Discussion
Maternally inherited diabetes and deafness was first described in 1992 [
4]. It is caused in the great majority of cases by an adenine to guanine substitution at position 3243 of the mitochondrial DNA (m.3243A>G) encoding the gene for leucine transfer RNA. MIDD is frequently misdiagnosed as type 1 or type 2 diabetes, depending on the age of the patient and mode of presentation. About 1% of cases of diabetes are thought to be caused by mutations in mitochondrial DNA, and the m.3243A>G mutation accounts for about 85% of these [
5].
The key clinical features are diabetes and deafness and their presence in maternal relatives. As the case of our patient demonstrates, these are not always present. The diabetes usually presents insidiously, but in 20% of cases it presents acutely, with ketoacidosis present in 8% of the latter cases [
6]. About 75% of diabetic patients with the m.3243A>G mutation have deafness, which is sensorineural and due to cochlear disease [
7].
Renal disease is common in patients with MIDD, especially females. In one series of 74 patients with MIDD, proteinuria was greater in these patients than in control diabetic patients, and chronic kidney disease (CKD) stage 3 or greater was four-to sixfold higher despite lower HbA
1C, lower blood pressure and a 3.7-fold lower incidence of diabetic retinopathy than controls [
8]. The commonest histological pattern in patients undergoing renal biopsy is FSGS, but tubulointerstitial disease and renal cysts have also been described. The combination of nephropathy and deafness may lead to a mistaken diagnosis of Alport syndrome [
9].
Several other features present in this patient are also explained by the m.3243A>G mutation, including short stature, which is common and caused by a deficiency of growth hormone–releasing hormone [
10,
11]. Gastro-intestinal complaints are also common, especially constipation and pseudo-obstruction [
5]. The inability to play competitive football is likely to reflect impaired energy utilization and skeletal myopathy. Cardiomyopathy, retinal macular dystrophy and stroke, features not present in our patient, are also described.
Our patient had early onset of several features. This is likely to reflect high levels of heteroplasmy for the mutant mitochondrial DNA. The severity and early onset of renal disease may also be due to the presence of a heterozygous mutation in NPHS1. This gene codes for nephrin, an important component of the glomerular slit diaphragm, and homozygous mutations cause Finnish type congenital nephrotic syndrome.
A diagnosis of MIDD has several important implications for treatment. Metformin should not be used for treating the diabetes because of the increased risk of lactic acidosis. Antibiotics such as tetracyclines and chloramphenicol which interfere with mitochondrial function should also be avoided. Coenzyme Q10 has been shown to be beneficial in preventing hearing loss and delaying progression of diabetes [
12]. In patients who progress to end stage renal failure, great caution should be exercised in using maternal relatives as kidney donors.
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