Human Molecular Genetics, Vol. 1, No. 6
427-432
Characterization of mutations in phenotypic variants of hypoxanthine phosphoribosyltransferase deficiency Karin Sege-Peterson*. James Chambers2, Theodore Page, Oliver W.Jones1 and William L.Nyhan Department of Pediatrics and department of Medicine, University of California, San Diego, La Jolla, CA 92093 and department of Immunology, The Scripps Research Institute, La Jolla, CA 92037, USA Received May 11, 1992; Revised and Accepted July 10, 1992
The Lesch-Nyhan disease is caused by an almost complete lack of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT). Partial HPRT-deficiency, associated with less severe phenotype, has also been identified. We have characterized mutations occurring in HPRT cDNA isolated from patients with HPRT-deficiency with an emphasis on examining the more unusual partial variants of HPRTdeficiency. HPRT cDNA was amplified by PCR, cloned and analyzed by automated DNA sequence analysis. Twenty-two, unrelated individuals with HPRT deficiency were studied including eight classic Lesch-Nyhan patients and fourteen patients representing the different groups of partial HPRT deficiency. We found a diverse pattern of mutations with point mutations accounting for the majority of abnormal HPRT genes. Nonsense mutations and exon deletions were only found in HPRT cDNA isolated from classic Lesch-Nyhan patients. Mutations associated with partial HPRT-deficiency were frequently located in the amino terminal part of the molecule. A CpG mutational hot spot was identified at the position for Arg-51 in the HPRT protein. Two hyperuricemk patients exhibited unusual splice site mutations: in one this led to the creation of an additional exon in the HPRT gene and in the other part of exon 6 was missing in a subpopulation of the transcripts, producing the effect of a dominant, negative mutation. INTRODUCTION The Lesch-Nyhan disease (1), an inherited, X-linked recessive disorder of purine metabolism is caused by an almost complete lack of hypoxanthine phosphoribosyltransferase (HPRT; EC 2.4.2.8.) activity (2,3). The disease is characterized by hyperuricemia, neurologic dysfunction, including severe spasticity and choreoathetoid movements, mental retardation, and a typical self-injurious behavior. Partial deficiency of HPRT has been described in a variety of patients with phenotypic differences ranging from isolated hyperuricemia to varying degrees of neurologic dysfunction. Notably, in HPRT deficiency there is an inverse correlation between residual HPRT enzyme activity, as measured in an intact cell assay, and the severity of the neurologic disease (4,5). Also, HPRT-deficient individuals from the same pedigree have a very similar disease. This is in contrast
* To whom correspondence should be addressed
to many other genetic disorders e.g. medium chain acyl-CoA dehydrogenase deficiency (6) and Gaucher's disease (7) in which patients with identical mutations, and thus similar levels of residual enzyme activity, can have very different clinical course of the disease. The most common phenotypes of partial deficiency of HPRT are gout or nephropathy and normal cerebral function. The Lesch-Nyhan syndrome occurs with an incidence of 1 in 100,000 to 1 in 380,000. The incidence of partial HPRT deficiency is not known but is lower than the classic Lesch—Nyhan disease. Human HPRT has been purified to homogeneity and its amino acid sequence determined (8). The native enzyme is comprised of four identical subunits, each containing 217 amino acids. Structural studies on the human HPRT gene (9,10) have identified 9 exons spanning about 44 kb of DNA on the long arm of the X chromosome. Molecular studies have facilitated the search for mutations underlying deficiency of HPRT, and with the introduction of the polymerase chain reaction (PCR) the number of mutations identified in the HPRT gene has rapidly increased (11 -16). In the majority of patients with HPRT deficiency there is no gross alteration of the gene. The spectrum of mutations reported to date is broad with the same mutation rarely being found in unrelated pedigrees and includes point mutations, deletions, insertions, and duplications. Most of the reported mutations in HPRT have been derived from patients with the classic Lesch—Nyhan syndrome. In this report we have extended the search for mutations in the HPRT gene with an emphasis on examining a spectrum of phenotypes from classic Lesch—Nyhan to classic partial with clinical gout, and including the more unusual variants that exhibit hyperuricemia and neurologic dysfunction without the complete Lesch—Nyhan picture. RESULTS This study includes eight classic Lesch—Nyhan patients, five neurologic variants, and nine patients with hyperuricemia as the only manifestation of the disease. HPRT enzyme activity was measured in fibroblast cell lines, established from each patient, using an intact cell assay (5). The data on enzyme activity, shown in Table I, were consistent with the inverse correlation between clinical severity of the disease and residual enzyme activity
Downloaded from http://hmg.oxfordjournals.org/ at University of California, Santa Barbara on June 24, 2015
ABSTRACT
428 Human Molecular Genetics, Vol. 1, No. 6 Table I. Mutations in the HPRT gene Nucleotide changes or rearrangements
Deduced effect on protein
