Experiment
The goal of the present study was to use a genetic backcross between C3HeB/FeJ (C3H/F) and SJL/J mice to map modifier loci that affect intima formation in response to low blood flow. Previous studies found dramatic strain dependent differences in carotid IMT (intima thickening) between SJL/J and C3H/F mice. SJL/J mice developed greater intima formation, associated with increased vascular inflammation and proliferation compared to C3H/F mice.
One hundred thirty four N2 progeny ((C3HeB/FeJ x SJL/J)F1 x SJL/J) were genotyped using 80 microsatellite markers spaced at approximately 18 cM intervals through 20 mouse autosomes. MapManager QTX20b was used for linkage analysis. Permutation tests were run on data at 1 cM intervals for 10,000 permutations to establish whole genome linkage thresholds. Suggestive (p=0.67), significant (p=0.05) and highly significant (p=0.001) levels were established.
Interval mapping identified two highly significant QTL linked to intima on Chromosomes 2 and 11; one significant QTL on Chromosome 18, and two suggestive QTL on Chromosomes 7 and 17. [Table 1].
QTL Intim1, intima modifier 1, mapped the Chr 2 peaking at marker D2Mit411, LOD=6.4, p=0.000, the CI spanned 20 cM and the locus accounted for 20% of trait variance.
QTL Intim2, intima modifier 2, mapped to Chr 11 peaking at marker D11Mit231, LOD=5.3, p=0.000, the CI interval spanned 24 cM and the locus accounted for 17% of trait variance.
QTL Intim3, intima modifier 3, mapped to Chr 18 peaking at marker D18Mit222, LOD=3.0, p=0.00021, the CI spanned 41 cM and the locus accounted for 10% of trait variance.
The significant QTL identified for intima trait co-localized with QTL that controlled the intima/media ratio trait, identified in the same backcross.
QTL Intmrm1, intima/media ratio modifier 1, mapped to Chr 2 peaking at marker D2Mit411, LOD=6.8, p=0.000, the CI spanned a 19 cM interval and the locus accounted for 21% of trait variance.
QTL Intmrm2, intima/media ratio modifier 2, mapped to Chr 11 peaking at marker D11Mit231, LOD=5.8, p=0.000, the CI spanned a 22 cM interval and the locus accounted for 18% of trait variance.
QTL Intmrm3, intima/media ratio modifier 3, mapped to Chr 18 peaking at marker D18Mit222, LOD=3.3, p=0.00021, the CI spanned a 37 cM interval and the locus accounted for 11% of trait variance. A suggestive QTL mapped to Chr 17.
The N2 mice with a homozygous genotype (SJL/SJL) at D2Mit411 (Intim1/Intmrm1) and D11Mit231 (Intim2/Intmrm2) exhibited greater intima and intima/media ratio values compared to heterozygous (C3H/F/SJL) littermates. However, an opposite trend was found between genotypes at D18Mit222 (Intim3/Intmrm3) in relation to both traits.
Analysis of two high-priority 1.4 Mb haplotype blocks revealed 40 known or predicted genes that represented candidate genes for flow-induced intima formation. Informative SNPs within the 40 Mb intervals on Chr 2 and Chr 11 for C3H/F and SJL mice were obtained from the Mouse Phenome Database and cross-referenced with the 40 candidate genes. Six genes were identified in which the SNPs differed between the two strains: Dsn1, Src, Ctnnbl1, Tti1, Rprd1b and Spred2.
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