Insulin resistance (IR) is likely to precede and to play a role in diabetic nephropathy (DN) in both type 1 (T1D) [1] and type 2 diabetes [2]. IR and DN may also share common genetic determinants [36]. The gene encoding PTP1B, a tyrosine phosphatase which inhibits insulin signalling, is an excellent candidate for IR and related disorders. We recently have identified a 1484insG variation in the 3'-untranslated region (UTR) of the gene which stabilizes PTP1B mRNA and associates with IR in the general population [7]. Our aim was to verify whether the PTP1B 1484insG variant [as evaluated by polymerase chain reaction (PCR) and SacII enzyme digestion] plays a role in the development of DN in patients with T1D. In a casecontrol study, 288 European patients with T1D were enrolled. There were 170 patients with microalbuminuria [albumin excretion rate (AER) = 30300 mg/24 h or albumin/creatinine ratio (ACR) = 2.5 males/3.5 females, 30 mg/mmol on two or more consecutive occasions] or persistent proteinuria (AER >300 mg/24 h or ACR >30 mg/mmol or a urine sample dipstick positive for protein on two or more consecutive occasions). There were 118 controls (i.e. no DN after 15 years since diabetes onset). Of the whole cohort, 186 subjects (65%) were recruited in Italy and the remainder in the UK. Cases and controls were of similar age (42.7±11 vs 44.2±13 years), gender distribution (103 males/67 females vs 64 males/54 females) and duration of diabetes (25.9±10 vs 25.6±9 years) but different HbA1c (8.6±1.7 vs 7.9±1.6%, P<0.01) and plasma creatinine (118±79 vs 79±13 µmol, P<0.01) levels. The PTP1B 1484insG genotype and allele frequencies were in HardyWeinberg equilibrium. The frequency of the less common 1484insG allele was similar in cases and controls (0.06 and 0.07;
2 = 0.38, P = 0.5). Similar data in terms of genotype and allele frequencies in both cases and controls were also observed when Italian and British cohorts were analysed separately (data not shown).
Data obtained in this first cohort were replicated in a second, more ethnically homogeneous, cohort recruited from Tuscany, Italy: 104 cases and 109 controls of similar age (37±10 vs 37±11 years), gender distribution (44 males/65 females vs 45 males/59 females), duration of diabetes (23.2 ± 7 vs 21.9±8 years) and frequency of the 1484insG variant (0.04 and 0.04; 2 = 1.52, P = 0.2). When the two cohorts were pooled and analysed together, carriers of the 1484insG variant did not show an increased risk of having DN; rather the risk tended to be reduced (age- and gender-adjusted odds ratio = 0.69, 95% confidence interval = 0.341.4, P = 0.3). In addition, the 1484insG variant was distributed similarly between patients with micro- (n = 124) or macroalbuminuria (n = 150) (data not shown). In a subset of 80 patients of the first cohort reported here, we have retrospective information on the rate of decline of the glomerular filration rate (GFR; as derived by the CockcroftGault formula), whose median value was 4.4 ml/min/year (range 3.8 to 16.6). Also in this case, no statistical difference in GFR decline was observed between patients carrying (n = 8) or not carrying (n = 72) the 1484insG variant (3.2 ml/min/year, range:1.411.7 vs 4.7 ml/min/year, range 3.8 to 16.6; P = 0.9). The low number of patients carrying the 1484insG variant does not allow us to draw a firm conclusion about a possible role for this variant in the rate of diabetic nephropathy progression, and indicates the need for a larger study to obtain insight into this specific issue.
Our study clearly indicates, therefore, the lack of association of the 1484insG variation in the PTP1B gene with the risk of developing DN in patients with T1D. Although a negative casecontrol study may be the consequence of insufficient statistical power, we believe this possibility is unlikely in our case. In fact, the number of patients we have studied is at least comparable with that of other major studies in this field (reviewed in [8]). Given a genotype frequency of the PTP1B 1484insG of 15% in the general population [7] and assuming the same frequency in control patients, the two cohorts pooled and analysed together have a power of 80% (P = 0.05) to detect twice as high a risk of developing DN in patients carrying the gene variant. A similar risk has been reported for other genetic determinants of DN (as indicated in [5,6,8]). In addition, data replication in two different cohorts minimizes the risk of a false-negative result due to population stratification bias. Overall, we believe, therefore, that the possibility of a false-negative result in our study is very unlikely. Our finding adds complexity to the potential relationship between IR genes and DN, indicating that not all functional genetic variations affecting insulin sensitivity play a role in DN development. An additional functional single nucleotide polymorphism (SNP) in the PTP1B gene (a missense SNP, P387L) has been identified recently and associated with type 2 diabetes among Caucasians [9]. However, no association was observed with IR, making the relevance of this observation to DN uncertain. In addition, due its extremely low frequency (i.e. 1% in the general population), the possibility that it may exert a relevant and detectable population-attributable risk of DN is trivial.
In conclusion, the 1484insG variant of the PTP1B gene, which increases the susceptibility to IR in the general population, is unlikely to play a role in modulating the risk of developing DN in patients with T1D. Whether this variant plays a role in other aspects or stages of DN pathophysiology as reported for other IR genes, including time to development since diabetes onset [10] or rate of disease progression [3,4] once the complication is established, is currently unknown and cannot be answered by our present study design. Further prospective and larger studies may clarify this issue.
Conflict of interest statement. None declared.
1 Unit of Endocrinology IRCSS CSS San Giovanni Rotondo2 Department of Metabolic Medicine University of Pisa, Pisa3 Unit of Diabetes Brotzu Hospital Cagliari4 Chair of Metabolic Disease University of Padova5 Institute of Internal Medicine University of Turin6 Unit for Metabolic Medicine KCL Guy's Hospital London. UK7 Department of Clinical Science University of Rome La Sapienza, Italy Email: sdecosm{at}tin.it
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