Type 2 diabetes locus on 12q15. Further mapping and mutation screening of two candidate genes

We recently reported evidence of a novel type 2 diabetes locus placed on chromosome 12q15 between markers D12S375 and D12S1684 (Diabetes 48:2246-2251, 1999). Four multigenerational families having logarithm of odds (LOD) scores >1.0 in the original analysis were genotyped for 11 additional markers in this interval to refine this mapping; this allowed us to narrow the linked region to the interval between markers D12S1693 and D12S326. In a multipoint parametric analysis using the VITESSE software, the LOD score for linkage at this location reached 3.1 in one of these families. This interval contains the gene for protein tyrosine phosphatase receptor type R (PTPRR)--a protein that may be involved in both insulin secretion and action. After determining PTPRR exon-intron structure, we identified several polymorphisms in this gene but no mutation segregating with diabetes. The search for mutations was also negative for carboxypeptidase M (CPM)--another candidate gene mapped to this region. In summary, our data provide further evidence for the existence of a type 2 diabetes locus on chromosome 12q15. This locus, however, does not appear to correspond to the PTPRR or CPM, although a contribution of mutations in regulatory regions cannot be excluded at this time.

Polymorphisms of human paraoxonase 1 gene (PON1) and susceptibility to diabetic nephropathy in type I diabetes mellitus

AIMS/HYPOTHESIS

Oxidative stress is a putative mechanism in the development of diabetic nephropathy. Paraoxonase gene 1 is an HDL-bound enzyme that protects tissues against oxidative damage. Three common polymorphisms of paraoxonase gene 1, T-107C in the promoter, Leu54Met and Gln192Arg, that modify paraoxonase activity have been associated with cardiovascular disease. This study aimed to find whether these polymorphisms also contribute to the development of diabetic nephropathy.

METHODS

The association between diabetic nephropathy and these three polymorphisms was examined in a case-control study. For this purpose, genomic DNA was collected from 188 patients with Type I (insulin-dependent) diabetes mellitus and diabetic nephropathy and from 179 unrelated patients with Type I diabetes but without diabetic nephropathy despite the duration of diabetes of 15 or more years.

RESULTS

The genotype and allele frequencies for each of the three polymorphisms (T-107C, Leu54Met and Gln192Arg) were similar in cases and control subjects.

CONCLUSION/INTERPRETATION

The three polymorphisms in paraoxonase gene 1 that have been associated with serum levels of paraoxonase are not associated with diabetic nephropathy. We show that this genetically determined component of the antioxidant capacity of HDL does not play a critical part in the development of diabetic nephropathy.

APOE polymorphisms and the development of diabetic nephropathy in type 1 diabetes: results of case-control and family-based studies

The goal of this study was to examine the association between known polymorphisms in the apolipoprotein E gene (APOE) and diabetic nephropathy (DN) in type 1 diabetes. We used both a case-control comparison and a family-based study design known as the transmission/disequilibrium test (TDT). For the case-control comparison, we collected DNA from 223 subjects with clinically diagnosed DN and 196 control subjects with normoalbuminuria and long-duration type 1 diabetes (> or = 15 years). For the family-based study, we obtained DNA from both parents of 154 DN subjects and 81 control subjects. The frequency of the epsilon2 allele of exon 4 of APOE was significantly higher in DN subjects than in control subjects. The risk of DN was 3.1 times higher (95% CI 1.6-5.9) in carriers of this allele than in noncarriers. In the family study, heterozygous parents for the E2 allele preferentially transmitted epsilon2 to DN offspring (64 vs. 36%, P < 0.03). Four additional polymorphisms (i.e., -491 A/T, -219 G/T, IE1 G/C, and APOCI insertion/deletion [I/D]) that flank the APOE locus were not associated with DN in either the case-control comparison or in the family-based study. In conclusion, the results of the case-control as well as the family-based study provide evidence that the epsilon2 allele of APOE increases the risk of DN in type 1 diabetes. The molecular mechanisms underlying this risk are unclear at present.

