Purine metabolism in heterozygous carriers of hypoxanthine-guanine phosphoribosyltransferase deficiency

Purine metabolism in female heterozygotes for hypoxanthine-guanine phosphoribosyltransferase deficiency

BACKGROUND

Female carriers of the X-linked recessive disorder hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency show somatic cell mosaicism, and this may cause an increased synthesis of purines. We have examined whether urinary oxypurines could be useful for carrier diagnosis.

METHODS

Carrier testing was performed in 35 women belonging to 16 unrelated Spanish families with at least one subject affected by the Lesch-Nyhan syndrome (11 families, 14 patients) or the Kelley-Seegmiller syndrome (five families, six patients) by means of HPRT and adenine phosphoribosyltransferase activities in hair follicles and/or molecular studies. Plasma and 24-h urinary concentrations of hypoxanthine, xanthine and uric acid were measured while subjects were on a purine-restricted diet.

RESULTS

Mean plasma urate concentrations and 24-h urinary hypoxanthine, xanthine and uric acid excretion rates were significantly higher in 22 heterozygotes than in 13 non-carriers (P < 0.02). Daily urinary oxypurine excretion rates were also significantly higher in heterozygotes than in 12 normal women (P = 0.0011). Cumulative 5-day radioactivity excretion after [8-14C]-adenine infusion was markedly increased in 10 carrier women compared with five normal women (P = 0.0369). The sensitivity of 24-h urinary hypoxanthine and xanthine excretion rates was 86% and 77%, respectively, and the specificity 100% for both tests.

CONCLUSION

Female heterozygotes for HPRT deficiency show an enhanced purine nucleotide degradation and purine overproduction. An elevated hypoxanthine and/or xanthine excretion rate differentiated most heterozygotes for HPRT deficiency from non-carrier women and thus could be useful for carrier diagnosis.

In vivo hprt mutant frequencies in T-cells of normal human newborns

Mutation at the hypoxanthine-guanine phosphoribosyl transferase locus (hprt; HPRT enzyme) in the human fetus was studied by clonal assay of placental cord blood samples from full-term newborns. Conditions for determining hprt mutant frequencies, as defined for adults, were also optimal for studies in newborns. The mean mutant frequency for 45 normal human newborns (37 male, 8 female) was 0.64 X 10(-6) (SD = 0.41 X 10(-6); median value = 0.58 X 10(-6). These values are approx. 10-fold lower than corresponding adult hprt mutant frequency values. Factors such as limiting-dilution cloning efficiencies, delay prior to study of sample, sex, cryopreservation or technician performing the assay did not significantly affect assay results. Maternal smoking did not result in elevated mutant frequency values. Most wild-type and mutant clones studied were CD4 surface antigen positive (helper/inducer). All hprt mutants analyzed lacked HPRT activity.

Use of selective media for distinguishing variant forms of hypoxanthine phosphoribosyl transferase

Growth properties of cultured fibroblasts in selective media were used to characterize the HPRT enzyme of a patient with a new variant of hypoxanthine phosphoribosyl transferase (HPRT) with deficient activity. The clinical phenotype of the patient was typical of the Lesch-Nyhan syndrome. However, cells of the patient were not selected for by growth in either 8-azaguanine or 6-thioguanine. Assay of the activity of the enzyme in erythrocyte lysates revealed values of approximately zero, while in the intact fibroblast assay the level of activity was 1.4% of normal. The heterozygous mother of the patient, unlike heterozygotes for the classic Lesch-Nyhan enzyme, had a level of activity in erythrocyte lysates that was 45% of control. In the presence of selective agents in vitro the cells of the patient retained sufficient HPRT activity to permit a degree of toxicity indistinguishable from that observed in normal cells although the degree of the deficiency was so great that it led to the complete Lesch-Nyhan phenotype. These findings call into question the use of selective agents for the identification of HPRT- cells in the detection of heterozygosity.

An enumerative assay of purine analogue resistant lymphocytes in women heterozygous for the Lesch-Nyhan Mutation

