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 Ggamma-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 beta-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.

The 5' regulatory region of the human fetal globin genes is a gene conversion hotspot.

The human fetal globin genes consist of the first mammalian genomic loci for which gene conversion was reported. To date, 14 gene conversions have been described in the human Ggamma- and Agamma-globin genes, the vast majority of which are restricted to the coding sequences. Here, we provide evidence for three new gene conversion events in the 5' regulatory region of the human fetal globin genes, identified during a large genetic screening effort in adult individuals with high fetal hemoglobin (Hb) levels. The sequence variations, resulting from these conversion events, are transcriptionally silent polymorphisms that do not contribute to increased fetal Hb levels. Our results suggest that the 5' regulatory region of the human fetal globin genes is a gene conversion hotspot that prevent globin gene promoter sequence diversification, further underlining the need for two functional fetal globin genes in fetal erythropoiesis.

Large-scale population genetic analysis for hemoglobinopathies reveals different mutation spectra in central Greece compared to the rest of the country.

We have undertaken a large population screening study to identify the molecular basis of hemoglobinopathies in the central Greece region. A total of 845 unrelated beta-thalassemia patients and alpha-, beta-, and deltabeta-thalassemia carriers have been recruited and screened for mutations in the alpha- and beta-globin gene clusters. The alpha(-MED) deletion and the Turkish inversion/deletion are the most frequent genetic rearrangements leading to alpha- and deltabeta-thalassemia respectively, contrary to the situation in the rest of the country, while the beta -101 (C>T) promoter mutation is surprisingly frequent in the central part of Greece. Our data indicate that determination of mutation frequencies in different regions is vital for accurate provision of genetic services and counseling and for precise estimation of genetic diversity.