Lesch Nyhan syndrome is an X-linked recessive disorder of purine metabolism resulting in abnormal mental behavior including mental retardation and a compulsive form of self mutilation. The disease is caused by a deficiency of hypoxanthine-guanine phosphoribosyl transferase (HGPRT), the enzyme that converts the free pases hypoxanthine and guanine to nucleotides. Little is known about how the HGPRT deficiency causes the unusual mental behavior, but there is evidence of dysfunction of dopaminergic nerve terminals, e.g. decreased levels of dopamine in brains of Lesch Nyhan patients. Our long term objective is to understand the molecular mechanism by which a deficiency of HGPRT causes neuronal dysfunction. Our approach to this objective will be to study the effect of an HGPRT deficiency on the ability of a clonal cell line to synthesize, store, and release neurotransmitter. The clonal line is PC12, a rat pheochromocytoma. PC12 cells contain dopamine and acetylcholine and they secrete each by a Ca++ dependent process. After treatment with nerve growth-factor, PC12 cells extend neurites and acquire the appearance of neurons. These properties make PC12 particularly well suited for studying the consequences of an HGPRT deficiency on neuronal function. We have obtained many PC12 variants deficient in HGPRT activity. Most of the HGPRT-deficient variants exhibit abnormal storage and release of dopamine. Initially, we will further characterize the variants with respect to their metabolism of dopamine and their storage and release of acetylcholine. Then, we will determine whether the HGPRT-deficiency itself causes the abnormal neurotransmitter metabolism. This will be done by examining whether the abnormality in neurotransmitter metabolism persists after insertion of a functional HGPRT gene into the genome of the variant in question. If the HGPRT-deficiency is responsible for a particular abnormality in neurotransmitter metabolism, we will begin to examine the mechanism by which the HGPRT deficiency causes that abnormality. We will also further examine the variants whose abnormality in neurotransmitter release is unrelated to a deficiency of HGPRT. We will attempt to identify the individual biochemical reactions that are involved in neurotransmitter release by normal cells but are defective in these variants.
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