Further evidence for a susceptibility locus for type 2 diabetes on chromosome 20q13.1-q13.2

We previously reported suggestive linkage between type 2 diabetes and markers in a region on chromosome 20q using data from a collection of 29 Caucasian families in which type 2 diabetes with middle-age-onset was segregated as an autosomal-dominant disorder. To map more precisely the susceptibility locus (or loci) within this broad region, we increased the family collection and genotyped all families for additional markers, both within the critical region and spaced over the rest of chromosome 20. Altogether 526 individuals (including 241 with diabetes) from the total collection of 43 families were included in the study. All individuals were genotyped for 23 highly polymorphic markers. Positive evidence for linkage was found for a 10-cM region on the long arm of chromosome 20q13.1-q13.2 between markers D20S119 and D20S428. The strongest evidence in two-point as well as multipoint linkage analysis (P = 1.8 x 10(-5)) occurred at the position corresponding to marker D20S196. The individuals with diabetes in the seven most strongly linked families had high serum insulin levels during fasting and 2-h post-glucose load periods. We did not find any evidence for linkage between type 2 diabetes and any other region on chromosome 20. In conclusion, our larger and more comprehensive study showed very strong evidence for a susceptibility gene for insulin-resistant type 2 diabetes located on the long arm of chromosome 20 around marker D20S196.

Segregation analysis of urinary albumin excretion in families with type 2 diabetes

Elevated urinary albumin excretion (UAE) is a predictor of the development of nephropathy and cardiovascular mortality. To study whether genetic factors may determine UAE, we examined familial aggregation of UAE in 96 large multigenerational pedigrees ascertained for type 2 diabetes. A total of 1,269 subjects had UAE measured as the urinary albumin-to-creatinine ratio (ACR). This included 630 subjects with type 2 diabetes and 639 subjects without diabetes. A significant correlation (Spearman's correlation 0.34, P < 0.001) was found between the median ACR values determined separately in nondiabetic and diabetic members of the same family. To determine whether this familial aggregation of ACR could be explained by the transmission of 1 or more major genes and thus be suitable for gene mapping studies, segregation analyses were performed. In these analyses, ACR was modeled as a continuous trait with the inclusion of age, sex, and duration of diabetes as covariates. Likelihood ratio tests were performed to test competing hypotheses, and Akaike's information criterion was used to determine the most parsimonious models. The Mendelian model with multifactorial inheritance was supported more strongly than Mendelian inheritance alone. These analyses suggested that the best model for ACR levels was multifactorial with evidence for a common major gene. When the analyses were repeated for diabetic subjects only, the evidence for Mendelian inheritance was improved, although a single major locus with additional multifactorial effects was more strongly supported. The results from the current study suggest that levels of UAE are determined by a mixture of genes with large and small effects as well as other measured covariates, such as diabetes.

Aldose reductase gene polymorphisms and susceptibility to diabetic nephropathy in Type 1 diabetes mellitus

AIMS

To investigate association and linkage between DNA sequence variants in the aldose reductase (AR) gene on chromosome 7q35 and diabetic nephropathy (DN) in Type 1 diabetes mellitus.

METHODS

By sequencing the promoter region and 10 exons in eight DN cases and eight controls, a frequent bi-allelic polymorphism (C-106T) was discovered. This polymorphism and the known 5'ALR2 dinucleotide repeat polymorphism were genotyped in unrelated cases with advanced nephropathy (n = 221) and unrelated controls with normoalbuminuria (n = 193). For a family based study, 166 case-trios (case and both parents) and 83 control-trios (control and both parents) were also genotyped.

RESULTS

In the case-control study, carriers of the Z-2 allele of the 5'ALR2 polymorphism had a significantly higher risk of DN than non-carriers (odds ratios: 1.6 for heterozygotes and 2.1 for homozygotes, P<0.05 for each). The same was true for carriers of the T allele of the C-106T polymorphism (odds ratios: 1.6 for heterozygotes and 1.9 for homozygotes, P<0.05 for each). Moreover, the haplotype carrying both risk alleles was in excess in DN cases. In the family study, transmission of risk alleles from heterozygous parents was consistent with the case-control study, excess transmission in case-trios and deficient in control-trios.

CONCLUSIONS

Association between DN and two DNA sequence variants in the promoter region of the AR gene implicates the polyol pathway in the development of kidney complications in Type 1 diabetes mellitus. Further examination of the molecular mechanisms underlying these findings may provide insight into the pathogenesis of DN.

Risk of advanced diabetic nephropathy in type 1 diabetes is associated with endothelial nitric oxide synthase gene polymorphism

Risk of advanced diabetic nephropathy in type 1 diabetes is associated with endothelial nitric oxide synthase gene polymorphism.

BACKGROUND

Polymorphisms in the endothelial nitric oxide synthase gene (eNOS) may be implicated in the development of nephropathy in patients with type 1 or insulin-dependent diabetes mellitus (IDDM).

