A polymorphism in the gene for the atrial natriuretic peptide and diabetic nephropathy. Diabetic Nephropathy Study Group

Genetic Polymorphisms of the Natriuretic Peptide System in the Pathogenesis of Cardiovascular Disease: What Lies on the Horizon?

Background: The natriuretic peptide hormone family includes various proteins characterized by similar chemical structure and shared biological functions, with important effects on the cardiovascular system. Accordingly, these molecules are widely recognized as key clinical biomarkers in the diagnosis and monitoring of heart failure, hypertension, and coronary heart disease.

Content: Several single-nucleotide polymorphisms have been recently identified in genes associated with the natriuretic system. This review provides an overview of new insights into the functional role of these genetic variants, as well as their impact on cardiovascular physiopathology and drug response.

Conclusions: Noteworthy relationships between some specific polymorphisms and clinical correlates of cardiovascular disease have emerged. Nevertheless, future confirming studies are needed to substantiate the clinical relevance of such variants.

Assessment of 115 candidate genes for diabetic nephropathy by transmission/disequilibrium test

Several lines of evidence, including familial aggregation, suggest that allelic variation contributes to risk of diabetic nephropathy. To assess the evidence for specific susceptibility genes, we used the transmission/disequilibrium test (TDT) to analyze 115 candidate genes for linkage and association with diabetic nephropathy. A comprehensive survey of this sort has not been undertaken before. Single nucleotide polymorphisms and simple tandem repeat polymorphisms located within 10 kb of the candidate genes were genotyped in a total of 72 type 1 diabetic families of European descent. All families had at least one offspring with diabetes and end-stage renal disease or proteinuria. As a consequence of the large number of statistical tests and modest P values, findings for some genes may be false-positives. Furthermore, the small sample size resulted in limited power, so the effects of some tested genes may not be detectable, even if they contribute to susceptibility. Nevertheless, nominally significant TDT results (P < 0.05) were obtained with polymorphisms in 20 genes, including 12 that have not been studied previously: aquaporin 1; B-cell leukemia/lymphoma 2 (bcl-2) proto-oncogene; catalase; glutathione peroxidase 1; IGF1; laminin alpha 4; laminin, gamma 1; SMAD, mothers against DPP homolog 3; transforming growth factor, beta receptor II; transforming growth factor, beta receptor III; tissue inhibitor of metalloproteinase 3; and upstream transcription factor 1. In addition, our results provide modest support for a number of candidate genes previously studied by others.

Investigation of the human ANP gene in type 1 diabetic nephropathy: case-control and follow-up studies

The atrial natriuretic peptide gene (PND) is a candidate gene for diabetic nephropathy. We systematically analyzed five nonsynonymous PND polymorphisms and tested the association of genotype and haplotype distributions with diabetic nephropathy in a cross-sectional study and a 6-year follow-up study (489 and 301 type 1 diabetic patients, respectively). For this purpose, a new maximum-likelihood method dealing with haplotype-based association analysis for survival data was developed. None of the genotypes or haplotypes were associated with the disease in the case-control study. In the follow-up study, C708T and T2238C showed a weak association with disease progression, but T2238C was strongly associated with progression in poorly controlled subjects (mean HbA(1c) during follow-up was more than the median value [8.5%]; log-rank [TC or CC versus TT], P = 0.007; adjusted hazard ratio, TC or CC versus TT, 2.28, 95% CI 1.10-4.74; P = 0.027). The raw effect of the 2238C allele (hazard risk ratio 1.93, 95% CI 1.15-3.24; P = 0.012) was further confirmed by the haplotype analysis, suggesting that the 2238C allele of PND may affect the course of nephropathy in inadequately controlled type 1 diabetic patients.

Genomic strategies for diabetic nephropathy

Insight into the molecular mechanisms that underlie the origin and progression of diabetic nephropathy remains limited in part because conventional research tools have restricted investigators to focus on single genes or isolated pathways. Microarray technologies provide opportunities for evaluating genetic factors and environmental effects at a genomic scale during the pathogenesis of diabetic nephropathy. Despite the enormous power of the microarray technology, there are several pitfalls that need to be considered. This article discusses conceptual, practical, statistical, and logistical considerations for the use of microarrays in studies of experimental and human diabetic renal disease. New knowledge in this field will facilitate new approaches for molecular diagnosis and drug discovery.

