The Diversity Outbred heterogeneous stock (J:DO) is a developing mouse population derived from progenitor lines of the Collaborative Cross (CC). The CC is a panel of recombinant inbred (RI) mouse strains that combines the genomes of eight genetically diverse founder strains - A/J, C57BL/6J, 129S1/SvImJ, NOD/ShiLtJ, NZO/HlLtJ, CAST/EiJ, PWK/PhJ, and WSB/EiJ - to capture nearly 90% of the known variation present in laboratory mice (Churchill et al. 2004). Animals from 160 incipient CC lines at early stages of inbreeding were used to establish the DO population, which is maintained by a randomized outbreeding strategy that avoids brother-sister matings. The DO and CC populations thus capture the same set of natural allelic variants derived from a common set of eight founder strains, with DO mice being outbred and the CC population being inbred.
CTC (2004), Churchill, G. A., et al.. The Collaborative Cross, a community resource for the genetic analysis of complex traits. Nat Genet. 36, 1133-7.
Heart size is an important factor in cardiac health and disease. In particular, increased heart weight is predictive of adverse cardiovascular outcomes in multiple large community-based studies. The authors used two cohorts of Diversity Outbred (DO) mice to investigate the role of genetics, sex, age, and diet on heart weight, independent of body size and age.
DO mice (n=289) of both sexes from generation 10 were fed a standard chow diet, and analyzed at 12-15 weeks of age. Another cohort of female DO mice (n=258) from generation 11 were fed either a high-fat, cholesterol-containing (HFC) diet or a low-fat, high-protein diet, and analyzed at 24-25 weeks. The authors observed a significant genetic effect on heart weight in the second DO population.
The authors aimed to identify QTL that influenced heart weight independent of body size and age. Indexing heart weight to long bone length is a standard approach to assessing cardiac hypertrophy in mice as body weight, composition, and systemic metabolism have direct effects on cardiac mass. QTL mapping was performed for heart weight as a function of femur length, with age, sex, and diet as covariates for 547 DO mice (mice that died over the course of the experiment or could not provide a full set of phenotypes were excluded from QTL mapping).
Mice were genotyped by MegaMUGA SNP array at 6 weeks of age. The Sanger Mouse Genomes Project REL-1505 was used to impute founder SNPs onto DO genomes during association mapping. QTL mapping was performed using the R package Rqtl2 (Broman 2014) and DOQTL version 1.10.0 (Gatti et al. 2014) for identifying SNP effects based on haplotype.
Two significant QTL for "heart weight by femur" were identified:
QTL Hwtf1 (heart weight by femur 1) maps to Chr15:71.8-73 Mbp (p <= 0.05) with a peak LOD score of 9.812 at 72.47 Mbp and explains 7.1% of the variance. WSB/EiJ and NZO/HlLtJ founder haplotypes were associated with larger heart size while 129S1/SvImJ and CAST/EiJ founder haplotypes were associated with smaller heart size.
QTL Hwtf2 (heart weight by femur 2) maps to Chr10:120-122 Mbp (p <= 0.1) with a peak LOD score of 7.199 at 120 Mbp and explains 6.2% of the variance. NZO/HlLtJ, NOD/ShiLtJ, A/J, and 129S1/SvImJ alleles were associated with increased heart size, and the PWK/PhJ, WSB/EiJ, CAST/EiJ, and C57BL/6J alleles were associated with a decreased size.
Hwtf1 contained collagen type XXII alpha 1 chain (Col22a1), and the NZO/HlLtJ and WSB/EiJ haplotypes were associated with larger hearts. This is consistent with heart tissue Col22a1 expression in DO founders and SNP patterns within Hwtf1 for Col22a1. Col22a1 has been previously associated with cardiac fibrosis in mice, suggesting that Col22a1 may be involved in pathological cardiac hypertrophy.