In females heterozygous for the Lesch-Nyhan (LN) mutation, there is mosaicism of peripheral blood lymphocytes (PBLs) with regard to sensitivity to 6-thioguanine (TG) inhibition of tritiated thymidine ([3H]Tdr) incorporation following phytohemagglutinin (PHA) stimulation. That there are two populations of PBLs, normal and mutant (LN-like), has been demonstrated by an autoradiographic enumerative assay. A single three-generation family containing six potentially heterozygous females was studied. Five of the six were mosaics with frequencies of TG-resistant (TGr) PBLs rangling from 1.4 x 10(-3) to 4.2 x 10(-3) when tested at 2 x 10(-4) M TG. The median frequency of TGr PBLs in 63 healthy non-LN individuals between the ages of 11 and 75 years was found to be 1.1 x 10(-4) (mean 1.3 x 10(-4)) (10th and 90th percentiles--6.1 x 10(-5) and 2.1 x 10(-4)) and was not age related. The sixth potentially heterozygous female in the current family had a TGr PBL frequency of 1.9 x 10(-4). In the five females with elevated TGr PBL frequencies, TGr skin fibroblasts with frequencies ranging from 26% to 100% of the sample tested were found; in the female with the normal TGr PBL frequency, no TGr skin fibroblasts were found. The former group was considered to be LN heterozygous. Four of the five had been previously diagnosed as such. The latter individual is considered to be genotypically normal. Females who are heterozygotes for the LN mutation have two populations of PBLs.

Adenine phosphoribosyltransferase: a simple spectrophotometric assay and the incidence of mutation in the normal population

The significance of partial deficiency of erythrocyte adenine phosphoribosyltransferase (APRT), reported in a number of subjects with gout, has been investigated by studying its incidence in 700 normal blood donors. Three clearly deficient subjects were found, an incidence not significantly different from that in patients with abnormalities of urate metabolism. A new assay method for APRT is described in which an erythrocyte lysate is incubated with adenine and phosphoribosylpyrophosphate (PRPP) for a given time; both hemoglobin and adenine nucleotide (AMP) are then precipitated with lanthanum phosphate; the change in absorbance of adenine at 260 nm reflects the extent of its conversion to AMP by APRT.

Urate production in heterozygotes for glucose-6-phosphatase deficiency

Urate production and excretion were studied in heterozygous parents of a child with glucose-6-phosphatase deficiency. Both parents demonstrated glucose-6-phosphatase concentrations in platelets intermediate between those in the homozygote and the normal. The miscible urate pool and turnover rate, the rate of incorporation of [14C]glycine into urate, the renal clearance of urate and the percentage excretion of labelled urate by the renal route were within the normal range in both heterozygotes, as were the serum cholesterol and triglyceride concentrations. Thus, a partial deficiency of glucose-6-phosphatase was not associated with the abnormalities of urate or lipoprotein metabolism which are features of homozygous glucose-6-phosphatase deficiency. Erythrocyte phosphoribosyl-pyrophosphate concentration, an increased concentration of which has been postulated as the mechanism responsible for the increased de novo purine biosynthesis in glucose-6-phosphatase deficiency, was found to be within the normal range in erythrocytes from both a homozygote and a heterozygote for this condition.

Hypoxanthine-guanine phosphoribosyl transferase deficiency

In man congential lack of enzyme of the purine salvage system, hypoxanthineguanine phosphoribosyl transferase (HG-PRT E.C. 2.4.2.8), is mostly accompanied by a picture known as the Lesch-Nyhan snydrome. The degree of deficiency may vary from zero to a few percent of normal activity but a correlation between the severity of HG-PRT deficiency and the clinical picture has not been observed, no more than a correlation HG-PRT deficiency and neurological dysfunction. But individuals with undetectable HG-PRT activity but without the Lesch-Nyhan syndrome have been described. Patients with partial HG-PRT defiency have clinically distinctive findings. Sometimes mild neurological abnormalities are observed. Because of marked overproduction of ric acid severe gouty arthritis and renal dysfunction are often encountered in both complete and partial deficiency. There is considerable molecular heterogeneity in HG-PRT deficiency in man. Mutant ebnzymes may exhibit different kinetic and electrophoretic properties, indicating that hterwe might be a mutation on the structural gene coding for HG-PRT. Lack of HG-PRT disturbs purine interconversions profoundly. In addition to an important function of HG-PRT in the uptake of the purine hypoxantine and guanine into the cell, the effective uptake of inosine, guanosine and adenosine also seems to be dependent on HG-PRT...

Adenine phosphoribosyltransferase deficiency: its inheritance and occurrence in a female with gout and renal disease

