Studies of kidney and muscle biopsy specimens from identical twins discordant for type I diabetes mellitus.

To distinguish metabolic from genetic factors in the development of microangiopathy in diabetes, we evaluated biopsy specimens of kidney and quadriceps muscle from seven pairs of identical twins who were discordant for Type I (insulin-dependent) diabetes mellitus. Two of the diabetic patients had clinical diabetic nephropathy, including hypertension, marked albuminuria, and a substantially reduced creatinine clearance; the other five had normal renal function and only minor clinical indications of complications. All the twins of the diabetic patients had normal glomerular basement membrane widths and normal fractional volumes of the glomerular mesangium. Values for glomerular basement membrane width, tubular basement membrane width, and mesangial volume in each diabetic twin exceeded the values in the respective sibling (P less than or equal to 0.0035), even if the value in the diabetic twin lay within established normal ranges. Values for muscle capillary basement membrane width in the diabetic twins did not differ from those in their siblings (P = 0.5). Our observations suggest that the metabolic abnormalities of diabetes are necessary, if not sufficient, for the development of glomerular abnormalities. We also conclude that in diabetic patients, alterations in muscle capillary basement membrane width do not necessarily accompany pathologic lesions in the kidney.