Combined liver-kidney transplantation in primary hyperoxaluria type 1
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Primary hyperoxaluria type 1 (PH1) is a rare autosomal recessive disorder characterised by an increased urinary excretion of calcium oxalate, leading to recurrent urolithiasis, nephrocalcinosis and accumulation of insoluble oxalate throughout the body (oxalosis) when the glomerular filtration rate falls to below 40–20 mL/min per 1.73 m2. The disease is due to a functional defect of the liver-specific peroxisomal enzyme alanine: glyoxylate aminotransferase (AGT), the gene of which is located on chromosome 2q37.3. The diagnosis is based on increased urinary oxalate and glycollate, increased plasma oxalate and AGT measurement in a liver biopsy. AGT mistargeting may be investigated by immuno-electron microscopy and DNA analysis. End-stage renal failure is reached by the age of 15 years in 50% of PH1 patients and the overall death rate approximates 30%. The conservative treatment includes high fluid intake, pyridoxine and crystallisation inhibitors. Since the kidney is the main target of the disease, isolated kidney transplantation (Tx) has been proposed in association with vigorous peri-operative haemodialysis in an attempt to clear plasma oxalate at the time of Tx. However, because of a 100% recurrence rate, the average 3-year graft survival is 15%–25% in Europe, with a 5–10-year patient survival rate ranging from 10% to 50%. Since the liver is the only organ responsible for the detoxification of glyoxylate by AGT, deficient host liver removal is the first rationale for enzyme replacement therapy. Subsequent orthotopic liver Tx aims to supply the missing enzyme in its normal cellular and subcellular location and thus can be regarded as a form of gene therapy. Because of the usual spectrum of the disease, isolated liver Tx is limited to selected patients prior to having reached an advanced stage of chronic renal failure. Combined liver-kidney Tx has therefore become a conventional treatment for most PH1 patients: according to the European experience, patient survival approximates 80% at 5 years and 70% at 10 years. In addition, the renal function of survivors remains stable over time, between 40 and 60 mL/min per 1.73 m2 after 5 to 10 years. In addition, liver Tx may allow the reversal of systemic storage disease (i.e. bone, heart, vessels, nerves) and provide valuable quality of life. Whatever the transplant strategy, the outcome is improved when patients are transplanted early in order to limit systemic oxalosis. According to the European experience, it appears that combined liver-kidney Tx is performed in PH1 patients with encouraging results, renal Tx alone has little role in the treatment of this disease, and liver Tx reverses the underlying metabolic defect and its clinical consequences.