Crossovers within a short DNA sequence indicate a long evolutionary history of the APRT*J mutation

Estimating carrier frequencies of newborn screening disorders using a whole-genome reference panel of 3552 Japanese individuals

Incidence rates of Mendelian diseases vary among ethnic groups, and frequencies of variant types of causative genes also vary among human populations. In this study, we examined to what extent we can predict population frequencies of recessive disorders from genomic data, and explored better strategies for variant interpretation and classification. We used a whole-genome reference panel from 3552 general Japanese individuals constructed by the Tohoku Medical Megabank Organization (ToMMo). Focusing on 32 genes for 17 congenital metabolic disorders included in newborn screening (NBS) in Japan, we identified reported and predicted pathogenic variants through variant annotation, interpretation, and multiple ways of classifications. The estimated carrier frequencies were compared with those from the Japanese NBS data based on 1,949,987 newborns from a previous study. The estimated carrier frequency based on genomic data with a recent guideline of variant interpretation for the PAH gene, in which defects cause hyperphenylalaninemia (HPA) and phenylketonuria (PKU), provided a closer estimate to that by the observed incidence than the other methods. In contrast, the estimated carrier frequencies for SLC25A13, which causes citrin deficiency, were much higher compared with the incidence rate. The results varied greatly among the 11 NBS diseases with single responsible genes; the possible reasons for departures from the carrier frequencies by reported incidence rates were discussed. Of note, (1) the number of pathogenic variants increases by including additional lines of evidence, (2) common variants with mild effects also contribute to the actual frequency of patients, and (3) penetrance of each variant remains unclear.

Adenine phosphoribosyltransferase (APRT) deficiency: identification of a novel nonsense mutation

Background

Adenine phosphoribosyltransferase deficiency (APRTD) is an under estimated genetic form of kidney stones and/or kidney failure, characterized by intratubular precipitation of 2,8-dihydroxyadenine crystals (2,8-DHA). Currently, five pathologic allelic variants have been identified as responsible of the complete inactivation of APRT protein.

Case presentation

In this study, we report a novel nonsense mutation of the APRT gene from a 47- year old Italian patient. The mutation, localized in the exon 5, leads to the replacement of a cytosine with a thymine (g.2098C > T), introducing a stop codon at amino acid position 147 (p.Gln147X).

This early termination was deleterious for the enzyme structural and functional integrity, as demonstrated by the structure analysis and the activity assay of the mutant APRT protein.

Conclusion

These data revealed that the p.Gln147X mutation in APRT gene might be a new cause of APRT disease.

Phenotype and genotype characterization of adenine phosphoribosyltransferase deficiency

Adenine phosphoribosyltransferase (APRT) deficiency is a rare autosomal recessive disorder causing 2,8-dihydroxyadenine stones and renal failure secondary to intratubular crystalline precipitation. Little is known regarding the clinical presentation of APRT deficiency, especially in the white population. We retrospectively reviewed all 53 cases of APRT deficiency (from 43 families) identified at a single institution between 1978 and 2009. The median age at diagnosis was 36.3 years (range 0.5 to 78.0 years). In many patients, a several-year delay separated the onset of symptoms and diagnosis. Of the 40 patients from 33 families with full clinical data available, 14 (35%) had decreased renal function at diagnosis. Diagnosis occurred in six (15%) patients after reaching ESRD, with five diagnoses made at the time of disease recurrence in a renal allograft. Eight (20%) patients reached ESRD during a median follow-up of 74 months. Thirty-one families underwent APRT sequencing, which identified 54 (87%) mutant alleles on the 62 chromosomes analyzed. We identified 18 distinct mutations. A single T insertion in a splice donor site in intron 4 (IVS4 + 2insT), which produces a truncated protein, accounted for 40.3% of the mutations. We detected the IVS4 + 2insT mutation in two (0.98%) of 204 chromosomes of healthy newborns. This report, which is the largest published series of APRT deficiency to date, highlights the underdiagnosis and potential severity of this disease. Early diagnosis is crucial for initiation of effective treatment with allopurinol and for prevention of renal complications.

