Synchronizing allelic effects of opposing quantitative trait loci confirmed a major epistatic interaction affecting acute lung injury survival in mice.

Increased oxygen (O(2)) levels help manage severely injured patients, but too much for too long can cause acute lung injury (ALI), acute respiratory distress syndrome (ARDS) and even death. In fact, continuous hyperoxia has become a prototype in rodents to mimic salient clinical and pathological characteristics of ALI/ARDS. To identify genes affecting hyperoxia-induced ALI (HALI), we previously established a mouse model of differential susceptibility. Genetic analysis of backcross and F(2) populations derived from sensitive (C57BL/6J; B) and resistant (129X1/SvJ; X1) inbred strains identified five quantitative trait loci (QTLs; Shali1-5) linked to HALI survival time. Interestingly, analysis of these recombinant populations supported opposite within-strain effects on survival for the two major-effect QTLs. Whereas Shali1 alleles imparted the expected survival time effects (i.e., X1 alleles increased HALI resistance and B alleles increased sensitivity), the allelic effects of Shali2 were reversed (i.e., X1 alleles increased HALI sensitivity and B alleles increased resistance). For in vivo validation of these inverse allelic effects, we constructed reciprocal congenic lines to synchronize the sensitivity or resistance alleles of Shali1 and Shali2 within the same strain. Specifically, B-derived Shali1 or Shali2 QTL regions were transferred to X1 mice and X1-derived QTL segments were transferred to B mice. Our previous QTL results predicted that substituting Shali1 B alleles onto the resistant X1 background would add sensitivity. Surprisingly, not only were these mice more sensitive than the resistant X1 strain, they were more sensitive than the sensitive B strain. In stark contrast, substituting the Shali2 interval from the sensitive B strain onto the X1 background markedly increased the survival time. Reciprocal congenic lines confirmed the opposing allelic effects of Shali1 and Shali2 on HALI survival time and provide unique models to identify their respective quantitative trait genes and to critically assess the apparent bidirectional epistatic interactions between these major-effect loci.

Multigenic control and sex bias in host susceptibility to spore-induced pulmonary anthrax in mice.

Mechanisms underlying susceptibility to anthrax infection are unknown. Using a phylogenetically diverse panel of inbred mice and spores of Bacillus anthracis Ames, we investigated host susceptibility to pulmonary anthrax. Susceptibility profiles for survival time and organ pathogen load differed across strains, indicating distinct genetic controls. Tissue infection kinetics analysis showed greater systemic dissemination in susceptible DBA/2J (D) mice but a higher terminal bacterial load in resistant BALB/cJ (C) mice. Interestingly, the most resistant strains, C and C57BL/6J (B), demonstrated a sex bias for susceptibility. For example, BALB/cJ females had a significantly higher survival time and required 4-fold more spores for 100% mortality compared to BALB/cJ males. To identify genetic regions associated with differential susceptibility, survival time and extent of organ infection were assessed using mice derived from two susceptibility models: (i) BXD advanced recombinant inbred strains and (ii) F2 offspring generated from polar responding C and D strains. Genome-wide analysis of BXD strain survival identified linkage on chromosomes 5, 6, 9, 11, and 14. Quantitative trait locus (QTL) analysis of the C×DF2 population revealed a significant QTL (designated Rpai1 for resistance to pulmonary anthrax infection, locus 1) for survival time on chromosome 17 and also identified a chromosome 11 locus for lung pathogen burden. The striking difference between genome-wide linkage profiles for these two mouse models of anthrax susceptibility supports our hypothesis that these are multigenic traits. Our data provide the first evidence for a differential sex response to anthrax resistance and further highlight the unlikelihood of a single common genetic contribution for this response across strains.

Reciprocal backcross mice confirm major loci linked to hyperoxic acute lung injury survival time.

Morbidity and mortality associated with acute lung injury (ALI) and acute respiratory distress syndrome remain substantial. Although many candidate genes have been tested, a clear understanding of the pathogenesis is lacking, as is our ability to predict individual outcome. Because ALI is a complex disease, single gene approaches cannot easily identify effectors that must be treated concurrently. We employed a strategy to help identify critical genes and gene combinations involved in ALI mortality. Using hyperoxia to induce ALI, a mouse model for genetic analyses of ALI survival time was identified: C57BL/6J (B) mice are sensitive (i.e., die early), whereas 129X1/SvJ (S) mice are significantly more resistant, but with low penetrance. Segregation analysis of reciprocal F(2) mice generated from B and S strains revealed significant sex, cross, and parent of origin effects. Quantitative trait locus (QTL) analysis identified five chromosomal regions significantly linked to hyperoxic ALI survival time (named Shali1-Shali5). Further analyses demonstrated that both parental strains contribute resistance alleles to their offspring and that the phenotype demonstrated parent of origin effects. To validate earlier findings, we generated and tested mice from all eight possible B-S-derived backcrosses. Results from segregation and QTL analyses of 935 backcrosses, alone and combined with the previous 840 B-S-derived F(2) population, further supported the highly significant QTLs on chromosomes 1 (Shali1) and 4 (Shali2) and confirmed that the sex, cross, and parent of origin all contribute to survival time with hyperoxic ALI.