METHODS

Three groups of IDDM patients were selected to study this hypothesis: cases with advanced diabetic nephropathy (N = 78), cases with overt proteinuria but normal serum creatinine (N = 74), and controls with normoalbuminuria despite 15 years of diabetes (N = 195). Parents of 132 cases and 53 controls were also examined and were used for the transmission disequilibrium test, a family-based study design to test association.

RESULTS

We examined four eNOS polymorphisms, and two were associated with diabetic nephropathy in the case-control comparisons: a T to C substitution in the promoter at position -786 and the a-deletion/b-insertion in intron 4. For the former, the risk of developing advanced nephropathy was higher for C allele homozygotes than for the other two genotypes (odds ratio 2.8, 95% CI, 1.4 to 5.6). For the latter polymorphism, it was the a-deletion carriers that had the higher risk (odds ratio 2.3, 95% CI, 1.3 to 4.0) in comparison with noncarriers. Both polymorphisms were analyzed together as haplotypes in a family-based study using the transmission disequilibrium test. The C/a-deletion haplotype was transmitted from heterozygous parents to cases with advanced diabetic nephropathy with a significantly higher frequency than expected (P = 0.004).

CONCLUSION

The findings of the case-control and family-based studies demonstrate clearly that DNA sequence differences in eNOS influence the risk of advanced nephropathy in type 1 diabetes.

Progression of microalbuminuria to proteinuria in type 1 diabetes: nonlinear relationship with hyperglycemia

While small clinical trials have shown that improved glycemic control reduces the risk of progression of microalbuminuria to proteinuria, two recent clinical trials did not confirm this finding. We sought to reconcile the contradictory evidence by examining the dose-response relationship between hyperglycemia and progression of microalbuminuria to proteinuria in individuals with type 1 diabetes and microalbuminuria (n = 312) who were followed for 4 years with repeated assessments of urinary albumin excretion. Since 33 patients did not participate in follow-up (10.6%), data for 279 patients were analyzed. Urinary albumin excretion level worsened to proteinuria in 40 (4.1 per 100 person-years). To examine the dose-response relationship, baseline HbA1c was divided into four roughly equal groups using the cut points 8, 9, and 10%. The incidence rate varied significantly among the four groups (P = 0.008). Among those with HbA1c <8.0%, the incidence rate of progression was only 1.3 per 100 person-years, while it was 5.1, 4.2, and 6.7 per 100 person-years in the three other groups. We used generalized additive models to examine the dose-response curve using HbA1c as a continuous variable and found that the risk of progression rises steeply between an HbA1c of 7.5-8.5% and then remains approximately constant across higher levels. In conclusion, the results of this study suggest that, in patients with microalbuminuria, the risk of progression to overt proteinuria can be reduced by improved glycemic control only if the HbA1c is maintained below 8.5%. Moreover, below that value, the risk declines as the level of HbA1c decreases.

Urinary albumin excretion in families with type 2 diabetes is heritable and genetically correlated to blood pressure

Urinary albumin excretion in families with type 2 diabetes is heritable and genetically correlated to blood pressure.

BACKGROUND

Levels of urinary albumin excretion (UAE) are used to define both the diagnosis of diabetic nephropathy and its progression. Predisposition to high blood pressure is also a risk factor for susceptibility to nephropathy, but its relationship with UAE in type 2 diabetes remains unclear. In this study, we have estimated heritabilities of UAE and blood pressure and their correlation attributable to genetic effects using 96 large families ascertained for type 2 diabetes.

METHODS

In these families, 630 individuals with type 2 diabetes and 639 individuals with normoglycemia were examined. All of them had a determination of UAE as the urinary albumin creatinine ratio (ACR), a convenient index of UAE, together with blood pressure and other variables, such as age, sex, and body mass index. A variance components approach was used to estimate heritability and genetic correlations for ACR and systolic and diastolic blood pressures (SBP and DBP) after adjustment for relevant covariates.

RESULTS

In the 96 pedigrees, 6481 usable pairs of relatives were identified. In the total collection of pairs, heritability for ACR (h2 = 0.27, P < 0.001) was similar to that for blood pressure. When only pairs of diabetic relatives were analyzed (N = 1732), the estimates of heritability increased slightly for ACR (h2 = 0.31), SBP (h2 = 0.33), and DBP (h2 = 0.23). A significant genetic correlation was found between ACR and SBP (rg 0.27) and DBP (rg 0.26) in all pairs of relatives (P < 0.001). In pairs of diabetic relatives, these values were higher for SBP and DBP, 0.38 and 0.52, respectively.

CONCLUSION

In families with type 2 diabetes, UAE is a heritable trait, with a heritability similar to that for blood pressure. A significant genetic correlation between UAE and blood pressure, particularly in the presence of diabetes, indicates that these traits share common genetic determinants.

