Molecular, biochemical, and genetic characterization of a female patient with Lesch–Nyhan disease

Lesch–Nyhan disease (LND) is a rare X-linked recessive disorder caused by virtually complete deficiency of activity of the purine salvage enzyme hypoxanthine phosphoribosyltransferase (HPRT; EC 2.4.2.8). Human HPRT is encoded by a single structural gene located on the long arm of the X-chromosome (Xq26). The classical LND phenotype occurs almost exclusively in males, manifested in excessive purine production and characteristic neurological manifestations, including compulsive self-mutilation, choreoathetosis, spasticity, and occasionally developmental delay. Heterozygous females are usually phenotypically normal, due to the random inactivation of the X chromosome (Lyonization mechanism). However, six females were reported to be affected with the full biochemical and clinical manifestations of LND. All these cases were heterozygous for an HPRT mutation. Absence of transcription of the normal HPRT allele was attributed in all of them to non-random inactivation of the X chromosome carrying the normal allele. Here we describe an additional LND female, who presented with acute renal failure at the age of two months, in whom absence of transcription of the two HPRT alleles occurred due to as yet undescribed mechanism in LND females: the transcription of one HPRT allele was blocked due to a de novo X chromosome–autosome translocation 46,XX,t(X:2)(q26:p25), with a breaking point encompassing the HPRT gene locus, whereas the transcription of the normal allele was inhibited due to non-random inactivation of the second X-chromosome. Cultured fibroblasts from this patient exhibited the biochemical alterations in purine nucleotide metabolism characteristic of male LND fibroblasts.