Mitochondrial diseases and the kidney

Mitochondrial disease can affect any organ in the body including the kidney. As increasing numbers of patients with mitochondrial disease are either surviving beyond childhood or being diagnosed in adulthood, it is important for all nephrologists to have some understanding of the common renal complications that can occur in these individuals. Mitochondrial proteins are encoded by either mitochondrial or nuclear DNA (mtDNA and nDNA, respectively); therefore, disease causing mutations may be inherited maternally (mtDNA) or autosomally (nDNA), or can arise spontaneously. The commonest renal phenotype in mitochondrial disease is proximal tubulopathy (Fanconi syndrome in the severest cases); however, as all regions of the nephron can be affected, from the glomerulus to the collecting duct, patients may also present with proteinuria, decreased glomerular filtration rate, nephrotic syndrome, water and electrolyte disorders, and renal tubular acidosis. Understanding of the relationship between underlying genotype and clinical phenotype remains incomplete in mitochondrial disease. Proximal tubulopathy typically occurs in children with severe multisystem disease due to mtDNA deletion or mutations in nDNA affecting mitochondrial function. In contrast, glomerular disease (focal segmental glomerulosclerosis) has been reported more commonly in adults, mainly in association with the m.3243A<G point mutation. Co-enzyme Q10 (CoQ10) deficiency has been particularly associated with podocyte dysfunction and nephrotic syndrome in children. Underlying mitochondrial disease should be considered as a potential cause of unexplained renal dysfunction; clinical clues include lack of response to conventional therapy, abnormal mitochondrial morphology on kidney biopsy, involvement of other organs (e.g. diabetes, cardiomyopathy, and deafness) and a maternal family history, although none of these features are specific. The diagnostic approach involves acquiring tissue (typically skeletal muscle) for histological analysis, mtDNA screening and oxidative phosphorylation (OXPHOS) complex function tests. A number of nDNA mutations causing mitochondrial disease have now been identified and can also be screened for if clinically indicated. Management of mitochondrial disease requires a multidisciplinary approach, and treatment is largely supportive as there are currently very few evidence-based interventions. Electrolyte deficiencies should be corrected in patients with urinary wasting due to tubulopathy, and CoQ10 supplementation may be of benefit in individuals with CoQ10 deficiency. Nephrotic syndrome in mitochondrial disease is not typically responsive to steroid therapy. Transplantation has been performed in patients with end-stage kidney disease; however, immunosuppressive agents such as steroids and tacrolimus should be used with care given the high incidence of diabetes in mitochondrial disease.