Mutations in NEUROD1 are associated with the development of type 2 diabetes mellitus

The helix-loop-helix (HLH) protein NEUROD1 (also known as BETA2) functions as a regulatory switch for endocrine pancreatic development. In mice homozygous for a targeted disruption of Neurod, pancreatic islet morphogenesis is abnormal and overt diabetes develops due in part to inadequate expression of the insulin gene (Ins2). NEUROD1, following its heterodimerization with the ubiquitous HLH protein E47, regulates insulin gene (INS) expression by binding to a critical E-box motif on the INS promoter. Here we describe two mutations in NEUROD1, which are associated with the development of type 2 diabetes in the heterozygous state. The first, a missense mutation at Arg 111 in the DNA-binding domain, abolishes E-box binding activity of NEUROD1. The second mutation gives rise to a truncated polypeptide lacking the carboxy-terminal trans-activation domain, a region that associates with the co-activators CBP and p300 (refs 3,4). The clinical profile of patients with the truncated NEUROD1 polypeptide is more severe than that of patients with the Arg 111 mutation. Our findings suggest that deficient binding of NEUROD1 or binding of a transcriptionally inactive NEUROD1 polypeptide to target promoters in pancreatic islets leads to the development of type 2 diabetes in humans.

Amino acid variants of the vitamin D-binding protein and risk of diabetes in white Americans of European origin

BACKGROUND

Genetic variants of vitamin D-binding protein (DBP) have been reported to be associated, not only with diabetes, but also with prediabetic traits, in several populations. There are two known polymorphisms in exon 11 of the DBP gene that result in amino acid variants: at codons 416 GAT-->GAG (Asp-->Glu) and 420 ACG-->AAG (Thr-->Lys).

OBJECTIVE

To examine the association of these polymorphisms with diabetes in white Americans of European origin.

METHODS

We studied unrelated individuals: 181 with type 1 diabetes, 215 with type 2 diabetes, and 163 healthy controls. Exon 11 was amplified using polymerase chain reaction and the two alleles were determined by digestion with specific endonucleases: HaeIII and StyI, respectively.

RESULTS

At codon 416, Asp/Glu allele frequencies were 45%/55% in patients with type 1 diabetes, 43%/57% in patients with type 2 diabetes, and 46%/54% in controls (chi(2)=0.69, 2 d.f., P<0.71). At codon 420, corresponding Lys/Thr frequencies were 27%/73%, 30%/70%, and 30%/70% (chi(2)=1.25, 2 d.f., P=0.53). Distributions of genotypes at both loci, and the haplotypes defined by the two loci, were also very similar in all groups.

CONCLUSION

DNA polymorphisms in the DBP gene are not associated with diabetes in white Americans of European origin.

Evidence of a novel type 2 diabetes locus 50 cM centromeric to NIDDM2 on chromosome 12q

To replicate the recent finding of a type 2 diabetes locus (NIDDM2) on 12q, families segregating early-onset autosomal-dominant type 2 diabetes were screened for linkage. Included were 26 Caucasian and 6 non-Caucasian pedigrees with an average age at diabetes diagnosis of 37 +/- 18 years. Affected (n = 233) and nonaffected (n = 152) family members were genotyped for 17 markers covering 90 cM on chromosome 12q. While no evidence for linkage was detected at the NIDDM2 locus, a linkage peak was observed 50 cM centromeric to NIDDM2 at markers D12S375 and D12S1052. In a nonparametric analysis, the Z(all) score was 2.9 (P = 0.015) at D12S375, and increased to 3.8 (P = 0.007) among Caucasian families. Further increase in significance was observed in pedigrees with poor insulin response, with a maximum Z(all) of 6.2 (P = 0.002) at D12S375. Suggestive evidence of linkage was also detected by the parametric analysis, with the heterogeneity logarithm of odds score peaking at 2.5 (alpha = 0.15) between D12S375 and D12S1052. In summary, our data indicate that the NIDDM2 locus does not play a major role in early-onset autosomal-dominant type 2 diabetes. Rather, they strongly suggest that a previously undetected type 2 diabetes locus exists 50 cM from NIDDM2 on 12q.