A deficiency of adenine phosphoribosyltransferase (APRT) enzyme activity to approximately 40% of normal has been found in erythrocytes from a young woman aged 24 years, who had suffered from recurrent gouty arthritis since 11 years of age. She also demonstrated considerable, although asymptomatic, renal impairment with a creatinine clearance of one-third normal. Her father had suffered from gouty arthritis but had a normal APRT activity; he was obese, had a high purine intake and was a regular beer drinker. The patient's mother was asymptomatic with a normal serum urate concentration, but demonstrated a similar reduction in APRT activity to that of her daughter. Eleven other asymptomatic members of the family also demonstrated a similar reduction in APRT activity in erythrocyte lysates. The pattern of inheritance was consistent with autosomal transmission. Concentrations of phosphoribosylpyrophospate (PRPP) in erythrocytes were within normal limits both in the subjects with deficient, and in those with normal, APRT activity. Partial purification of APRT enzyme from erythrocytes of the index case did not reveal any difference from the normal enzyme as far as Michaelis constants, heat stability, or mobility in polyacrylamide gel was concerned. No primary abnormality of lipoprotein metabolism was demonstrated either in the propositus or in other members of her family. Study of urate metabolism in the propositus indicated that, although urate production was within the normal range in absolute terms, there was increased incorporation of glycine into produced urate, usually taken as one index of de novo urate production. Impaired renal excretion of urate was also shown. Although detailed study of urate metabolism has not been undertaken in other family members with APRT deficiency, no conclusive relationship has yet been demonstrated between APRT deficiency and disordered urate metabolism.

The kinetics of intramolecular distribution of 15N in uric acid after administration of (15N) glycine. A reappraisal of the significance of preferential labeling of N-(3+9) of uric acid in primary gout

THE CONCEPT OF AN ABNORMALITY OF GLUTAMINE METABOLISM IN PRIMARY GOUT WAS FIRST PROPOSED ON THE BASIS OF ISOTOPE DATA: when [(15)N]glycine was administered to gouty subjects, there was disproportionately great enrichment of N-(3 + 9) of uric acid, which derive from the amide-N of glutamine. An unduly high concentration of (15)N in glutamine was postulated, and attributed to a hypothetical defect in catabolism of glutamine. Excess glutamine was proposed as the driving force of uric acid overproduction. WE HAVE REEXAMINED THIS PROPOSITION IN FOUR GOUTY SUBJECTS: one mild overproducer of uric acid with "idiopathic gout," one marked overproducer with high-grade but "partial" hypoxanthine-guanine phosphoribosyl-transferase deficiency, and two extraordinary overproducers with superactive phosphoribosylpyrophosphate synthetases. In the last three, the driving force of excessive purine biosynthesis is a known surplus of alpha-5-phosphoribosyl-1-pyrophosphate. Disproportionately high labeling of N-(3 + 9) was present in all four gouty subjects, most marked in the most flamboyant overproducers. The precursor glucine pool was sampled by periodic administration of benzoic acid and isolation of urinary hippuric acid. Similarly, the precursor glutamine pool was sampled by periodic administration of phenylacetic acid and isolation of the amide-N of urinary phenylacetylglutamine. The time course of (15)N enrichment of hippurate differed from that of the amide-N of glutamine. Whereas initial enrichment values of hippurate were very high, those of glutamine-amide-N were low, increasing to a maximum at about 3 h, and then declining less rapidly than those of hippurate. However, enrichment values of hippurate and of phenacetyl glutamine were normal in all of the gouty subjects studied. Thus, preferential enrichment of N-(3 + 9) in gouty overproducers given [(15)N]glycine does not necessarily reflect a specific abnormality of glutamine metabolism, but rather appears to be a kinetic phenomenon associated with accelerated purine biosynthesis per se.In addition, greater enrichment of N-9 than of N-3 on days 1 and 2 provided suggestive evidence for a second pathway for synthesis of the initial precursor of purine biosynthesis, phosphoribosylamine, perhaps utilizing ammonia rather than the amide-N of glutamine as nitrogen donor. In this limited study, the activity of this potential second pathway did not appear to be selectively increased in gout.

Activation of variants of hypoxanthine-guanine phosphoribosyl transferase by the normal enzyme

Deficient hypoxanthine-guanine phosphoribosyl transferase (HGPRT; EC 2.4.2.8) enzymes from erythrocytes of patients with hyperuricemia and with the Lesch-Nyhan syndrome migrate 15% faster in polyacrylamide gel disc electrophoresis than the normal enzyme. A half-sister of two males with partial deficiency, who had 34% of normal HGPRT activity in her erythrocytes, yielded profiles containing two distinct zones of activity; one corresponded to the enzyme found in normal individuals and one to the variant of her half-brothers. However, in her profile her variant enzyme showed notably greater activity than that observed in her half-brothers. This increase was due to an activation of the variant by normal enzyme. Electrophoresis of mixtures of normal enzyme with partially deficient enzymes from patients with hyperuricemia and with the Lesch-Nyhan syndrome also led to activation of deficient HGPRT variants by normal enzymes. Deficient variants were also activated by normal enzyme on filtration through Sephadex G-25. Experiments in which deficient variant enzymes were activated with purified normal enzyme labeled with (125)I indicated that deficient enzymes incorporate components of the normal enzyme. No such activation of deficient enzymes was ever obtained when mixtures of deficient and normal enzymes were put together in a test tube.