Tyrosinemia II: lessons in molecular pathophysiology.

Tyrosinemia II is caused by a deficiency of hepatic tyrosine aminotransferase. With the deficiency of this key enzyme of tyrosine catabolism there is an increase in plasma tyrosine and then an increase in tyrosine metabolites in the urine. The increased plasma tyrosine causes tyrosine to crystallize in the cornea, producing corneal ulcerations and sometimes proliferation of corneal epithelium. In the epidermis of the palms and soles, tyrosine leads to erosions, crusting, and then hyperkeratosis. The human disease is due to an autosomal recessive gene, and similar genetic diseases have been found in mink and in dogs. A nutritional model for the disease, in which a high-tyrosine low-protein diet is fed to rats, produces almost identical features. The features of this disorder and some of the implications of this disease for the study of other genetic diseases is discussed in this review.