Hepatocyte nuclear factor-4gamma: cDNA sequence, gene organization, and mutation screening in early-onset autosomal-dominant type 2 diabetes

The aim of this study was to investigate whether mutations in hepatocyte nuclear factor (HNF)-4gamma, a transcription factor homologous to HNF-4alpha, contribute to the etiology of early-onset type 2 diabetes. Linkage between diabetes and two polymorphic markers at the HNF-4gamma locus (D8S286 and D8S548) was evaluated in 32 multigenerational families with early-onset autosomal-dominant type 2 diabetes unlinked to known maturity-onset diabetes of the young genes. Total logarithm of odds (LOD) scores were strongly negative (-50.3 at D8S286 and -46.2 at D8S548), but linkage could not be excluded in 15 families having LOD scores >-2.0. To screen these pedigrees for HNF-4gamma mutations, the gene structure was defined. Because reverse transcriptase-polymerase chain reaction experiments indicated that the first 1,674 bp of the published cDNA sequence (3,248 bp) were a cloning artifact, the correct cDNA sequence was determined by 5' rapid amplification of cDNA ends (RACE) and primer extension assay. Based on the new cDNA sequence (1,731 bp), 11 exons were found. After screening the 5' flanking region and all coding exons for mutations, we identified several polymorphisms, one of which affected the amino acid sequence (M190I). However, no mutations segregating with diabetes could be found in these families. We conclude that genetic variability in the HNF-4gamma gene is unlikely to play a major role in the etiology of early-onset autosomal-dominant type 2 diabetes.

Identification of new mutations in the hepatocyte nuclear factor 4alpha gene among families with early onset Type 2 diabetes mellitus

AIMS

Mutations in hepatocyte nuclear factor (HNF)-4alpha gene located on chromosome 20q have been found to be responsible for the development of early onset Type 2 diabetes mellitus (DM). Through a national campaign, 53 families with autosomal dominant, early onset Type 2 DM (n=654) were assembled to determine the frequency of mutations in the HNF-4alpha gene and their contribution to the development of diabetes.

METHODS

Twelve exons and the promoter region of the HNF-4alpha gene were screened in probands of the families by a double gradient, denaturing gradient gel electrophoresis (DG-DGGE) protocol combined with automated bi-directional sequencing of the PCR products of all heterozygous individuals.

RESULTS

We detected two new mutations in the HNF-4alpha gene that changed the amino-acid sequence. The first mutation was a Gly-->Ser substitution in codon 115 within a highly conserved DNA binding domain, and all six carriers of this mutation had diabetes and low insulin secretion. The second mutation was an Ile-->Val substitution in codon 454 within the transactivation domain. It was carried by four family members, two of whom also carried a mutation in the HNF-1alpha gene. Of those having only the mutation in HNF-4alpha one had diabetes and the other had normal glucose tolerance and both were obese and hyperinsulinaemic. Thus, it is uncertain that this mutation is responsible for any of the diabetes in this family.

CONCLUSION

We have found that mutations in the HNF-4alpha gene account for a small proportion, about 2-4%, of families with early onset, autosomal dominant, Type 2 DM in US Caucasians.

Exclusion of insulin receptor substrate 2 (IRS-2) as a major locus for early-onset autosomal dominant type 2 diabetes

We investigated whether variability at the insulin receptor substrate (IRS)-2 locus plays a role in the etiology of early-onset autosomal dominant type 2 diabetes. By means of radiation hybrid mapping, we placed the human IRS-2 gene on 13q at 8.6 cRays from SHGC-37358. Linkage between diabetes and two polymorphic markers located in this region (D13S285 and D13S1295) was then evaluated in 29 families with early-onset autosomal dominant type 2 diabetes. Included were 220 individuals with diabetes, impaired glucose tolerance, or gestational diabetes (mean age at diabetes diagnosis 36 +/- 17 years) and 146 nondiabetic subjects. Overall, strongly negative logarithm of odds (LOD) scores for linkage with diabetes were obtained by multipoint parametric analysis (LOD score -45.4 at D13S285 and -40.9 at D13S1295). No significant evidence of linkage was obtained under the hypothesis of heterogeneity or by nonparametric methods. Fourteen pedigrees for which linkage could not be excluded (LOD score > -2.0) were screened for mutations in the IRS-2 coding region by dideoxy fingerprinting. However, no mutations segregating with diabetes could be detected in these families. These data indicate that IRS-2 is not a major gene for early-onset autosomal dominant type 2 diabetes, although a role of mutations in the promoter region cannot be excluded at this time.

Phenotypic characteristics of early-onset autosomal-dominant type 2 diabetes unlinked to known maturity-onset diabetes of the young (MODY) genes

OBJECTIVE

To investigate whether there are forms of early-onset autosomal-dominant type 2 diabetes that are distinct from typical maturity-onset diabetes of the young (MODY) and to characterize their phenotypic characteristics.

