Studies on hypertension in spontaneously hypertensive rats

Susceptibility to Hypertensive Renal Disease in the Spontaneously Hypertensive Rat Is Influenced by 2 Loci Affecting Blood Pressure and Immunoglobulin Repertoire

High blood pressure exerts its deleterious effects on health largely through acceleration of end-organ diseases. Among these, progressive loss of renal function is particularly important, not only for the direct consequences of kidney damage but also because loss of renal function is associated with amplification of other adverse cardiovascular outcomes. Genetic susceptibility to hypertension and associated end-organ disease is non-Mendelian in both humans and in a rodent model, the spontaneously hypertensive rat (SHR). Here, we report that hypertensive end-organ disease in the inbred SHR-A3 line is attributable to genetic variation in the immunoglobulin heavy chain on chromosome 6. This variation coexists with variation in a 10 Mb block on chromosome 17 that contains genetic variation in 2 genes involved in immunoglobulin Fc receptor signaling. Substitution of these genomic regions into the SHR-A3 genome from the closely related, but injury-resistant, SHR-B2 line normalizes both biomarker and histological measures of renal injury. Our findings indicate that genetic variation leads to a contribution by immune mechanisms hypertensive end-organ injury and that, in this rat model, disease is influenced by differences in germ line antibody repertoire.

Hypertensive renal disease: susceptibility and resistance in inbred hypertensive rat lines

BACKGROUND

Spontaneously hypertensive rat (SHR) lines differ in their susceptibility to hypertensive end-organ disease and may provide an informative model of genetic risk of disease. Lines derived from the original SHR-B and SHR-C clades are highly resistant to hypertensive end-organ disease, whereas lines derived from the SHR-A clade were selected for stroke susceptibility and experience hypertensive renal disease.

METHOD

Here we characterize the temporal development of progressive renal injury in SHR-A3 animals consuming 0.3% sodium in the diet and drinking water. SHR-A3 rats demonstrate albuminuria, glomerular damage, tubulointerstitial injury, and renal fibrosis that emerge at 18 weeks of age and progress.

RESULTS AND CONCLUSION

Mortality of SHR-A3 animals was 50% at 40 weeks of age, and animals surviving to this age had reduced renal function. In contrast SHR-B2, which are 87% genetically identical to SHR-A3, are substantially protected from renal injury and demonstrate only moderate changes in albuminuria and renal histological injury over this time period. At 40 weeks of age, electron microscopy of the renal glomerulus revealed severe podocyte effacement in SHR-A3, but slit diaphragm architecture in SHR-B2 at this age was well preserved. Renal injury traits in the F1 and F2 progeny of an intercross between SHR-A3 and SHR-B2 were measured to determine heritability of renal injury in this model. Heritability of albuminuria, glomerular injury, and tubulointerstitial injury were estimated at 48.9, 66.5 and 58.6%, respectively. We assessed the relationship between blood pressure and renal injury measures in the F2 animals and found some correlation between these variables that explain up to 26% of the trait variation. Quantitative trait locus (QTL) mapping was performed using over 200 single nucleotide polymorphism markers distributed across the 13% of the genome that differs between these two closely related lines. Mapping of albuminuria, tubulointerstitial injury, and renal fibrosis failed to identify loci linked with disease susceptibility, suggesting a complex inheritance of disease risk. We detected a single QTL conferring susceptibility to glomerular injury that was confined to a small haplotype block at chromosome 14:70-76Mb.

Immunoglobulin locus associates with serum IgG levels and albuminuria

The interaction between IgG and Fc-γ receptors in glomeruli contributes to the development of several types of proteinuric glomerular disease, but the involvement of immunological mechanisms in hypertensive renal injury is incompletely understood. Here, we investigated serum IgG levels in SHR-A3 rats, which develop hypertensive injury, and compared them with the injury-resistant SHR-B2 line. At 18 weeks old, SHR-A3 rats had serum total IgG levels nearly twice those of SHR-B2 rats, although subclass IgG2b was undetectable in SHR-A3 rats compared with mean levels (± SEM) of 80.7 ± 12.8 mg/dl (18 weeks) and 116.6 ± 19.0 mg/dl (30 weeks) in SHR-B2 rats. In addition, these two strains had significantly different serum levels of IgG1, IgG2a, and IgG2c; differences persisted at 30 weeks for all subclasses except IgG2a. Genetic mapping revealed that a locus on chromosome 6 linked to IgG subclass levels that affected IgG1, IgG2b, and IgG2c but not IgG2a. The mapped haplotype block contains IgH, suggesting regulation of three of four serum IgG subclass levels in cis. Resequencing revealed variation in the sequence of the Fc portion of the IgG heavy chain, which predicts important functional changes. To examine whether there is any relationship between this haplotype block and susceptibility to renal injury, we examined the effect of SHR-A3 and SHR-B2 alleles at this block on albumin excretion in an F2 intercross. Albuminuria doubled with inheritance of SHR-A3 alleles. In summary, allelic variation in IgH or nearby genes may modulate the susceptibility to hypertensive renal injury in SHR-A3 rats.