Figure 1: Fine mapping and functional validation of Tgfbm2 in congenic mice.

(a) F1.C57/129.Tgfb1^+/− mice were backcrossed >10 generations into inbred C57, at each generation selecting for both the Tgfb1^tm1N allele and D1Mit362¹²⁹, using genetic markers. When mice were >99.9% C57, except for a 26 Mb 129 interval at the telomeric end of chromosome 1, littermates were used to generate C57. Tgfb1^+/− congenics that were either homozygous C57 (Tgfbm2^C57/C57) or homozygous 129 (Tgfbm2^129/129) at Tgfbm2. Tgfb1^+/− intercrosses within the indicated mouse strains were assayed for viable Tgfb1^−/− pups as a percentage of wild-type neonates. Whereas Tgfb1^−/− mice on a C57 (n>400) or congenic C57.Tgfbm2^C57/C57 (n=84) background always die in utero from vascular dysgenesis, Tgfbm2^129/129 (n=139) rescued C57.Tgfb1^−/− mice from invariable prenatal lethality (P=0.02; χ² test ), almost to the extent observed on a pure 129 genetic background (n>100, and see earlier publication ³⁷). (b) Tgfb1^+/− mice backcrossed >20 generations to NIH selecting only for the Tgfb1^tm1N allele were unexpectedly found to harbour 129 genomic variants at Tgfbm2. These mice were segregated by Tgfbm2 genotype and assayed for frequency of viable Tgfb1^−/− pups born on a NIH.Tgfbm2^129/129 background, compared with that on a NIH.Tgfbm2^NIH/NIH background. A similar comparative intercross analysis was undertaken on a panel of NIH.Tgfbm3^C57/C57 mice (lines 3, 4, 6 and 8; n>100 mice for each genotype) that are sensitized to Tgfb1^−/− prenatal lethality³⁸, showing a statistically significant association between pup viability and Tgfbm2^129/129 (P=0.0002; logistic regression). All data in (b) are normalized relative to pup viability of the partner mouse strain homozygous for the NIH allele of Tgfbm2^NIH/NIH (light grey bars). Dark grey bar shows data for NIH mice heterozygous for Tgfbm2^129/NIH. Black bars show data for mice homozygous 129 for Tgfbm2^129/129.