Hepatic lentiviral gene transfer prevents the long-term onset of hepatic tumours of glycogen storage disease type 1a in mice.

Glycogen storage disease type 1a (GSD1a) is a rare disease due to the deficiency in the glucose-6-phosphatase (G6Pase) catalytic subunit (encoded by G6pc), which is essential for endogenous glucose production. Despite strict diet control to maintain blood glucose, patients with GSD1a develop hepatomegaly, steatosis and then hepatocellular adenomas (HCA), which can undergo malignant transformation. Recently, gene therapy has attracted attention as a potential treatment for GSD1a. In order to maintain long-term transgene expression, we developed an HIV-based vector, which allowed us to specifically express the human G6PC cDNA in the liver. We analysed the efficiency of this lentiviral vector in the prevention of the development of the hepatic disease in an original GSD1a mouse model, which exhibits G6Pase deficiency exclusively in the liver (L-G6pc(-/-) mice). Recombinant lentivirus were injected in B6.G6pc(ex3lox/ex3lox). SA(creERT2/w) neonates and G6pc deletion was induced by tamoxifen treatment at weaning. Magnetic resonance imaging was then performed to follow up the development of hepatic tumours. Lentiviral gene therapy restored glucose-6 phosphatase activity sufficient to correct fasting hypoglycaemia during 9 months. Moreover, lentivirus-treated L-G6pc(-/-) mice presented normal hepatic triglyceride levels, whereas untreated mice developed steatosis. Glycogen stores were also decreased although liver weight remained high. Interestingly, lentivirus-treated L-G6pc(-/-) mice were protected against the development of hepatic tumours after 9 months of gene therapy while most of untreated L-G6pc(-/-) mice developed millimetric HCA. Thus the treatment of newborns by recombinant lentivirus appears as an attractive approach to protect the liver from the development of steatosis and hepatic tumours associated to GSD1a pathology.

Lessons from new mouse models of glycogen storage disease type 1a in relation to the time course and organ specificity of the disease.

Patients with glycogen storage diseases type 1 (GSD1) suffer from life-threatening hypoglycaemia, when left untreated. Despite an intensive dietary treatment, patients develop severe complications, such as liver tumors and renal failure, with aging. Until now, the animal models available for studying the GSD1 did not survive after weaning. To gain further insights into the molecular mechanisms of the disease and to evaluate potential treatment strategies, we have recently developed novel mouse models in which the catalytic subunit of glucose-6 phosphatase (G6pc) is deleted in each glucose-producing organ specifically. For that, B6.G6pc(ex3lox/ex3lox) mice were crossed with transgenic mice expressing a recombinase under the control of the serum albumin, the kidney androgen protein or the villin promoter, in order to obtain liver, kidney or intestine G6pc(-/-) mice, respectively. As opposed to total G6pc knockout mice, tissue-specific G6pc deficiency allows mice to maintain their blood glucose by inducing glucose production in the other gluconeogenic organs. Even though it is considered that glucose is produced mainly by the liver, liver G6pc(-/-) mice are perfectly viable and exhibit the same hepatic pathological features as GSD1 patients, including the late development of hepatocellular adenomas and carcinomas. Interestingly, renal G6pc(-/-) mice developed renal symptoms similar to the early human GSD1 nephropathy. This includes glycogen overload that leads to nephromegaly and morphological and functional alterations in the kidneys. Thus, our data suggest that renal G6Pase deficiency per se is sufficient to induce the renal pathology of GSD1. Therefore, these new mouse models should allow us to improve the strategies of treatment on both nutritional and pharmacological points of view.

Targeted deletion of kidney glucose-6 phosphatase leads to nephropathy.

Renal failure is a major complication that arises with aging in glycogen storage disease type 1a and type 1b patients. In the kidneys, glucose-6 phosphatase catalytic subunit (encoded by G6pc) deficiency leads to the accumulation of glycogen, an effect resulting in marked nephromegaly and progressive glomerular hyperperfusion and hyperfiltration preceding the development of microalbuminuria and proteinuria. To better understand the end-stage nephropathy in glycogen storage disease type 1a, we generated a novel kidney-specific G6pc knockout (K-G6pc(-/-)) mouse, which exhibited normal life expectancy. After 6 months, K-G6pc(-/-) mice showed glycogen overload leading to nephromegaly and tubular dilation. Moreover, renal accumulation of lipids due to activation of de novo lipogenesis was observed. This led to the activation of the renin-angiotensin system and the development of epithelial-mesenchymal transition process and podocyte injury by transforming growth factor ß1 signaling. The K-G6pc(-/-) mice developed microalbuminuria caused by the impairment of the glomerular filtration barrier. Thus, renal G6pc deficiency alone is sufficient to induce the development of the early-onset nephropathy observed in glycogen storage disease type 1a, independent of the liver disease. The K-G6pc(-/-) mouse model is a unique tool to decipher the molecular mechanisms underlying renal failure and to evaluate potential therapeutic strategies.