RESEARCH DESIGN AND METHODS

The study included 220 affected subjects from 29 families in which early-onset type 2 diabetes occurred in multiple generations and was not linked to known MODY genes (MODY gene-negative families). All individuals underwent an oral glucose tolerance test and other clinical measurements aimed at investigating the underlying metabolic defect and the presence of diabetic complications. For comparison, 79 affected carriers of MODY3 (hepatocyte nuclear factor [HNF]-1 alpha) mutations were similarly examined.

RESULTS

Subjects from MODY gene-negative pedigrees were diagnosed with diabetes at an older age (36 +/- 17 vs. 21 +/- 10 years, P = 0.0001) and were more frequently obese (52 vs. 18%, P = 0.0001) than MODY3 individuals. MODY gene-negative patients who were insulin treated required more exogenous insulin than did MODY3 subjects (0.7 +/- 0.4 vs. 0.45 +/- 0.2 U.kg-1.day-1, P = 0.04), despite similar C-peptide levels. Among subjects not treated with insulin, MODY gene-negative subjects had significantly higher serum insulin levels, both fasting (16.5 +/- 15 vs. 6.5 +/- 5 microU/ml, P = 0.027) and 2 h after a glucose load (53 +/- 44 vs. 11 +/- 10, P = 0.002). They also had higher serum triglycerides (P = 0.02), higher cholesterol levels (P = 0.02), more hypertension (P = 0.0001), and more nephropathy (P = 0.001). Differences persisted when families were matched for age at diagnosis.

CONCLUSIONS

Our findings indicate the existence of forms of early-onset autosomal-dominant type 2 diabetes that are distinct from MODY and are frequently characterized by insulin resistance, similar to later-onset type 2 diabetes. Because of the Mendelian pattern of inheritance, the goal of identifying the genes involved in these forms of diabetes appears to be particularly feasible.

The role of aldose reductase gene in the susceptibility to diabetic nephropathy in Type II (non-insulin-dependent) diabetes mellitus

The dinucleotide repeat polymorphism (5'-ALR2) in the promoter region of the aldose reductase gene on chromosome 7q35 has been implicated in the development of diabetic nephropathy in Type I (insulin-dependent) diabetes mellitus, and markers flanking the aldose reductase locus have given evidence suggestive of a linkage between diabetic nephropathy and Type II (non-insulin-dependent) diabetes mellitus in Pima Indians. To examine whether the 5'-ALR2 polymorphism in the aldose reductase gene is involved in the development of diabetic nephropathy in Caucasians with Type II diabetes, we carried out a large association study. Patients with Type II diabetes from one outpatient clinic were screened for diabetic nephropathy and divided into three groups according to the degree of this disease: 179 patients with normoalbuminuria, 225 patients with microalbuminuria and 70 patients with proteinuria. Patients with normoalbuminuria were included in the study only if they had had Type II diabetes for 10 or more years. DNA from all patients was genotyped for the 5'-ALR2 polymorphism using a previously established polymerase chain reaction protocol. The frequency of the putative risk allele Z-2 was 34.6%, 34.2% and 33.6% in the normoalbuminuria, microalbuminuria and proteinuria groups, respectively. Similarly no difference among groups was found for the frequency of the putative protective allele Z + 2. In conclusion, the results of our association study in Caucasian patients with Type II diabetes do not support the hypothesis that the 5'-ALR2 polymorphism in the aldose reductase gene contributes to susceptibility to diabetic nephropathy.

The C825T polymorphism in the human G-protein beta3 subunit gene is not associated with diabetic nephropathy in Type I diabetes mellitus

In Type I (insulin-dependent) diabetes mellitus a genetic predisposition exists to nephropathy and is related to parental hypertension. Enhanced G-protein activation, a cellular phenotype observed in cultured cells from patients with essential hypertension, was recently documented in Type I diabetic subjects with nephropathy. This enhanced G-protein activation has been associated with a genetic variant in the G-protein beta3 subunit, GNB3. A C-->T polymorphism at position 825 in exon 10 is associated with G-protein activation, the T allele associated with enhanced activity. Furthermore the T allele was observed more frequently in a group with essential hypertension. In this report we have analysed the role of the C825T polymorphism in the predisposition to diabetic nephropathy in Type I diabetes. We have investigated the frequency of this polymorphism in a large case-control study and found no association of the T allele with diabetic nephropathy. Specifically carriage of the T allele as CT or TT was observed in 49% of 200 Type I diabetic control subjects with normoalbuminuria (diabetes duration 24 years) compared with 53% of 216 Type I diabetic subjects with nephropathy (overt proteinuria or end-stage renal failure). Within this group we have also examined the inheritance of C825T alleles in a family study and found no evidence for excess transmission of the T allele to Type I diabetic offspring with nephropathy (T allele transmitted to 51% of nephropathy offspring, C allele transmitted to 49% of nephropathy offspring, p = 0.79). In none of the Type I diabetic datasets examined was there any effect of genotype on variation in systolic or diastolic blood pressure. In conclusion we can find no evidence for the C825T polymorphism of the beta3 G-protein subunit as a major gene in the susceptibility to diabetic nephropathy in Type I diabetes.

