CREATION OF A MOUSE MODEL OF TYROSINEMIA, TYPE 1

Tyrosinemia type I is an autosomal recessive metabolic disease caused by
deficiency of the enzyme fumarylacetoacetate hydrolase (FAH). Its main
clinical features include progressive liver failure, renal tubular
damage, intermittent porphyria like neurologic crisis and the development
of hepatocellular carcinomas.The mortality rate is high and death usually
occurs in childhood unless liver transplantation is performed. Although
this disease is rare in most populations, there are ethnic groups in
which the incidence is high, especially in French Canadians. Because of
the severity of the disorder and the positive clinical experience with
liver transplantation, FAH deficiency is a candidate disorder for gene
therapy attempts directed at the liver. Gene transfer into other target
tissues such as bone marrow might also be beneficial. However, a natural
animal model in which to test such strategies is not available.We propose
to create such an animal model by disrupting the gene by homologous
recombination in murine embryonic stem cells (ES cells) and subsequently
generating mice -homozygous for the FAH gene disruption event. In the
initial phase of the project the mouse FAH cDNA will be cloned and
employed to isolate and characterize the mouse gene. This information
will be used to create a replacement vector capable of disrupting the FAH
gene in ES cells by homologous recombination. Chimeric mice will be
generated from these ES cells and the mice bread to homozygosity.
Hopefully the FAH deficiency mouse will express a phenotype similar to
the human disorder and then would be used for 3 main purposes in the
future. First it would be utilized to test strategies of delivering a
functional FAH cDNA for therapeutic purposes. The main target tissue is
liver, but kidney, hemopoietic stem cells and possibly intestine are also
potential locations in which expression of the enzyme might alleviate the
biochemical defect. Second, the effects of dietary and pharmacologic
manipulations could be systematically evaluated for their efficiency.
These include phenylalanine, tyrosine and methionine restriction and
supplementation with cysteine, glutathione or penicillamine. The third
goal is the detailed study of the pathophysiology of tyrosinemia,
especially the development of hepatocellular carcinoma. In addition
several biochemical features of the disorder such as the elevated
methionine and depletion of intracellular glutathione are only poorly
understood and could be studied in more detail in the FAH deficient
animal.