with previous observations using DNA footprinting analysis, suggesting that guanine at position HBG2/1:g.-109 is critical for NF-E3 binding. These data suggest that the HBG2:g-109G>T mutation has a functional role in increasing HBG2 transcription and is responsible for the HPFH phenotype observed in our index cases.