Genetic aspects of diabetic nephropathy

BACKGROUND

Diabetic nephropathy from type 2 diabetes mellitus is the most common cause of end-stage renal disease (ESRD) in the United States and in Western Europe. Although patients with overt nephropathy generally experience greater cumulative glycemic exposure, the difference in glycemic control between patients developing nephropathy compared to those who did not, could not be demonstrated. This observation is consistent with the notion that perhaps other factors, in addition to glycemic control, have a bearing on the development of nephropathy. Genetic factors, which specifically increase the susceptibility to nephropathy in patients with diabetes, have been proposed.

METHODS

A range of linkage and association studies has been performed for revealing the genetic background of diabetic nephropathy.

RESULTS

Until now, no mutation has been identified which could explain the development of diabetic nephropathy in the majority of diabetic patients.

CONCLUSIONS

Because of relatively small case numbers of all studies being performed so far, conclusions from those studies are limited. With the development of better technologies for an affordable genome-wide association study using thousands of markers, it might become possible to unravel the genetic causes of diabetic nephropathy.

Association between polymorphisms of the atrial natriuretic peptide gene and proteinuria: a population-based study

AIMS/HYPOTHESIS

In case-control studies, polymorphisms at the atrial natriuretic peptide gene (ANP) locus have been associated with presence of albuminuria in Type 1 and Type 2 diabetes. We evaluated the relationship between the ScaIand BstxI polymorphisms and albuminuria in the general population of the Mexico City Diabetes Study.

METHODS

Allele/genotype frequencies were analysed by PCR-RFLP analysis using ScaI (wild, A(2) vs mutated, A(1)) and BstxI (wild, C(708) vs mutated, T(708)) enzyme.

RESULTS

Among 1288 subjects, hypertension was present in 112 subjects, Type 2 diabetes in 191 and impaired glucose tolerance in 136; microalbuminuria was present in 464 subjects, and clinical proteinuria in 199. General frequencies were 0.93 and 0.96 for the wild alleles, and 0.07 and 0.04 for the mutated alleles, respectively for ScaI and BstxI. Frequency of A(1)was 0.08 in normoalbuminuric, 0.05 in microalbuminuric, and 0.05 in proteinuric patients (chi(2)=7.3, p=0.025). Frequency of T(708) was 0.06 in normoalbuminuric and 0.03 microalbuminuric and 0.03 in proteinuric subjects (chi(2)=8.1, p=0.017). By multivariate analysis, the associations between A(1)or T(708) allele and albuminuria were independent of age, sex, BMI, diabetes, and hypertension, (odds ratio (OR) 0.60, p=0.01, (OR) 0.51, p=0.004, respectively).

CONCLUSION/INTERPRETATION

In the general population of Mexico City, both polymorphisms of ANP are associated with albuminuria independently of hypertension, and could play a role in protecting subjects against development of albuminuria.

Genetics of diabetic nephropathy

Diabetic nephropathy can develop in up to one-third of patients with type 1 diabetes and approximately 25% of patients with type 2 diabetes. This complication is important as it not only leads to renal failure but is associated with a high risk of coronary artery disease and other vascular complications. Although hyperglycaemia is necessary for the development of diabetic nephropathy, it is not sufficient, genetic factors also being important. This is evidenced by studies showing that only a subgroup of patients are at risk of nephropathy and that nephropathy clusters in families. The genes involved in susceptibility to diabetic nephropathy have yet to be identified. Most studies to date have been case-control in design, and there have been conflicting results. Genes suggested as having a role include those encoding angiotensin-1 converting enzyme, apolipoprotein E, heparan sulphate and aldose reductase. In order to clarify the role of these and other candidate genes in nephropathy, association studies in families are necessary. Because of the large number required, this will require international collaboration. A genetic marker for nephropathy would enable the earlier detection of this complication, thus facilitating screening and targeted intervention. An understanding of the role of susceptibility genes will ultimately allow the development of novel therapeutic strategies.

