Identification of the modifier of Min 2 (Mom2) locus, a new mutation that influences Apc-induced intestinal neoplasia

Abstract

Min (Multiple intestinal neoplasia) mice carry a dominant mutation in the adenomatous polyposis coli (Apc) gene and develop multiple adenomas throughout their intestinal tract (Moser et al. 1990; Su et al 1992). Polyp multiplicity in Min mice is greatly influenced by genetic background. A modifier locus, Mom1 (Modifier of Min 1), was identified and localized to distal mouse chromosome 4 (Moser et al. 1992; Dietrich et al. 1993), and accounts for some of the genetic variance in polyp multiplicity. Mom1 is a semidominant modifier of polyp size and multiplicity in Min mice (Gould and Dove 1997), and encodes the secretory type II nonpancreatic phospholipase A2 (Pla2g2a) gene (MacPhee et al. 1995; Cornier et al. 1997, 2000). We now report the identification of a second Modifier of Min 2 (Mom2) locus that is the result of a spontaneous mutation. One resistant Mom2 allele can suppress 88%-95% of polyps detected in Apc(Min)/+ mice, indicating that Mom2 acts in a dominant fashion. Linkage analysis has localized Mom2 to distal mouse chromosome 18. The effects of the Mom2 locus on reducing polyp multiplicity are stronger than the effects of the Mom1 locus, in both the small and large intestines. Some Apc(Min)/+ mice that carried one resistant Mom2 allele were tumor-free at 21 weeks of age, even in the absence of a resistant Mom1 allele. Thus, the resistant Mom2 allele can, in some cases, completely suppress the penetrance of the Apc(Min) mutation.

Figure 1

Polyp multiplicity of offspring from a DBA/2J X C57BL/6J − Apc^Min/+ intercross. Apc^Min/+ progeny from each mating cage were aged for 130–150 days and then analyzed for polyp number in the small intestines. Black bars represent male progeny, and white bars represent female progeny. (A) Polyp counts of the small intestine in D2B6F1 Apc^Min/+ offspring from 4 of 5 mating cages showing the expected results. Average polyp multiplicity ± standard deviation was 48.2 ± 13.1. (B) Polyp counts of the small intestine in D2B6F1 Apc^Min/+ offspring from the exceptional mating cage reveal an apparent bimodal distribution in polyp multiplicity. (C) Polyp counts of the small intestine in B6 Apc^Min/+ offspring aged for 165–190 days; average polyp multiplicity ± standard deviation was 89.9 ± 25.0.

Figure 2

Mapping and chromosomal localization of the Mom2 locus in the mouse genome. (A) Summary of the results of the N2 backcross analysis. Loci mapped in the analysis are listed to the left. Black boxes represent the C57BL/6J (B6) allele, and white boxes represent the DBA/2J (D2) allele. The Mom2 allele was determined by phenotype: black boxes represent the resistant B6 allele where mice had ≤24 polyps, and white boxes represent the susceptible (wild-type) D2 allele where mice had ≥25 polyps in the small intestine. The numbers of each type of chromosome identified in the backcross progeny are listed at the bottom. (B) Genetic localization of the Mom2 locus. The region of chromosome 18 analyzed in our backcrosses (left) is shown adjacent to a partial consensus linkage map of mouse chromosome 18 (Radice 2000). The loci mapped are listed to the right, and the genetic distances (in centimorgans) between adjacent loci are listed to the left of the chromosomes. The two maps were arbitrarily aligned at the D18Mit186 and D18Mit213 loci. The data from our backcross analysis gives a genetic distance of 14 ± 3.5 cM between D18Mit186 and D18Mit213, whereas the genetic distance given in the consensus map shows the two loci as being 10 cM apart. The locations of the human homologues of mouse genes are listed to the right.

Figure 3

Distribution of N2 progeny from the B6 and D2 backcrosses to D2B6F1 Apc^Min/+ mice based on polyp number in the small intestine and genotypes at the Mom1 and Mom2 loci. N2 offspring (Table 1) were genotyped for the Pla2g2a gene (see Methods) to determine whether they carried the B6 susceptible Mom1^S allele and/or the D2 resistant Mom1^R allele. N2 offspring were phenotyped for polyp number to determine whether they were homozygous for susceptible Mom2 (Mom2⁺/Mom2⁺) alleles (≥25 polyps = high-polyp phenotype) or whether they were heterozygous for the resistant Mom2^R (Mom2^R/Mom2⁺) allele (≤24 polyps = low-polyp phenotype); molecular markers on mouse chromosome 18 were consistent with the determination of Mom2 allele status (Fig. 2). Genotypes at the Mom1 and Mom2 loci are shown to the right. (A) A total 79 N2 offspring were generated from the backcross to B6, and (B) a total of 21 N2 offspring were generated from the backcross to D2 (Table 1).