Multiple point mutations in a shuttle vector propagated in human cells: evidence for an error-prone DNA polymerase...

Area of Science:

• Molecular Biology
• Genetics
• Cell Biology

Background:

• DNA mutagenesis is a critical process influencing genetic stability and evolution.
• Understanding DNA repair mechanisms is key to comprehending mutation patterns.

Purpose of the Study:

• To investigate the mechanisms of DNA mutagenesis at the DNA sequence level in human cells.
• To determine the role of DNA repair pathways in generating multiple base substitutions.

Main Methods:

• Utilized a shuttle vector plasmid (pZ189) with a suppressor tRNA marker gene.
• Introduced DNA damage (UV treatment, single-strand nicks) to the plasmid.
• Propagated the plasmid through repair-proficient and repair-deficient human cell lines (fibroblasts, lymphoid cells, xeroderma pigmentosum cells).
• Sequenced recovered plasmids to identify mutations within the marker gene and adjacent regions.

Main Results:

• Multiple base substitutions (2-6 per plasmid) were observed in 20-30% of mutants from repair-proficient cells.
• Excision-repair-deficient xeroderma pigmentosum cells yielded significantly fewer multiple base substitution mutants (2%).
• Introducing a single-strand nick followed by passage through xeroderma pigmentosum cells dramatically increased multiple mutations (66%).
• Mutations were localized to a specific region (160-base-pair marker gene), rarely occurring in adjacent areas.

Conclusions:

• DNA repair processes, particularly gap-filling by error-prone polymerases, can generate clustered multiple base substitutions.
• These mutation patterns resemble those observed in immunoglobulin hypervariable regions.
• Defective DNA repair pathways can influence the spectrum and frequency of mutagenesis.