DNA polymorphisms in the ACE gene, serum ACE activity and the risk of nephropathy in insulin-dependent diabetes mellitus

BACKGROUND

To determine the relationship between DNA polymorphisms in the angiotensin I converting enzyme (ACE) gene, serum ACE activity and the risk of diabetic nephropathy.

METHODS

A case-control study was carried out in a population of Jewish insulin-dependent diabetes mellitus (IDDM) patients. Cases (77 IDDM patients with diabetic nephropathy) and controls (89 IDDM patients with normoalbuminuria) were genotyped with PCR protocols for detecting two DNA polymorphisms in the ACE gene: one in intron 7 detected with the restriction enzyme PstI and the other in intron 16 identified as an insertion/deletion (I/D).

RESULTS

The risk of nephropathy was increased only in patients homozygous for the allele with the PstI site. These homozygotes had a nephropathy risk that was 2.3 times (95% C.I.: 1.2-4.5) that of the other genotypes. Furthermore, these individuals did not have elevated serum ACE activity.

CONCLUSIONS

The results of this study are evidence that the risk of diabetic nephropathy in IDDM is influenced by genetic variability at the ACE locus, but the responsible variant is not the I/D polymorphism in intron 16. Our findings require further studies in other populations.

Major susceptibility locus for nephropathy in type 1 diabetes on chromosome 3q: results of novel discordant sib-pair analysis

Diabetic nephropathy (DN) clusters in families with type 1 diabetes and the degree of clustering suggests that a major gene having a common disease allele may be responsible. To investigate the chromosomal regions containing genes for the renin-angiotensin system, we performed a linkage study using pairs of siblings with type 1 diabetes who were discordant for DN. Theoretical considerations supported by simulation studies indicated that such discordant pairs, rather than the usual concordant pairs, would be more effective in detecting a major susceptibility gene for DN. We applied this novel strategy to test for linkage between DN and chromosomal regions containing genes for the ACE, angiotensinogen (AGT), and angiotensin II type 1 receptor (AT1). Two polymorphic markers were genotyped in the vicinity of each of the three loci in 66 discordant sib pairs and were analyzed with multipoint methods. The regions containing ACE and AGT loci were not linked with DN, while the region containing the AT1 locus showed linkage with DN. As a result of these positive findings, eight additional polymorphic markers spanning a 63-cM region around AT1 locus were genotyped. Linkage was demonstrated between DN and a 20-cM region that includes AT1 (P = 7.7 x 10(-5)), an obvious candidate gene for DN. To investigate whether AT1 could account for the observed linkage, we sequenced all exons, splicing junctions, and the promoter region and examined the identified polymorphisms/mutations for association with DN using the transmission disequilibrium test. Four new polymorphisms in the gene were found, but neither these nor previously described polymorphisms were associated with DN. Thus, while our study does not implicate AT1 itself in the etiology of DN, it provides very strong evidence that a 20-cM region around AT1 contains a major locus for susceptibility to DN.

Gender-specific association of M235T polymorphism in angiotensinogen gene and diabetic nephropathy in NIDDM

This study examined the association between the development of nephropathy in non-insulin-dependent diabetes mellitus (NIDDM) patients and M235T polymorphism in the angiotensinogen gene. White NIDDM patients with diabetic nephropathy (case subjects, n = 117) and patients without any evidence of nephropathy and > or = 10 years of NIDDM (control subjects, n = 125) were selected from among patients of the Joslin Diabetes Center and examined. In addition to a standardized examination, blood was drawn for DNA and determination of M235T genotypes at the angiotensinogen locus. For the angiotensinogen gene, the frequency of the genotype 235T/235T, known to be associated with essential hypertension, was higher among case subjects with nephropathy than in control subjects without this complication. This difference, expressed as the odds ratio for nephropathy among 235T/235T homozygotes in comparison with all other genotypes, was 2.2 (95% confidence interval, 1.1 to 4.4). The difference, however, was confined to men (odds ratio, 4.8; 95% confidence interval, 1.5 to 14.9), with the distribution of genotypes in case and control subjects being equal among women (odds ratio, 1.1). DNA polymorphism M235T in the angiotensinogen gene, which is associated with higher expression of this gene, contributes to the risk of diabetic nephropathy in NIDDM men but not in women.

