Why the DNA self-depurination mechanism operates in HB-β but not in β-globin paralogs HB-δ, HB-ɛ1, HB-γ1 and HB-γ2.

The human β-globin, δ-globin and ɛ-globin genes contain almost identical coding strand sequences centered about codon 6 having potential to form a stem-loop with a 5'GAGG loop. Provided with a sufficiently stable stem, such a structure can self-catalyze depurination of the loop 5'G residue, leading to a potential mutation hotspot. Previously, we showed that such a hotspot exists about codon 6 of β-globin, with by far the highest incidence of mutations across the gene, including those responsible for 6 anemias (notably Sickle Cell Anemia) and β-thalassemias. In contrast, we show here that despite identical loop sequences, there is no mutational hotspot in the δ- or ɛ1-globin potential self-depurination sites, which differ by only one or two base pairs in the stem region from that of the β-globin gene. These differences result in either one or two additional mismatches in the potential 7-base pair-forming stem region, thereby weakening its stability, so that either DNA cruciform extrusion from the duplex is rendered ineffective or the lifetime of the stem-loop becomes too short to permit self-catalysis to occur. Having that same loop sequence, paralogs HB-γ1 and HB-γ2 totally lack stem-forming potential. Hence the absence in δ- and ɛ1-globin genes of a mutational hotspot in what must now be viewed as non-functional homologs of the self-depurination site in β-globin. Such stem-destabilizing variants appeared early among vertebrates and remained conserved among mammals and primates. Thus, this study has revealed conserved sequence determinants of self-catalytic DNA depurination associated with variability of mutation incidence among human β-globin paralogs.