Mining the genome for susceptibility to diabetic nephropathy: the role of large-scale studies and consortia

Carnosinases, their substrates and diseases

Carnosinases are Xaa-His dipeptidases that play diverse functions throughout all kingdoms of life. Human isoforms of carnosinase (CN1 and CN2) under appropriate conditions catalyze the hydrolysis of the dipeptides carnosine (β-alanyl-l-histidine) and homocarnosine (γ-aminobutyryl-l-histidine). Alterations of serum carnosinase (CN1) activity has been associated with several pathological conditions, such as neurological disorders, chronic diseases and cancer. For this reason the use of carnosinase levels as a biomarker in cerebrospinal fluid (CSF) has been questioned. The hydrolysis of imidazole-related dipeptides in prokaryotes and eukaryotes is also catalyzed by aminoacyl-histidine dipeptidases like PepD (EC 3.4.13.3), PepV (EC 3.4.13.19) and anserinase (EC 3.4.13.5). The review deals with the structure and function of this class of enzymes in physiological and pathological conditions. The main substrates of these enzymes, i.e., carnosine, homocarnosine and anserine (β-alanyl-3-methyl-l-histidine) will also be described.

Diabetic nephropathy in a sibling and albuminuria predict early GFR decline: a prospective cohort study

BACKGROUND

Diabetic nephropathy is a growing clinical problem, and the cause for >40% of incident ESRD cases. Unfortunately, few modifiable risk factors are known. The objective is to examine if albuminuria and history of diabetic nephropathy (DN) in a sibling are associated with early DN progression or mortality.

METHODS

In this longitudinal study of adults >18 yrs with diabetes monitored for up to 9 yrs (mean 4.6 ± 1.7 yrs), 435 subjects at high risk (DN family history) and 400 at low risk (diabetes >10 yrs, normoalbuminuria, no DN family history) for DN progression were evaluated for rate of eGFR change using the linear mixed effects model and progression to ESRD. All-cause mortality was evaluated by Kaplan-Meier analyses while controlling for baseline covariates in a Cox proportional hazards model. Covariates included baseline eGFR, age, gender, race, diabetes duration, blood pressure, hemoglobin A1c and urine albumin:creatinine ratio. Propensity score matching was used to identify high and low risk group pairs with balanced covariates. Sensitivity analyses were employed to test for residual confounding.

RESULTS

Mean baseline eGFR was 74 ml/min/1.73 m2 (86% of cohort >60 ml/min/1.73 m2). Thirty high risk and no low risk subjects developed ESRD. eGFR decline was significantly greater in high compared to low risk subjects. After controlling for confounders, change in eGFR remained significantly different between groups, suggesting that DN family history independently regulates GFR progression. Mortality was also significantly greater in high versus low risk subjects, but after controlling for baseline covariates, no significant difference was observed between groups, indicating that factors other than DN family history more strongly affect mortality. Analyses of the matched pairs confirmed change in eGFR and mortality findings. Sensitivity analyses demonstrated that the eGFR results were not due to residual confounding by unmeasured covariates of a moderate effect size in the propensity matching.

CONCLUSIONS

Diabetic subjects with albuminuria and family history of DN are vulnerable for early GFR decline, whereas subjects with diabetes for longer than 10 years, normoalbuminuria and negative family history, experience slower eGFR decline, and are extremely unlikely to require dialysis. Although we would not recommend that patients with low risk characteristics be neglected, scarce resources would be more sensibly devoted to vulnerable patients, such as the high risk cases in our study, and preferably prior to the onset of albuminuria or GFR decline.

Purification and identification of two carnosine-cleaving enzymes, carnosine dipeptidase I and Xaa-methyl-His dipeptidase, from Japanese eel (Anguilla japonica)

Three enzymes, carnosine dipeptidase I (EC 3.4.13.20, CNDP1), carnosine dipeptidase II (EC 3.4.13.18, CNDP2), and Xaa-methyl-His dipeptidase (or anserinase: EC 3.4.13.5, ANSN), are known to be capable of catalyzing the hydrolysis of carnosine (β-alanyl-l-histidine), in vertebrates. Here we report the purification and identification of two unidentified carnosine-cleaving enzymes from Japanese eel (Anguilla japonica). Two different dipeptidases were successfully purified to homogeneity from the skeletal muscle; one exhibited a broad substrate specificity, while the other a narrow specificity. N-terminal amino-acid sequencing, deglycosylation analysis, and genetic analysis clearly revealed that the former is a homodimer of glycosylated subunits, encoded by ANSN, and the latter is another homodimer of glycosylated subunits, encoded by CNDP1; that is, Xaa-methyl-His dipeptidase, and carnosine dipeptidase I respectively. This is the first report on the identification of carnosine dipeptidase I from a non-mammal. Database search revealed presence of a CNDP1 ortholog only from salmonid fishes, including Atlantic salmon and rainbow trout, but not from other ray-finned fish species, such as zebrafish, fugu, and medaka whose genomes have been completely sequenced. The mRNAs of CNDP1 and ANSN are strongly expressed in the liver of Japanese eel, compared with other tissues, while that of CNDP2 is widely distributed in all tissues tested.

