The Y Chromosome
Epistatic and Ecogenetic Interactions in Genetic Hypertension
Previous studies have revealed conflicting evidence concerning a Y-chromosome effect on blood pressure (BP) in genetic crosses involving different strains of spontaneously hypertensive rats (SHR or SHRSP). We had previously found an ≈16 mm Hg difference in systolic BP (P<10−7) at baseline but not after dietary salt loading (P=.82) between F2 males derived from an SHRSPHD grandfather and a Wistar-Kyoto (WKYHD-0) grandmother and F2 males from a reciprocal cross (WKYHD-0 grandfather). When we examined F2 animals from reciprocal crosses between SHRSPHD and a congenic strain, WKYHD-1, which carries a 6-centimorgan-long SHRSPHD-homologous genomic fragment on chromosome 10 that contains a quantitative trait locus linked to BP (BP/SP-1a), we found no significant differences either at baseline (P=.39) or after salt loading (P=.51) in the two reciprocal F2 cohorts. To test the hypothesis that Y-chromosome–autosomal epistasis accounts for the discrepant Y-chromosome effects on BP, we analyzed the interaction between BP/SP-1a and reciprocal cross status on BP in the two crosses. In the F2 (WKYHD-0×SHRSPHD) cross, no significant interaction was found for basal systolic BP (P=.89), arguing against a major influence of BP/SP-1a on the Y-chromosome effects on basal BP. However, a significant interaction between zygosity at the BP/SP-1a locus and reciprocal cross status for systolic BP after salt loading (P=.022) indicated that the BP/SP-1a–SHRSPHD allele exhibited a significant effect on BP after dietary excess salt only in males that inherited the SHRSP Y chromosome. These results support the relevance of a Y-chromosome effect on BP and suggest that a complex interplay of epistatic and ecogenetic interactions governs its effect on phenotype.
Sexual dimorphism is a well recognized characteristic of BP distribution, with males exhibiting higher levels than do females.1 This trend is observed for BP in the normal range as well as in essential hypertension and in animal models of genetic hypertension.1 Genetic studies in the SHR and SHRSP have provided evidence that sexual dimorphism, as it pertains to hypertension, may be at least partially attributable to BP-raising effects of a Y-chromosome locus.2 3 4 5 6 These observations, however, have not been confirmed in all crosses.7 We had previously found that basal systolic BP in F2 (WKYHD-0×SHRSPHD) rats that had inherited the Y chromosome from an SHRSPHD grandfather were significantly higher than those in F2 males from the reciprocal cross.5 There was, however, no genetic influence of grandparental status on elevated BP levels after excess dietary salt intake. In a second study involving reciprocal crosses between SHRSPHD and a congenic strain, WKYHD-1, which carries a 6-cM-long SHRSPHD homologous genomic fragment that contains a BP quantitative trait locus, BP/SP-1a, introgressed on chromosome 10,8 we found no significant effects of parental status on BP either at baseline or after salt loading in F2 males. The detailed genetic analysis in the crosses reported herein provide new information about the potential role of gene-gene interactions between a putative Y-chromosome locus and effects on BP and BP/SP-1a. In addition, the influence of ecogenetic factors, ie, dietary salt exposure, on the Y-chromosome BP effects was assessed.
Animals and Genetic Crosses
Animals were obtained from our colonies of SHRSPHD, WKY-0HD, and WKY-1HD rats at the University of Heidelberg, Heidelberg, Germany.8 9 Experimental procedures for animal housing and breeding have been previously reported.9 The WKY-1HD strain has been characterized elsewhere.8 In brief, WKY-1HD represents a congenic lineage of WKYHD animals that carry a ≤6-cM-long SHRSPHD homologous chromosome fragment on chromosome 10 that contains a BP-relevant locus, BP/SP-1a.8 The first reciprocal F2 population used in the present study originated from a cross of SHRSPHD and WKY-0HD and has been described in detail.9 This F2 population consisted of 28 males with a male WKY-0HD founder (cross I) and 36 males with a male SHRSPHD founder (cross II). The second reciprocal cross analyzed for this article was generated from WKY-1HD and SHRSPHD and has also been described previously.8 This F2 population consisted of 34 males with a male WKY-1HD founder (cross I) and 33 males with a male SHRSPHD founder (cross II).
