Abstract P5-05-04: The APOBEC3B transgenic mouse
Authors: Choi M , Lyle LT , Song C , Clare SE 
Published: February 2018

Abstract
Background: Massively parallel DNA sequencing efforts have facilitated the production of catalogs of the somatic mutations present in cancer genomes. Examining the sequence context of the mutations it is possible to identify patterns that recur frequently. These mutational patterns or “signatures” enable inference into the mutational processes responsible for the patterns. Investigators at the Sanger Institute in the UK have produced a compendium of 30 “Signatures of Mutational Processes in Human Cancer”. Two of these signatures have been attributed to the actions of APOBEC cytidine deaminases. Data from other investigators suggest that APOBEC3B (A3B) is more likely to play a role than are other members of the APOBEC family. The evidence to support the hypothesis that APOBEC activation is responsible for the mutation patterns is limited at present. It is important to establish whether these enzymes play a role in oncogenesis as the APOBEC mutational signature had been noted to be present in 1 of every 6 cancer specimens and second only to aging in prevalence.
Methods: In order to determine if overexpression of APOBEC3B in the mammary gland of mice is tumorigenic and produces the mutational signatures attributed to its activity, we contracted with a commercial vendor for the production of an APOBEC3B transgenic mouse on a 129:C57BL/6 background. Of note, in the course of the production of the transgenic, the intron between exons 5 and 6 was retained in the construct to prevent the expression of the highly mutatgenic protein in E. coli. The transgene was placed 3' of a stop sequence flanked by LoxP sites within the ROSA26 locus. Mating with MMTV-Cre mice restricts the excision of the stop cassette and expression of APOBEC3B to the mammary tissue. A3B heterozygotes from the F1 generation were bred to produce A3B homozygotes. Homozygotes have been allowed to undergo multiple pregnancies and have been observed for up to 12 months as of June 2017.
Results: 3 mice have developed malignancies. Two mice appear to have developed lymphomas and the epithelial cells of a mammary gland of a third mouse (homozygote) at 11 months of age are diffusely hyperplastic. The cells are arranged in rows ranging from 2-4 cells in thickness. Periodic acid–Schiff (PAS) staining confirms that the basement membrane is intact around the hyperplasic cells. There is no anisocytosis, anisokaryosis, desmoplasia, necrosis or mitotic figures noted in the gland.
Conclusions: At 11 month of age, the mammary glands of APOBEC3B transgenic mice are beginning to display histologic abnormalities associated with breast oncogenesis. Observation will continue. With the passage of time we expect to observe more advanced and larger mammary lesions. DNA will be extracted from any invasive mammary tumors, sequenced and the mutation signature determined using the EMu: Expectation-Maximisation inference of mutational signatures software available from Sanger.
Citation Format: Choi M, Lyle LT, Song C, Clare SE. The APOBEC3B transgenic mouse [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P5-05-04.