Complex trait analysis of the hippocampus: mapping and biometric analysis of two novel gene loci with specific effects on hippocampal structure in mice

Abstract

Notable differences in hippocampal structure are associated with intriguing differences in development and behavioral capabilities. We explored genetic and environmental factors that modulate hippocampal size, structure, and cell number using sets of C57BL/6J (B6) and DBA/2J (D2) mice; their F1 and F2 intercrosses (n = 180); and 35 lines of BXD recombinant inbred (RI) strains. Hippocampal weights of the parental strains differ by 20%. Estimates of granule cell number also differ by approximately 20%. Hippocampal weights of RI strains range from 21 to 31 mg, and those of individual F2 mice range from 23 to 36 mg (bilateral weights). Volume and granule cell number are well correlated (r = 0.7-0.8). Significant variation is associated with differences in age and sex. The hippocampus increases in weight by 0.24 mg per month, and those of males are 0.55 mg heavier (bilateral) than those of females. Heritability of variation is approximately 50%, and half of this genetic variation is generated by two quantitative trait loci that map to chromosome 1 (Hipp1a: genome-wide p < 0.005, between 65 and 100 cM) and to chromosome 5 (Hipp5a, p < 0.05, between 15 and 40 cM). These are among the first gene loci known to produce normal variation in forebrain structure. Hipp1a and Hipp5a individually modulate hippocampal weight by 1.0-2.0 mg, an effect size greater than that generated by age or sex. The Hipp gene loci modulate neuron number in the dentate gyrus, collectively shifting the population up or down by as much as 200,000 cells. Candidate genes for the Hipp loci include Rxrg and Fgfr3.

Fig. 1.

An example of a dissected hippocampus. This dissection is from the left hemisphere and is oriented properly with respect to the septotemporal (S toT) axis of the small inset of a mouse brain. The internal anatomy of the hippocampus is referenced by five small insets of Nissl-stained sections in the coronal plane (a–e). Scale bar (for the image of the dissected hippocampus), 1 mm.

Fig. 2.

Regression analysis of hippocampal weight (bilateral sum). A, Approximately half of the variation in hippocampal weight is associated with variation in brain weight.B, Corresponding plot of hippocampal weight against brain weight minus hippocampal weight. C, Body weight is significantly correlated with hippocampal weight. As shown inD, after correction for brain weight by multiple linear regression, body weight residuals (Body Res) have no independent association with hippocampal residual weight (Hippocampus Res). E,xand y axes represent the logarithm of age residuals (Log age Res) and hippocampus residuals (Hippocampus Res), respectively. F, Age remains significantly correlated with hippocampal weight residuals even after accounting for variation in forebrain minus hippocampus (see Results for details on calculation methods). There is a modest sex difference. m is regression line for males;f is regression line for females. See Table 2 for correlations among these and other variables.

Fig. 3.

Relations between volume of hippocampus and (A) volume of the entire dentate gyrus, (B) volume of the pyramidal cell layer, including CA1 through CA3, and (C) the volume of the granule cell layer of the dentate gyrus. D illustrates the relation between granule cell layer volume and granule cell density. All volumetric data are corrected for differential shrinkage.

Fig. 4.

Interval maps of QTL on Chr 1 (A, B) and Chr 5 (C, D,E, F) in BXD (left) and F2 (right). In each of the six panels, theleft axis and bold black line represent values of the LRS computed at 1 cM intervals. The right axis and the thin red line represent values for the additive effect of the substitution of a single Dallele with a B allele. The x-axis represents the entire genetic lengths of Chr 1 (A, B) and Chr 5 (C–F). Positions of several markers used for mapping both QTLs (e.g., D1Mit200,D5Mit356) are labeled on the xaxes. The approximate 2-LOD confidence band is represented by agray horizontal bar. A–D are simple interval maps, whereas E and F are composite interval maps that control for the Hipp1alocus (D1Mit145) on Chr 1.

Fig. 5.

Interval maps for five different hippocampal traits on maps of Chr 1 derived from the BXD strains. Thex axes represent the entire genetic length of Chr 1.y axes represent values for the LRS computed at 1 cM intervals. A, Hippocampal volume; B, pyramidal cell layer volume (CA1–CA3); C, granule cell layer volume of the dentate gyrus; D, volume of the dentate gyrus (excludes part of the hilus); E,cross-section area of the mossy fiber projection (from Lassalle et al., 1999).

Fig. 6.

Interval map of QTL on Chr 5. xaxes represent the entire genetic length of Chr 5. yaxes represent values for the LRS computed at 1 cM intervals. Conventions as in Figure 5.