Hypertension in diabetic nephropathy: epidemiology, mechanisms, and management

Hypertension is highly prevalent in patients with diabetic nephropathy. Diabetic nephropathy is the leading cause of CKD and end-stage kidney disease in the United States. The etiology of hypertension in diabetic nephropathy involves mechanisms with multiple inter-related mediators that result in renal sodium reabsorption and peripheral vasoconstriction. The management of hypertension in these patients is focused on treatments that target these mediators. Clinical trials have established that drugs that inhibit the renin-angiotensin-aldosterone system should be used as first-line agents on the basis of their ability to slow down progression of kidney disease and lower albuminuria. There is further interest into how the combination of drugs that inhibit this pathway at multiple steps will contribute further to the management of hypertension and diabetic nephropathy. This article presents an updated review of the mechanisms involved in hypertension in patients with diabetic nephropathy. It also reviews the past clinical trials using single agents as therapeutics and the more recent trials involving novel drugs or drug combinations used to treat these patients. Retrospective analyses of multiple studies are included to better examine the significance of the currently proposed blood pressure targets for patients with diabetic nephropathy.

Distinct genetic regulation of progression of diabetes and renal disease in the Goto-Kakizaki rat

Goto-Kakizaki (GK) rats develop early-onset type 2 diabetes (T2D) symptoms, with signs of diabetic nephropathy becoming apparent with aging. To determine whether T2D and renal disease share similar genetic architecture, we ran a quantitative trait locus (QTL) analysis in the F2 progeny of a GK x Brown Norway (BN) rat cross. Further, to determine whether genetic components change over time, we ran the QTL analysis on phenotypes collected longitudinally, at 3, 6, 9 and 12 mo, from the same animals. We confirmed three chromosomal regions that are linked to early diabetes phenotypes (chromosomes 1, 5, and 10) and a single region involved in the late progression of the disorder (chromosome 4). A single region was identified for the onset of the renal phenotype proteinuria (chromosome 5). This region overlaps the diabetic QTL, although it is not certain whether similar genes are involved in both phenotypes. A second QTL linked to the progression of the renal phenotype was found on chromosome 7. Linkage for triglyceride and cholesterol levels were also identified (chromosomes 7 and 8, respectively). These results demonstrate that, in general, different genetic components control diabetic and renal phenotypes in a diabetic nephropathy model. Furthermore, these results demonstrate that, over time, different genetic components are involved in progression of disease from those that were involved in disease onset. This observation would suggest that clinical studies collecting participants over a wide age distribution may be diluting genetic effects and reducing power to detect true effects.

Association of genetic variants at 3q22 with nephropathy in patients with type 1 diabetes mellitus

Diabetic nephropathy (DN) is the primary cause of morbidity and mortality in patients with type 1 diabetes mellitus (T1DM) and affects about 30% of these patients. We have previously localized a DN locus on chromosome 3q with suggestive linkage in Finnish individuals. Linkage to this region has also been reported earlier by several other groups. To fine map this locus, we conducted a multistage case-control association study in T1DM patients, comprising 1822 cases with nephropathy and 1874 T1DM patients free of nephropathy, from Finland, Iceland, and the British Isles. At the screening stage, we genotyped 3072 tag SNPs, spanning a 28 Mb region, in 234 patients and 215 controls from Finland. SNPs that met the significance threshold of p < 0.01 at this stage were followed up by a series of sample sets. A genetic variant, rs1866813, in the noncoding region at 3q22 was associated with increased risk of DN (overall p = 7.07 x 10(-6), combined odds ratio [OR] of the allele = 1.33). The estimated genotypic ORs of this variant in all Finnish samples suggested a codominant effect, resulting in significant association, with a p value of 4.7 x 10(-5) (OR = 1.38; 95% confidence interval = 1.18-1.62). Additionally, an 11 kb segment flanked by rs62408925 and rs1866813, two strongly correlated variants (r(2) = 0.95), contains three elements highly conserved across multiple species. Independent replication will clarify the role of the associated variants at 3q22 in influencing the risk of DN.

Genome-wide scan for estimated glomerular filtration rate in multi-ethnic diabetic populations: the Family Investigation of Nephropathy and Diabetes (FIND)

OBJECTIVE

Diabetic nephropathy, the most common cause of end-stage renal disease, aggregates in families and specific ethnic groups. Deconstructing diabetic nephropathy into intermediate, quantitative phenotypes may increase feasibility of detecting susceptibility loci by genetic screens. Glomerular filtration rate (GFR), which characterizes diabetic nephropathy, was employed as a quantitative trait in a preliminary whole-genome scan.

RESEARCH DESIGN AND METHODS

Estimated GFR (eGFR) was calculated for 882 diabetic sibpairs (mean age 57 years) of African-American (25.6% of total), American Indian (8.6%), European-American (14.2%), and Mexican-American (51.6%) descent enrolled in the initial phase of the Family Investigation of Nephropathy and Diabetes (FIND). A whole-genome scan was performed using 404 microsatellite markers (average spacing 9 cM) and model-free linkage analysis.

RESULTS

For all ethnicities combined, strong evidence for linkage was observed on chromosomes 1q43 (P = 3.6 x 10(-3)), 7q36.1 (P = 2.1 x 10(-4)), 8q13.3 (P = 4.6 x 10(-4)), and 18q23.3 (P = 2.7 x 10(-3)). Mexican-American families, who comprised the major ethnic subpopulation in FIND, contributed to linkage on chromosomes 1q43, 2p13.3, 7q36.1, 8q13.3, and 18q23.3, whereas African-American and American-Indian families displayed linkage peaks on chromosomes 11p15.1 and 15q22.3, respectively.

CONCLUSIONS

We have demonstrated multiple chromosomal regions linked to eGFR in a multi-ethnic collection of families ascertained by a proband with diabetic nephropathy. Identification of genetic variants within these loci that are responsible for the linkage signals could lead to predictive tests or novel therapies for subsets of patients at risk for diabetic nephropathy.