Complex germline and somatic mutation processes at a haploid human minisatellite shown by single-molecule analysis

Area of Science:

• Genetics
• Molecular Biology
• Human Evolution

Background:

• Most human minisatellite mutations result from interallelic processes, leading to length and structural variations.
• MSY1, located on the Y chromosome's non-recombining region, is unique as a haploid minisatellite maintaining high diversity without interallelic interactions.

Purpose of the Study:

• To investigate the mutation processes underlying MSY1's high diversity in the absence of interallelic interactions.
• To compare mutation patterns in sperm and blood DNA to understand tissue-specific differences.

Main Methods:

• Unbiased structural analysis of over 500 single-molecule MSY1 PCR products.
• Comparison of MSY1 alleles from matched sperm and blood samples of a single donor.
• Analysis of mutant structures against phylogenetically matched population samples.

Main Results:

• Overall mutation frequencies in sperm (2.68%) and blood (1.88%) were not significantly different.
• Sperm DNA showed a higher frequency of length mutants compared to blood DNA.
• Both tissues exhibited small-scale length changes and isomeric mutations (boundary switches, modular structure mutants), with significant differences in the proportions of these mutants between sperm and blood.

Conclusions:

• MSY1 mutation likely involves gene conversion via synthesis-dependent strand annealing.
• Observed differences between sperm and blood may stem from relaxed sister chromatid alignment constraints in blood.
• The findings provide insights into the mutation mechanisms of haploid genetic elements.