HPRT1 and arthritic joint disease: Mutations of the enzyme hypoxanthine‐guanine phosphoribosyl transferase (HPRT) for purine salvage or the hyperactivity of enzyme 5‐phosphoribosyl‐1‐pyrophospate (PRPP) synthetase as a co‐substrate for HPRT usually lead to gouty arthritis during childhood and early adulthood.15