Branch site haplotypes that control alternative splicing

Area of Science:

• Molecular Biology
• Genetics
• Immunology

Background:

• Alternative splicing of DQB1 transcripts is crucial for soluble HLA-DQbeta chain expression.
• Branchpoint sequences (BPS) in intron 3 are known regulatory elements in splicing.

Purpose of the Study:

• To elucidate the role of BPS haplotypes in DQB1 intron 3 on allele-dependent transcript expression.
• To investigate the mechanism of BPS recognition by splicing factors and its impact on disease.

Main Methods:

• Utilized a multi-allelic reporter system to study DQB1 pre-mRNA splicing.
• Assessed RNA-binding affinities of splicing factor 1 (SF1) to different BPS variants.
• Analyzed the effect of naturally occurring BPS point mutations and coding DNA variants on exon inclusion.

Main Results:

• DQB1 intron 3 BPS haplotypes determine allele-dependent expression of soluble HLA-DQbeta transcripts.
• Impaired SF1 binding to BPS RNAs correlates with low transmembrane exon inclusion, indicating a role in spliceosome assembly.
• Disease-associated BPS mutations cluster near the branchpoint, suggesting impaired SF1-BPS interactions contribute to phenotypes.
• Coding DNA variants showed less impact on exon inclusion than random mutations, implying selection against mutations affecting exonization.

Conclusions:

• The study provides the molecular basis for haplotype-specific soluble DQbeta expression.
• Findings improve the prediction of disease-causing intronic point mutations.
• Highlights how DNA variability in HLA influences pre-mRNA splicing through selection.