Increased urinary albumin and retinol-binding protein in type I diabetes. A study of identical twins

Indexes of early renal glomerular and tubular dysfunction have been demonstrated in type I diabetes, but it remains uncertain whether such changes are genetically determined or are secondary to the disease process. We therefore undertook to study whether early markers of renal dysfunction are a consequence of type I diabetes or inherited.

We estimated both urinary albumin excretion (UAE) and urinary retinol-binding protein (RBP) in 51 identical twin pairs discordant for type I diabetes and in 51 matched control subjects.

UAE and RBP were significantly higher in the diabetic twins than in their nondiabetic co-twins (P < 0.0001 and P < 0.0002, respectively). Seven diabetic twins had elevated UAE, but none of the nondiabetic co-twins did. In a subgroup of 44 twins with normal UAE (albumin excretion rate < 20 micrograms/min), diabetic twins had both a higher albumin excretion function (median [range]; 0.64 [0.18-2.74] mg/mmol creatinine) than their nondiabetic co-twins (0.48 [0.24-1.40], P < 0.01) and higher levels of RBP excretion (10.4 [4.0-167.0] micrograms/mmol creatinine) than their nondiabetic co-twins (7.5 [0.97-23.0], P < 0.05). Values between twins of a pair were significantly correlated for RBP (r = 0.36, P < 0.05) but not for UAE (r = 0.13).

These results suggest that in type I diabetes, an index of renal tubular function (RBP), but not glomerular function (UAE), is influenced by shared genetic and nongenetic factors. Type I diabetes can affect renal tubular function even when glomerular function is normal. We conclude that neither the increased UAE nor urinary RBP found in type I diabetes is inherited independently of the diabetes process.