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SPONTANEOUS REVERSION AT THE HPRT LOCUS IN CELLS FROM A PATIENT WITH A MILD VARIANT OF THE LESCH-NYHAN SYNDROME

We have studied cells from a patient who was followed with a diagnosis of spastic cerebral palsy until the age of 9 years when he was found to have a deficiency of hypoxanthine phosphoribosyltransferase (HPRT). He has dysarthria, choreathetoid movements, and an IQ in the normal range. He is nonambulatory because of the spasticity and has not engaged in self-mutilation other than some scratching and nail biting. Autoradiography of his cultured skin fibroblasts after exposure to 3H-hypoxanthine revealed a frequency of HPRT+ cells in the range of 10−5 to 10−4. His cells were cloned in 6-thioguanine and then grown in the absence of selective medium. The frequency of HPRT+ cells ranged from 10−6 to 3×10−5 after approximately 23 cell doublings. Cells exposed to 3H-hypoxanthine in the presence of aminopterin showed significant low level uptake in all cells indicating that these cells all contain some HPRT activity.Cells were grown in HAT medium and clones of HAT-resistant cells were isolated. A sensitive HPLC assay for HPRT has been developed to study enzyme activity in these cells. The milder phenotype of the patient may be related to cells that are spontaneously reverting to HPRT+ as well as to the low background level of HPRT in all cells.