The protocol for hemodynamic characterization of F2 rats by femoral artery cannulation and intermittent on-line recording has been reported elsewhere.9 This protocol included two consecutive femoral artery cannulations to measure BP at baseline at the age of 16 weeks and after a period of 12 days of dietary salt loading with 1% NaCl in the drinking water.9
Statistical evaluation was carried out by two-factor ANOVA that accounted for the origin of the Y chromosome (reciprocal cross status) and for the two F2 populations. Analysis of the interaction between BP/SP-1a and reciprocal cross status in the F2 (WKY-0HD×SHRSPHD) population was performed by two-factor ANOVA, under the assumption of a dominant effect of BP/SP-1a, which we had previously shown to be operative.8 Individual group comparisons were carried out by appropriate post hoc testing with Bonferroni's correction.
Mean BP values for F2 males in the reciprocal crosses of the F2 (WKY-0HD×SHRSPHD) and F2 (WKY-1HD×SHRSPHD) cohorts are presented in Table 1⇓. Significantly higher systolic and diastolic BP values were found in male F2 (WKY-0HD×SHRSPHD) rats carrying the SHRSPHD Y chromosome (cross I) than in those carrying the WKY-0HD Y chromosome (cross II).5 Excess dietary salt led to a significant increase in BP in this cross. Reciprocal cross status had a highly significant influence on the salt-induced increase in systolic BP. Male rats of cross I exhibited a twofold higher BP increase compared with those of cross II; this increase eliminated the ≈16 mm Hg difference seen at baseline between the two groups. An analogous but less pronounced effect of reciprocal cross status was seen for increases in diastolic BP after salt loading.
Basal systolic and diastolic BP in F2 (WKY-1HD×SHRSPHD) males was significantly higher than that in F2 (WKY-0HD×SHRSPHD) males. Basal BP was, in fact, not significantly different from that observed after dietary salt exposure (P=.54 and P=.22, respectively) in F2 (WKY-0HD×SHRSPHD). In F2 (WKY-1HD×SHRSPHD) no significant differences between reciprocal crosses I and II were observed for basal BP or salt-stimulated BP. Furthermore, there was no significant influence of reciprocal cross status on salt-induced increases in systolic or diastolic BP (Table 1⇑).
We tested for epistatic interactions between a previously characterized BP quantitative trait locus, BP/SP-1a, and Y-chromosome effects in the F2 (WKY-0HD×SHRSPHD) population. A two-factor-ANOVA that accounted for zygosity at BP/SP-1a and the origin of the Y chromosome (ie, reciprocal cross status) was carried out (Table 2⇓). While no interactions were seen with regard to basal BP, we observed a significant effect on systolic BP after salt loading. Only in those males that had inherited the SHRSPHD-derived Y chromosome did the presence of one or two copies of the SHRSPHD-derived BP/SP-1a allele result in BP values that were significantly higher than in animals homozygous for the WKY-0HD–derived BP/SP-1a allele (Table 2⇓). A tendency toward an interaction between BP/SP-1a and Y-chromosome gene(s) was also seen for diastolic BP after salt loading. However, the magnitude of this effect failed to reach statistical significance (Table 2⇓).
