Genetic analysis of hematological parameters in incipient lines of the collaborative cross

Abstract

Hematological parameters, including red and white blood cell counts and hemoglobin concentration, are widely used clinical indicators of health and disease. These traits are tightly regulated in healthy individuals and are under genetic control. Mutations in key genes that affect hematological parameters have important phenotypic consequences, including multiple variants that affect susceptibility to malarial disease. However, most variation in hematological traits is continuous and is presumably influenced by multiple loci and variants with small phenotypic effects. We used a newly developed mouse resource population, the Collaborative Cross (CC), to identify genetic determinants of hematological parameters. We surveyed the eight founder strains of the CC and performed a mapping study using 131 incipient lines of the CC. Genome scans identified quantitative trait loci for several hematological parameters, including mean red cell volume (Chr 7 and Chr 14), white blood cell count (Chr 18), percent neutrophils/lymphocytes (Chr 11), and monocyte number (Chr 1). We used evolutionary principles and unique bioinformatics resources to reduce the size of candidate intervals and to view functional variation in the context of phylogeny. Many quantitative trait loci regions could be narrowed sufficiently to identify a small number of promising candidate genes. This approach not only expands our knowledge about hematological traits but also demonstrates the unique ability of the CC to elucidate the genetic architecture of complex traits.

Keywords: Mouse Collaborative Cross; Mouse Genetic Resource; QTL; complex traits; hematology; hemoglobin β; mean red cell volume; mouse genetics; shared ancestry.

Figure 1

(A) Distribution of MCV in CC founder strains and pre-CC mice. (B) Genome scan for MCV (maroon) and genome scan conditioned on Hbb genotype (blue). The dashed line denotes 95% significance threshold determined by permutation. (C) Allele effects plots for Mcvq4 on Chr 7 (left) and Mcvq5 on Chr 14 (right). Shaded areas correspond to the QTL confidence interval, and the dark bars on the x-axis denote the regions identified by analysis of shared ancestry as described in text. Color-coding of allele effects is as follows: dark gray, C57BL/6J; light blue, NZO/HlLtJ; blue, NOD/ShiLtJ; green, CAST/EiJ; yellow, A/J; salmon, 129S1/SvImJ; red, PWK/PhJ; purple, WSB/EiJ. (D) Phylogenetic trees for regions of shared ancestry in Mcvq4 (left) and Mcvq5 (right). Trees were constructed using SNP data from Sanger MGP (see Materials and Methods), and the numbers indicate bootstrap support estimates for each branch.

Figure 2

MCV as a function of Hbb genotype in pre-CC mice (light gray) and founder strains. The color scheme is the same as that used in Figure 1. The regression line shows the effect of the s vs. the d allele. Heterozygotes showed intermediate phenotypes.

Figure 3

(A) Distributions of MO, %NE, and WBC. (B) Genome scans for MO (maroon), %NE (blue), and WBC (black). Dashed lines represents 95% significance threshold determined by permutation for MO (maroon), %NE and WBC (black). (C) QTL allele effects plots for Moq1 (left), NE_pct1 (center), and Wbcq7 (right). The color scheme is the same as that used in Figure 1. For Moq1, the CAST/EiJ allele effect (green) is intermediate to the low-MO and the high-MO group. Shaded areas correspond to the QTL confidence interval, and the dark bar on the x-axis for Moq1 denotes the region identified by analysis of shared ancestry as described in (D). Phylogenetic tree for the region in Moq1of shared ancestry among the low-MO group, spanning 92.8 to 93.0 Mb on Chr 1.