Lesch-Nyhan GM AS MS PW DW RT DL*
427-432
Characterization of mutations in phenotypic variants of hypoxanthine phosphoribosyltransferase deficiency Karin Sege-Peterson*. James Chambers2, Theodore Page, Oliver W.Jones1 and William L.Nyhan Department of Pediatrics and department of Medicine, University of California, San Diego, La Jolla, CA 92093 and department of Immunology, The Scripps Research Institute, La Jolla, CA 92037, USA Received May 11, 1992; Revised and Accepted July 10, 1992
The Lesch-Nyhan disease is caused by an almost complete lack of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT). Partial HPRT-deficiency, associated with less severe phenotype, has also been identified. We have characterized mutations occurring in HPRT cDNA isolated from patients with HPRT-deficiency with an emphasis on examining the more unusual partial variants of HPRTdeficiency. HPRT cDNA was amplified by PCR, cloned and analyzed by automated DNA sequence analysis. Twenty-two, unrelated individuals with HPRT deficiency were studied including eight classic Lesch-Nyhan patients and fourteen patients representing the different groups of partial HPRT deficiency. We found a diverse pattern of mutations with point mutations accounting for the majority of abnormal HPRT genes. Nonsense mutations and exon deletions were only found in HPRT cDNA isolated from classic Lesch-Nyhan patients. Mutations associated with partial HPRT-deficiency were frequently located in the amino terminal part of the molecule. A CpG mutational hot spot was identified at the position for Arg-51 in the HPRT protein. Two hyperuricemk patients exhibited unusual splice site mutations: in one this led to the creation of an additional exon in the HPRT gene and in the other part of exon 6 was missing in a subpopulation of the transcripts, producing the effect of a dominant, negative mutation. INTRODUCTION The Lesch-Nyhan disease (1), an inherited, X-linked recessive disorder of purine metabolism is caused by an almost complete lack of hypoxanthine phosphoribosyltransferase (HPRT; EC 2.4.2.8.) activity (2,3). The disease is characterized by hyperuricemia, neurologic dysfunction, including severe spasticity and choreoathetoid movements, mental retardation, and a typical self-injurious behavior. Partial deficiency of HPRT has been described in a variety of patients with phenotypic differences ranging from isolated hyperuricemia to varying degrees of neurologic dysfunction. Notably, in HPRT deficiency there is an inverse correlation between residual HPRT enzyme activity, as measured in an intact cell assay, and the severity of the neurologic disease (4,5). Also, HPRT-deficient individuals from the same pedigree have a very similar disease. This is in contrast
* To whom correspondence should be addressed
to many other genetic disorders e.g. medium chain acyl-CoA dehydrogenase deficiency (6) and Gaucher's disease (7) in which patients with identical mutations, and thus similar levels of residual enzyme activity, can have very different clinical course of the disease. The most common phenotypes of partial deficiency of HPRT are gout or nephropathy and normal cerebral function. The Lesch-Nyhan syndrome occurs with an incidence of 1 in 100,000 to 1 in 380,000. The incidence of partial HPRT deficiency is not known but is lower than the classic Lesch—Nyhan disease. Human HPRT has been purified to homogeneity and its amino acid sequence determined (8). The native enzyme is comprised of four identical subunits, each containing 217 amino acids. Structural studies on the human HPRT gene (9,10) have identified 9 exons spanning about 44 kb of DNA on the long arm of the X chromosome. Molecular studies have facilitated the search for mutations underlying deficiency of HPRT, and with the introduction of the polymerase chain reaction (PCR) the number of mutations identified in the HPRT gene has rapidly increased (11 -16). In the majority of patients with HPRT deficiency there is no gross alteration of the gene. The spectrum of mutations reported to date is broad with the same mutation rarely being found in unrelated pedigrees and includes point mutations, deletions, insertions, and duplications. Most of the reported mutations in HPRT have been derived from patients with the classic Lesch—Nyhan syndrome. In this report we have extended the search for mutations in the HPRT gene with an emphasis on examining a spectrum of phenotypes from classic Lesch—Nyhan to classic partial with clinical gout, and including the more unusual variants that exhibit hyperuricemia and neurologic dysfunction without the complete Lesch—Nyhan picture. RESULTS This study includes eight classic Lesch—Nyhan patients, five neurologic variants, and nine patients with hyperuricemia as the only manifestation of the disease. HPRT enzyme activity was measured in fibroblast cell lines, established from each patient, using an intact cell assay (5). The data on enzyme activity, shown in Table I, were consistent with the inverse correlation between clinical severity of the disease and residual enzyme activity
Downloaded from http://hmg.oxfordjournals.org/ at University of California, Santa Barbara on June 24, 2015
ABSTRACT
428 Human Molecular Genetics, Vol. 1, No. 6 Table I. Mutations in the HPRT gene Nucleotide changes or rearrangements
Deduced effect on protein
Lesch-Nyhan GM AS MS PW DW RT DL*
