Tyrosinemia: the Quebec experience.

Tyrosinemia, a genetic disorder of the liver and kidneys, is caused by reduced activity of fumarylacetoacetate hydrolase (FAH), the final enzyme in the degradation of tyrosine. The consequent presence of succinylacetone in urine or blood is pathognomonic of tyrosinemia and is used as a confirmatory test in the Quebec neonaral screening program. Due to a complex founder effect, the province of Quebec has an unusually high prevalence of tyrosinemia, particularly in the Saguenay-Lac Saint-Jean region (where the prevalence is 1 in 1850). Tyrosinemia has several different clinical presentations, ranging from acute liver failure with severe coagulopathy early in life, to slowly progressing cirrhosis with multiple nodules and variable renal dysfunction, to normal liver function with renal failure. Hepatocarcinoma has been found in approximately one third of cases. FAH complementary DNA has been cloned and mapped to chromosome 15q23-q25. The mutation observed in Quebec is a splice mutation at intron 12. This mutation is common and has been observed in other areas of the world as well, although more than 20 mutations causing tyrosinemia have now been described. Liver transplantation remains the definitive treatment. The author's team has carried out 28 liver transplantations (including 2 combined liver-kidney transplantations) in 25 children. The overall survival rate has been 92%; two children died as a result of primary nonfunction. The primary indications for transplantation were hepatic nodules (in 14 cases), neurological crises (6) and hepatic (3) or renal failure (2). An abnormal glomerular filtration rate (GFR) of less than 80 mL/min per 1.73 m2 was documented before transplantation in 54% of the cases. The rate normalized after liver transplantation in most patients, with rapid improvement in tubular function. However, patients with a severely low rate (less than 55 mL/min per 1.73 m2) before transplantation still had borderline renal function and poor growth after the transplantion, despite normal liver function. Therefore, for children with a consistently low GFR, careful consideration should be given to performing a combined liver-kidney transplantation, and a renal biopsy should form part of the pretransplantation evaluation.

A conformational model for the human liver microsomal glucose-6-phosphatase system: evidence from rapid kinetics and defects in glycogen storage disease type 1.

Rapid kinetics of glucose-6-phosphate (G6P) uptake and hydrolysis as well as of orthophosphate uptake were investigated in microsomes prepared from normal and glycogen storage disease type 1a (GSD 1a) human livers using a fast sampling, rapid filtration apparatus and were compared to those of rat liver microsomes. As shown before with rat microsomes, the production of [U-14C]glucose from 0.2 mmol/L [U-14C]G6P by untreated normal human microsomes was characterized by a burst in activity during the first seconds of incubation, followed by a slower linear rate. The initial velocity of the burst was equal to the rate of glucose production in detergent-treated microsomes. In untreated and detergent-treated GSD 1a microsomes, no glucose-6-phosphatase activity was observed. When untreated normal human or rat microsomes were incubated in the presence of 0.2 mmol/L [U-14C]G6P, an accumulation of [U-14C]glucose was observed, whereas no radioactive compound (G6P and/or glucose) was taken up by GSD 1a microsomes. Orthophosphate uptake was, however, detectable in both GSD 1a and normal untreated vesicles. These results do not support a rate-limiting transport of G6P in untreated normal human microsomes and further show that in this case of GSD 1a, no distinct G6P transport activity is present.