Date of Award

2026

Document Type

Honors Thesis (Colby Access Only)

Department

Colby College. Biology Dept.

Advisor(s)

Yee Mon Thu

Second Advisor

Robert Augustine

Third Advisor

Dave Angelini

Abstract

Genomic instability is detrimental to cells’ ability to resist DNA damage and is prominent in many conditions such as Bloom syndrome and primordial dwarfism. One process involved in combating genomic instability is sumoylation. Some important enzymes involved in sumoylation are Ubc9 (E2 Conjugating enzyme) and Mms21 (E3 ligase) or Nsmce2 (human ortholog). The importance of Mms21 for protecting against genomic instability is highlighted in a human genetic condition caused by compound heterozygous mutations in NSMCE2 (Payne F, et al. 2014). One mutation involves truncation of the C-terminus (mms21Δ22) and the other mutation has a truncation of the SP-RING (ligase domain) and the C-terminus (mms21Nterm). The C-terminus truncated mutant is less well studied and there is a lack of genetic evidence of the functional significance of Mms21’s C-terminus. We hypothesize the Mms21’s C-terminus has a role in regulating ligase activity and other independent functions. (Li C, Vo A, Baadi N, and Thu YM, 2026). We performed two projects 1) growth survival comparison of all possible heterozygous and homozygous wild-type and mutant genotypes, and 2) overexpression of MMS21 Ubc9 fusion and mms21Δ22 Ubc9 fusion proteins in MMS21 and mms21Δ22 cells. We found that the homozygous mms21Δ22 genotype and the compound heterozygous genotype had similar DNA damage sensitivities. Both genotypes also exhibited higher DNA damage sensitivity compared to the homozygous mms21Nterm genotypes. The differences between the genotypes suggest the mms21Δ22 allele is stronger than the mms21Nterm allele. MMS21_Ubc9 and mms21Δ22_Ubc9 partially rescued mms21Δ22 severe DNA damage phenotype when overexpressed, suggesting mms21Δ22’s severe phenotype is partially due to a lack of ligase activity. Collectively, both projects show that the C-terminus is involved in regulating ligase activity during DNA repair and has other independent functions.

Keywords

Sumoylation, NSMCE2/MMS21 mutations, Compound Heterozygous, Genomic instability, Saccharomyces cerevisiae

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