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

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• Human globin genes (β, δ, ɛ) share similar sequences around codon 6, enabling stem-loop formation with a self-depurinating 5'GAGG loop.
• Previous research identified a mutation hotspot at codon 6 of the β-globin gene, linked to anemias like Sickle Cell Anemia and β-thalassemias.

Purpose of the Study:

• To investigate why δ- and ɛ1-globin genes, despite similar sequences, lack a mutation hotspot at their potential self-depurination sites.
• To identify conserved sequence determinants of self-catalytic DNA depurination and their role in mutation variability among globin paralogs.

Main Methods:

• Comparative sequence analysis of human β, δ, and ɛ-globin genes.
• Assessment of stem-loop stability and self-catalytic depurination potential in different globin gene sequences.
• Evolutionary conservation analysis of stem-destabilizing variants in vertebrates, mammals, and primates.

Main Results:

• The δ- and ɛ1-globin genes exhibit reduced stem stability due to one or two additional mismatches in the stem region compared to β-globin.
• This destabilization prevents effective DNA cruciform extrusion or shortens stem-loop lifetime, inhibiting self-catalytic depurination.
• Paralogs HB-γ1 and HB-γ2 lack stem-forming potential entirely, explaining the absence of hotspots in these genes as well.

Conclusions:

• Conserved sequence differences in the stem region dictate the stability of potential self-depurination sites, explaining differential mutation incidence.
• The findings highlight conserved determinants of self-catalytic DNA depurination and their impact on mutation variability across human β-globin paralogs.
• Stem-destabilizing variants, conserved across mammals and primates, emerged early in vertebrate evolution.