Basic Studies Characteristics of murine histidinaemia and its potential for genetic manipulation N. Mellor 1 , M. Themis 2 , C. Selden 1 , M. Jones 3 and H. J. F. Hodgson 1 1 Centre for Hepatology, Department of Medicine, Royal Free and University College School of Medicine, London, UK, 2 Cystic Fibrosis Gene Therapy Research Group, Department of Biomedical Sciences, Imperial College School of Medicine, London, UK, 3 Department of Infectious Diseases, Hammersmith Campus, Imperial College School of Medicine, London, UK Mellor N, Themis M, Selden C, Jones M, Hodgson HJF. Characteristics of murine histidinaemia and its potential for genetic manipulation. Liver International 2004: 24: 354–360. r Blackwell Munksgaard 2004 Abstract: Background: Histidinaemia is an autosomal recessive disorder affecting the hepatic enzyme histidine ammonia lyase (histidase) resulting in elevated plasma and urinary histidine and is prototypic of a series of hepatic cytosolic enzyme defects. Aims: To characterise the physiology of murine histidinaemia with respect to histidine excretion and catabolism, and explore the potential for manipulating cellular and whole body histidase metabolism by gene transfer. Materials and Methods: We studied his/his mice which have a G to A substitution in the gene encoding histidase, using both in vitro transduction of isolated hepatocytes by lipofection with wild-type histidase cDNA, and in vivo transduction of whole liver using a retroviral construct. Results and Conclusion: Histidase cDNA expression restored histidase activity in vivo and in vitro towards normal levels, demonstrated both at the cellular level and by whole body metabolic studies, establishing the potential of this model for the development of new gene therapeutic approaches. Key words: histidase – histidinaemia – liver Prof. H.J.F. Hodgson, Centre for Hepatology, Department of Medicine (Upper Third Floor), Royal Free and University College Medical School, University College London, Royal Free Campus, Rowland Hill Street, Hampstead, London NW3 2PF, UK. Tel: 144 207 433 2862 Fax: 144 207 433 2852 e-mail: h.hodgson@rfc.ucl.ac.uk Received 27 September 2003, accepted 1 March 2004                                                                         Histidinaemia in man is an autosomal recessive condition, occurring in approximately 1 in 11 000 of the general population (1), reflecting decreased activity of the hepatic cytosolic enzyme histidase. Diminished histidase (histidine ammonia lyase, HAL (EC4.2.1.3)) enzyme activity in the liver leads to elevated blood, urine and CSF histidine (2), subnormal levels of the product urocanic acid, and compensatory elevations of other me- tabolites generated by minor pathways, due to a G to A transition in exon 22 of the HAL locus on chromosome 12 (2). There is no marked clinical dysfunction associated with the defect, although there are reports of individuals and series of patients in whom psychomotor and neurological abnormalities have been reported (3, 4) suggest- ing the defect may interact with other metabolic conditions to perturb neural or mental develop- ment. A similar condition occurs in mice due to a G to A transition in the HAL locus on chromosome 10 (5). In mice this has been shown to lead to an unstable form of the enzyme: hepatic histidase mRNA is as abundant as in wild-type animals, but histidase protein content and enzyme activity are greatly reduced (5, 6). Mice born to his/his mothers may demonstrate neurological abnorm- alities in particular affecting balance, but homo- zygotes born from heterozygote parents appear grossly normal. Murine histidinaemia thus represents a model system of a hepatic cytosolic enzyme detects that can be exploited for investigating new ther- apeutic approaches. We previously demonstrated that transplantation of wild-type hepatocytes into the peritoneum of his/his animals reverted the biochemical phenotype, demonstrated by a return of the urinary histidine excretion to normal, whilst transplantation of his/his hepatocytes had no effect (7). It is characteristic of all non-lethal genetic defects that secondary changes occur (e.g. up- regulation of alternative pathways), or structural change in cells due to accumulation of abnormal proteins. To ensure that hepatocytes from histi- dase-deficient animals were capable of supporting histidase degradation if normal enzyme activity was restored, and thus explore the potential for utilising this condition to develop new therapeu- tic approaches for hepatic cytosolic enzyme Liver International 2004: 24: 354–360 Printed in Denmark. All rights reserved Copyright r Blackwell Munksgaard 2004 DOI: 10.1111/j.1478-3231.2004.0929.x 354