Mapping of quantitative trait loci with knockout/congenic strains

Abstract

Recently we have explored the use of knockout/congenic mouse strains for isolating and mapping quantitative trait loci (QTLs). Because most knockout strains have been bred to be B6.129 congenic strains, they can be used to test for QTLs in the targeted chromosomal area as long as there is a genetic difference between B6 and 129. Thus, we have tested a number of knockout/congenic strains in a series of behavioral tests in which mouse performance has a significant genetic component. We have also developed a breeding scheme for distinguishing the effects of background flanking genes from the targeted ablation. In screening several knockout/congenics, we have found at least one that harbors a behavioral QTL in the 129 chromosomal segment. The position of this QTL was confirmed subsequently by several F1 crosses.

Figure 1

Strategy for determining whether a specific phenotype is the result of the ablated target gene or the flanking 129 genes. Comparisons are made to determine the cause of the genetic effect (target or flanking) as well as its mode of inheritance — dominant, additive or recessive. Lines represent chromosomes; target indicates the chromosome with the mapped target locus; background indicates the other 19 chromosomes. KO indicates that the chromosome contains the targeted ablation. Heavy lines indicate that the origin of the chromosomal segment is 129. Thin lines indicate B6 chromosomal segment origin. B6-KO indicates that a strain is homozygous for the targeted ablated or knocked out region. In a B6 congenic/knockout strain, after N9, >99.8% of the unlinked genes are of B6 origin. Arrows indicate the next step to be taken in scheme.

Figure 2

Open field performance of inbred and knockout/congenic mouse strains. The knockout mouse strains tested included B6-Il10^tm1Cgn (N10F14), B6-Fmr1^tm1Cgr (N9), B6-Apoe^tm1Unc (N10F19), B6-Selp^tm1Bay (N10F14), B6-Lck^tm1Mak (N12), B6-Cd4^tm1Knw (N10F8), B6-Il6^tm1Kopf (N11F7), and B6-Cd28^tm1Mak (N12F8). Knockout mice are all on a B6 background and in at least the N9 generation of backcrossing. They are indicated by the name of the ablated target gene. Twelve male mice of each strain were tested. Results are shown + s.e.m. There was a significant effect of strain (F_8,99 = 4.236, P<.001). Fisher's posthoc tests were then used to make pairwise comparisons. Single asterisks indicate P<0.05; double asterisks indicate P<0.01 in ANOVA analysis using STATVIEW.

Figure 3

Open field performance of F1 and knockout/congenic mice. Twenty-four male mice of each strain were tested. Results are shown + s.e.m. There was a significant effect of strain (F_4,115 = 7.733, P<.001). Fisher's posthoc tests were then used to make pairwise comparisons. Double asterisks indicate P<0.01 in ANOVA analysis using STATVIEW.