Hypertension and nephropathy in diabetes mellitus: what is inherited and what is acquired?

Prolonged duration of diabetes mellitus, poor long term glycemic control and raised blood pressure have all been clearly related to the development of diabetic nephropathy. Evidence exists to suggest that a subset of individuals with diabetes have a genetic predisposition to diabetic nephropathy. Cases of diabetic nephropathy cluster in families and a parental history of hypertension is more common in patients with diabetic nephropathy. Current evidence suggests an important role for hypertension in the genetic susceptibility to diabetic nephropathy but the extent of this is unknown. While cellular and animal studies have generated a plethora of data regarding mechanisms involved in the role of hypertension and diabetic nephropathy, these are not helpful for drawing conclusions in humans. In the following review, we examine the available clinical, epidemiologic and family studies to assess the relationship between the development of hypertension and diabetic nephropathy in IDDM and NIDDM. We will demonstrate the differences in the epidemiology of hypertension in diabetes depending on the type of diabetes and thus, move the emphasis of nephropathy susceptibility away from hypertension per se. We hope to emphasize instead the homogeneity of nephropathy risk in both IDDM and NIDDM and also the idea that a common genetic susceptibility exists for all types of diabetes and is conditional on cumulative exposure to hyperglycemia. Regarding the interaction of hypertension and nephropathy in diabetes mellitus, any conclusions at this time about what is inherited and what is acquired must be regarded as speculative. However we will discuss some potential mechanisms of hypertension in the evolution of nephropathy and we will allude to the role for novel genetic studies in the search for nephropathy susceptibility gene(s).

Shortened telomere length in white blood cells of patients with IDDM

IDDM is a polygenic and autoimmune disorder in which subsets of white blood cells (WBCs) are engaged in the destruction of beta-cells of the pancreas. The mechanisms that account for the abnormal behavior of these cells in IDDM are not fully understood. By measuring the mean length of telomeres of WBCs from patients with IDDM, we tested the concept that telomeres might play a role in IDDM. We examined the lengths of the terminal restriction fragments (TRFs) of DNA of WBCs from 234 white men comprising 54 patients with IDDM, 74 patients with NIDDM, and 106 control subjects. When adjusted for age, the TRF length from WBCs of patients with IDDM was significantly shorter than that of nondiabetic control subjects (mean +/- SE: 8.6 +/- 0.1 vs. 9.2 +/- 0.1, P = 0.002). No significant difference was observed between the TRF length from WBCs of patients with NIDDM versus nondiabetic subjects. Neither the duration nor the complications of IDDM (i.e., nephropathy and hypertension) had an effect on the TRF length of WBCs from patients with IDDM. The shortened TRF length of WBCs of patients with IDDM likely reflects a marked reduction in the TRF length of subsets of WBCs that play a role in the pathogenesis of IDDM.

Diabetic nephropathy is associated with AGT polymorphism T235: results of a family-based study

Diabetic nephropathy is a serious and frequent complication of insulin-dependent diabetes mellitus (IDDM) that has a strong genetic component. Several case-control studies have reported conflicting results with regard to the role of angiotensinogen gene polymorphisms, specifically the M235T T allele, in the development of diabetic nephropathy. The primary limitation of the case-control approach is that bias may be introduced by unrecognized differences in the populations selected for cases and control subjects. In contrast, family-based approaches, such as the transmission/disequilibrium test, assess whether a particular variant, or allele, is transmitted preferentially from a parent having a single copy of that allele. Thus each family provides its own control, thereby eliminating spurious results caused by mismatched population samples. To take advantage of this study design for further investigation of M235T, we collected from the Joslin Diabetes Center in Boston 148 IDDM patients with diabetic nephropathy, 62 nephropathy-free patients with long-duration IDDM, and, very importantly, parents of all these individuals. We found that among males (but not females) the T allele of the M235T polymorphism was transmitted preferentially to those with nephropathy compared with IDDM patients without nephropathy (P=.05). Moreover, the T allele was transmitted preferentially to patients with the most severe manifestation of nephropathy, end-stage renal disease (P=.04). In conclusion, results obtained in our family-based study support a role of the angiotensinogen gene M235T polymorphism, and specifically the T allele, in the development of diabetic nephropathy in IDDM.