BASES OF PATHOPHYSIOLOGY IN GALACTOSEMIA Profound impairment of galactose-1-phosphate uridylyltransferase (GALT) results in the inborn error of metabolism classic galactosemia. Although the acute and potentially lethal sequelae of this disorder can be resolved or prevented by early detection and lifelong dietary restriction of galactose, a large proportion of patients on dietary management nonetheless experience serious long-term complications. These complications include cognitive disabilities and a speech disorder in almost half of all patients, and primary or premature ovarian failure in more than 80% of females. Despite more than 50 years of investigation, the underlying bases of pathophysiology in galactosemia remain unclear, and animal models have failed to recapitulate the patient phenotype. The long-term goal of this proposal is to determine the biochemical bases of pathophysiology in galactosemia, and ultimately to exploit that knowledge to develop novel and more effective treatments for this disorder. Our short-term objective is to define the roles of specific biochemical factors, including aberrant glycosylation, as potential mediators of galactose sensitivity in GALT and/or GALE-impaired yeast or mammalian cells. We further propose to test the roles of these same candidate factors as potential mediators of outcome in a large cohort of patients with classic galactosemia. Our Specific Aims are: (1) to define the roles of GALK, GALE, and UGP1 as candidate modifiers of galactose sensitivity in yeast and human cell model systems of galactosemia, (2) to define the nature and underlying cause(s) of aberrant glycosylation in fibroblasts from patients with classic galactosemia and generalized epimerase-deficiency galactosemia, and (3) to define modifiers of outcome in a cohort of patients with classic galactosemia.