Previous studies have arrived at variable results concerning the presence of a specific Y-chromosome hypertensive effect.7 These discrepancies have been attributed to possible differences among SHR lines10 or variable epistatic interactions between the SHR Y chromosome and the cross-specific genetic background.6 10 Our initial observation in the F2 (WKY-0HD×SHRSPHD) population5 suggested that a Y-chromosome locus influenced BP at baseline but not after dietary salt excess. The reason for this discordant Y-chromosome effect on BP before and after salt loading remained unclear. We hypothesized that specific ecogenetic interactions between increased dietary salt intake and the putative hypertension-related gene(s) on the Y chromosome or a BP “ceiling” reached after salt loading (which halted recognition or phenotypic expression of graded differences) might explain this phenomenon. In fact, both hypotheses may be correct. The observation that the BP increase in response to salt was more than twice as high in males that inherited the WKY-0HD Y chromosome would favor the presence of ecogenetic effects, ie, salt sensitivity conferred by the WKY-0HD Y chromosome (or salt resistance conferred by the SHRSPHD Y chromosome). On the other hand, the overall smaller increase in BP seen after salt loading in F2 (WKY-1HD×SHRSPHD) rats, which had significantly higher baseline BPs than did F2 (WKY-0HD×SHRSPHD), is consistent with a “ceiling,” or saturation, phenomenon. A recently reported cosegregation study involving WKY and SHRSP animals from the Ann Arbor colonies6 tends to support the latter idea. The data obtained in that study argue against the relevance of specific ecogenetic interactions between dietary salt intake and manifestation of Y-chromosome BP effects, since the authors reported a hypertensinogenic Y-chromosome BP effect of the SHRSP Y allele at baseline as well as after salt loading.6
While this interpretation accommodates all of our observations and has the potential to reconcile, at least theoretically, previously discrepant observations, its generalizability must be viewed with some reservations. Strictly speaking, the observations apply only to the three strains studied, to the developmental phase, and to specific environmental variables investigated in our experiments. While the differences appear to be statistically robust, the subgroups from which they were derived were small; thus, larger experimental groups will be necessary to confirm them. This is also true for those results wherein we failed to recognize statistically significant differences due to limitations of statistical power inherent in the studies. Last, explanations other than those considered here may account for our findings, such as environmental effects of phenotype ascertainment (stress) or autosomal parent-of-origin effects.
In summary, our current analyses indicate the presence of rather intricate and complex interactions of Y-chromosome genes with environmental factors as well as with autosomal genes that govern BP regulation. We have demonstrated that phenotypic expression of the Y-chromosome BP effect modification may depend on both environmental and gene-gene interactions and on interactions that depend on both components. Moreover, the phenotypic effects of all interactions (or their recognition) may be subject to the level of underlying basal BP.
While our data await confirmation in larger experimental cohorts, they indicate the complexity of remote phenotype expression in complex polygenic traits, even in the case of reductionist models derived from genetically homogeneous inbred strains.
Selected Abbreviations and Acronyms
|SHR||=||spontaneously hypertensive rat|
|SHRSP||=||stroke-prone spontaneously hypertensive rat|
This work was supported in part by the Deutsche Forschungsgemeinschaft (to R.K., Kr1152/1-2) and by the EURHYPGEN concerted action of the European Economic Community. R.K. is the recipient of a Howard Hughes Medical Institute Research Fellowship for Physicians, and K.L. is the recipient of a National Institutes of Health (Bethesda, Md) Research Career Development Award K04HL03039.
- Received June 21, 1996.
- Revision received July 11, 1996.
- Accepted August 15, 1996.
Ely DL, Turner ME. Hypertension in the spontaneously hypertensive rat is linked to the Y chromosome. Hypertension. 1990;16:277-281.
Turner ME, Johnson ML, Ely DL. Separate sex-influenced and genetic components in spontaneously hypertensive rat hypertension. Hypertension. 1991;17:1097-1103.
Ely DL, Daneshvar H, Turner ME, Johnson ML, Salisbury RL. The hypertensive Y chromosome elevates blood pressure in F11 normotensive rats. Hypertension. 1993;21:1071-1075.
Hilbert P, Lindpaintner K, Beckmann JS, Serikawa T, Soubrier F, Dubay C, Cartwright P, DeGouyon B, Julier C, Takahasi S, Vincent M, Ganten D, Georges M, Lathrop GM. Chromosomal mapping of two genetic loci associated with blood-pressure regulation in hereditary hypertensive rats. Nature. 1991;353:521-529.
Davidson AO, Schork N, Jaques BC, Kelman AW, Sutcliffe RG, Reid JL, Dominiczak AF. Blood pressure in genetically hypertensive rats: influence of the Y chromosome. Hypertension. 1995;26:452-459.
Vincent M, Kaiser MA, Orea V, Lodwick D, Samani NJ. Hypertension in the spontaneously hypertensive rat and the sex chromosomes. Hypertension. 1994;23:161-166.
Kreutz R, Hübner N, James MR, Bihoreau MT, Gaugier D, Lathrop GM, Ganten D, Lindpaintner K. Dissection of a quantitative trait locus for genetic hypertension on rat chromosome 10. Proc Natl Acad Sci U S A. 1995;92:8778-8782.
Turner ME, Qadri A, Montgomery M, Ely DL. Y chromosome polymorphism in WKY and SHR isolates. Hypertension. 1994;24:373. Abstract.