Rare independent mutations in renal salt handling genes contribute to blood pressure variation

Abstract

The effects of alleles in many genes are believed to contribute to common complex diseases such as hypertension. Whether risk alleles comprise a small number of common variants or many rare independent mutations at trait loci is largely unknown. We screened members of the Framingham Heart Study (FHS) for variation in three genes-SLC12A3 (NCCT), SLC12A1 (NKCC2) and KCNJ1 (ROMK)-causing rare recessive diseases featuring large reductions in blood pressure. Using comparative genomics, genetics and biochemistry, we identified subjects with mutations proven or inferred to be functional. These mutations, all heterozygous and rare, produce clinically significant blood pressure reduction and protect from development of hypertension. Our findings implicate many rare alleles that alter renal salt handling in blood pressure variation in the general population, and identify alleles with health benefit that are nonetheless under purifying selection. These findings have implications for the genetic architecture of hypertension and other common complex traits.

Figure 1. Proven and inferred mutations in NCCT, NKCC2 and ROMK in the Framingham Heart Study offspring cohort

The predicted structure of each protein is shown, with locations of putatively functional missense variants found in FHS indicated in single letter code (X = premature termination; FS = frameshift).

Figure 2. Heterozygous mutations in NCCT, NKCC2 and ROMK lower blood pressure

a) and b) show distributions of standardized residual values for age and sex-adjusted long-term average blood pressure among carriers of putative functional mutations. Adjusted values for the cohort have a mean value of zero and a standard deviation 1. Mean effect size and parametric P-values are indicated. a) Systolic blood pressure. b) Diastolic blood pressure. Panels c) and d) show the mean and standard error of systolic (c) and diastolic (d) blood pressures among mutation carriers (n ranges from 31 to 43 in different decades) and non-carriers (n= 2094-2753) at the last examination in different age groups. P-values comparing values of mutation carriers and non-carriers by t-tests are shown. Asterisks indicate biochemically proven loss of function mutations.

Figure 3. Correlation of blood pressures among sibling pairs in Framingham Heart Study

The distribution of standardized SBP residuals of all 1369 sibling pairs in the FHS cohort is shown. The blood pressures of sibling pairs concordant for putative functional mutations (red), discordant sibling pairs (blue) and those with no mutations (gray) are indicated. For discordant pairs, the sibling carrying the mutation is represented on the y-axis (sib2).

Figure 4. Reduced prevalence of hypertension among mutation carriers

a) Kaplan-Meier plot of time to onset of hypertension in the 49 carriers and 3076 non-carriers. b) The prevalence of hypertension at the last exam within age 25-40, 41-50 and 51-60 years is shown for mutation carriers and non-carriers. The genotype relative risk (GRR) for mutation carriers is shown.