Polymorphisms in the hANP (human atrial natriuretic peptide) gene, albuminuria, and hypertension

Atrial natriuretic peptide (ANP) jointly affects kidney function and blood pressure homeostasis and is a candidate susceptibility gene for both essential hypertension and kidney disease. We evaluated the relation between the ScaI and BstXI polymorphisms of the human ANP (hANP) gene, hypertension, and albuminuria in a clinical cohort of 1033 subjects, including type 1 and type 2 diabetic patients, nondiabetic subjects with essential hypertension, and nondiabetic normotensive control subjects. Microalbuminuria was present in 15%, 29%, and 2%, respectively, of type 1 diabetic, type 2 diabetic, and nondiabetic patients. Macroalbuminuria was present in 9% of type 1 diabetics, 21% of type 2 diabetics, and 31% of nondiabetics. Prevalence of hypertension was 31%, 58%, and 61% in normoalbuminuric, microalbuminuric, and macroalbuminuric subjects, respectively (P<0.0001). Genotype distributions were in Hardy-Weinberg equilibrium in all 4 patient subgroups. The frequency of the ScaI mutated allele (A(1)) was significantly lower in hypertensive than in control subjects (11% versus 19%, P=0.018) and in patients with macroalbuminuria (5%) as compared with normoalbuminuric subjects (16%; P<0.0001). In a nominal logistic model adjusting for gender, age, obesity, diabetes, micro/macroalbuminuria, and hypertension, the A(1) allele was independently associated with macroalbuminuria (odds ratio, 0.57; confidence interval, 1.39 to 3.59; P=0.003) but not with hypertension. In the same model, the frequency of the BstXI mutated allele (T(708)) was increased in the presence of microalbuminuria (odds ratio, 2.25; confidence interval, 1.39 to 3.59; P<0.001). We conclude that the mutated genotypes of the ScaI polymorphism are negatively associated with overt nephropathy, whereas the mutated genotypes of BstXI polymorphism are positively associated with microalbuminuria. hANP gene variants may exert a protective effect against the development and progression of kidney damage in diabetes.

Pronatriodilatin gene polymorphisms, microvascular permeability, and diabetic nephropathy in type 1 diabetes mellitus

Approximately 30% of diabetic patients develop nephropathy, the appearance of which is partially under genetic control. Atrial natriuretic peptide (ANP) has associated physiologic effects on the kidney. This study was conducted to examine the relationship between a newly identified and known polymorphism at the pronatriodilatin (PND) gene locus and renal involvement in type 1 diabetic subjects. Of 454 type 1 diabetic patients (219 men, 235 women), 323 showed no sign of nephropathy, 79 had incipient renal involvement, and 52 established nephropathy; 58 healthy control subjects were examined for comparison. Allele frequencies (C708 versus T708) were: 0.95 and 0.05 in normoalbuminuric patients, respectively; 0.88 and 0.12 in microalbuminuric patients; 0.96 and 0.04 both in those with overt nephropathy and in healthy control subjects (P = 0.011). Patients with incipient nephropathy were in disequilibrium compared with the total diabetic cohort (P = 0.02). In the same populations, an additional genotype for ScaI polymorphism of the PND gene was tested. The A1 and A2 allele frequencies were: 0.21 and 0.79 in normoalbuminuric patients; 0. 13 and 0.87 in microalbuminuric patients; 0.06 and 0.94 in type 1 diabetic subjects with overt nephropathy; and 0.20 and 0.80 in healthy control subjects, respectively (P < 0.0001). A subset of 55 normotensive patients with type 1 diabetes, well matched for clinical features, plasma ANP levels, and microvascular permeability to macromolecules, was investigated on the basis of the C708/T and A2/A1 polymorphisms. Both transcapillary escape rate of albumin (TERalb) and plasma ANP levels were significantly lower in patients with the T708 than with C708 allele, as well as in the A1 than in A2 allele (TERalb: T708 versus C708: 5.5+/-1.7 versus 7.8+/-2.0%/h, P = 0.0001; plasma ANP levels: 8.3+/-3.9 versus 15.3+/-7.7 pg/ml, P = 0.0003; A1 versus A2: 6.05+/-2.2 versus 7.3+/-2.1%/h, P = 0.044; 8.53+/-4.6 versus 14.5+/-7.4 pg/ml, P = 0.0024, respectively). Thus, in a large ethnically homogeneous cohort of diabetic subjects, our data show: (1) a significant association of C708/T polymorphism with microalbuminuria in long-term diabetes and with both lower plasma ANP levels and widespread albumin leakage; and (2) a strong association between ScaI polymorphism and both diabetic nephropathy and plasma ANP concentrations. These results suggest a possible role of PND gene in conferring protection from nephropathy and microvascular damage in type 1 diabetes.