# A somatic hypermutation defective UNG mutant in antibody deficiency through destabilization of RPA interaction.

> **NIH NIH R03** · UNIVERSITY OF TX MD ANDERSON CAN CTR · 2024 · $81,000

## Abstract

PROJECT SUMMARY:
B cells require somatic hypermutation (SHM) and class switch recombination (CSR) to develop high affinity,
isotype switched immunoglobulins (Igs). Human patients deficient in these processes suffer from hyper IgM
syndrome, a primary immunodeficiency characterized by recurrent and severe infections. They have low IgG
levels and are often unable to develop high-affinity antibodies. Both CSR and SHM are initiated by activation-
induced cytidine deaminase (AID) which deaminates cytidines, creating uracils in Ig genes. Uracil is removed
by Uracil DNA glycosylase (UNG) and repaired in an error-prone fashion to complete SHM and CSR. UNG is
normally associated with high-fidelity base excision repair. The mechanism that directs uracil removal to error-
prone repair during SHM is not understood. However, mis regulation could threaten genome integrity through
inappropriate mutation or suppress SHM and the generation of high-affinity antibodies. To determine if UNG
directly influences repair outcome, we analyzed various mutants of UNG in cell-line, AID induced mutation
reporter assay. We found that UNG with point mutations in the Replication Protein (RPA) binding domain
efficiently supported high-fidelity repair, while suppressing the frequency of mutation. Furthermore ,RPA
mutants unable to interact with UNG also suppressed mutation frequency. Our hypothesis is that UNG-RPA
interaction governs error-prone repair during SHM in B cells. The cell line models available do not completely
recapitulate B cell SHM in terms of frequency and mutational spectrum. Therefore, our objective is to create a
mouse model with a UNG mutant deficient in RPA binding to investigate error-prone repair. We will accomplish
this in Aim 1 by using CRISPR gene editing technology to create a mouse with mutant UNG. In Aim2 this
mouse will be used to define how UNG-RPA interaction in B cells results in SHM. We expect with the
completion of these goals to have a mouse model deficient in normal SHM and a system to mechanistically
investigate how UNG and RPA regulate error-free or mutation repair outcome during base excision repair of
uracils.

## Key facts

- **NIH application ID:** 10809391
- **Project number:** 1R03AI175719-01A1
- **Recipient organization:** UNIVERSITY OF TX MD ANDERSON CAN CTR
- **Principal Investigator:** Kevin Michael McBride
- **Activity code:** R03 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2024
- **Award amount:** $81,000
- **Award type:** 1
- **Project period:** 2024-06-01 → 2026-04-30

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10809391

## Citation

> US National Institutes of Health, RePORTER application 10809391, A somatic hypermutation defective UNG mutant in antibody deficiency through destabilization of RPA interaction. (1R03AI175719-01A1). Retrieved via AI Analytics 2026-05-26 from https://api.ai-analytics.org/grant/nih/10809391. Licensed CC0.

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