Comparison between various strategies for the disease-gene mapping using linkage disequilibrium analyses: studies on adenine phosphoribosyltransferase deficiency used as an example

Recently, linkage disequilibrium analyses have been used to detect disease-causing loci based on the common disease-common variant hypothesis. To see what methods can effectively identify the genes, we have to apply them to the practical data obtained from the human population. We extensively performed linkage disequilibrium and haplotype analyses on adenine phosphoribosyltransferase ( APRT) genes in both control and deficient subjects. To examine the power to detect disease-causing loci, we analyzed SNPs, STRPs, and VNTR within and around the APRT gene. When only SNPs were used, P values did not necessarily show significant difference, even at loci close to the mutation site for APRT*J that is exclusively observed among Japanese. However, the examination of the same samples with haplotypes based on the haplotype block data gave sufficient significance. In the case of STRP and VNTR, some single-marker loci showed significant difference. Our study suggested that the use of haplotype analysis based on the haplotype-block structure is more powerful than single-marker locus analysis for the detection of disease-related loci.

Adenine phosphoribosyltransferase deficiency identified by urinary sediment analysis: cellular and molecular confirmation

Adenine phosphoribosyltransferase deficiency is an autosomal recessive purine enzyme defect that causes urolithiasis and, in severe cases, renal failure. Most homozygotes with this disorder were identified by analyses of excreted or surgically removed urinary stones, but some were identified only because they were family members of symptomatic individuals. We report here the detection of adenine phosphoribosyltransferase deficiency in two cases by routine analysis of urinary sediments. 2,8-Dihydroxyadenine-like spherical crystals were observed in the urinary sediment, and a diagnosis of homozygous adenine phosphoribosyltransferase deficiency was confirmed by cellular and molecular methods. A molecular diagnostic system using the polymerase-chain reaction and single-strand conformational polymorphism analysis proved to be a rapid and sensitive method to identify the APRT*J allele, a common mutant allele among the Japanese people. These methods will facilitate identification of symptomatic and asymptomatic individuals with homozygous adenine phosphoribosyltransferase deficiency.

Analysis of germline and in vivo somatic mutations in the human adenine phosphoribosyltransferase gene: mutational hot spots at the intron 4 splice donor site and at codon 87

We have characterized 18 germline and 10 in vivo somatic mutations in the human adenine phosphoribosyltransferase (APRT) gene. Both germline and in vivo somatic mutations were clustered at the intron 4 splice donor site and at codon 87. In vitro somatic mutations in human APRT do not appear to show this clustering. These findings suggest that the spectrum of germline mutations in APRT may be similar to that incurred by somatic cells in vivo, but different from that seen in cultured cells. Thus, in vivo, rather than in vitro, somatic mutations in this gene may be more representative of mutational events occurring in the germline.

Only three mutations account for almost all defective alleles causing adenine phosphoribosyltransferase deficiency in Japanese patients

We analyzed mutant alleles of adenine phosphoribosyltransferase (APRT) deficiency in Japanese patients. Among 141 defective APRT alleles from 72 different families, 96 (68%), 30 (21%), and 10 (7%) had an ATG to ACG missense mutation at codon 136 (APRT*J allele), TGG to TGA nonsense mutation at codon 98, and duplication of a 4-bp sequence in exon 3, respectively. The disease-causing mutations of only four (3%) of all the alleles among Japanese remain to be elucidated. Thus, a diagnosis can be made for most of the Japanese APRT-deficient patients by identifying only three disease-causing mutations. All of the different alleles with the same mutation had the same haplotype, except for APRT*J alleles, thereby suggesting that alleles with the same mutation in different families were derived from the same ancestral gene. Evidence for a crossover or gene conversion event within the APRT gene was observed in an APRT*J mutant allele. Distribution of mutant alleles encoding APRT deficiency among the Japanese was similar to that seen in cystic fibrosis genes among Caucasians and Tay-Sachs genes among the Ashkenazi Jews.

A non-radioactive method for the detection of a common mutant allele of aldehyde dehydrogenase 2

About 50% of Japanese have been estimated to possess at least one ALDH2*2 allele with a substitution of AAA for GAA at codon 487 of the aldehyde dehydrogenase gene. This mutation is tightly associated with the sensitivity of an individual to alcohol. We developed a method of identifying the ALDH2*2 allele by a non-radioactive technique. DNA from individuals was subjected to polymerase chain reaction in which part of the aldehyde dehydrogenase 2 gene was amplified. After dot-blotting onto nylon membranes, the DNA was hybridized with biotin-labelled allele-specific oligonucleotides. Determination of genotypes on 77 unrelated healthy Japanese individuals, using the conventional method and our new method with gradient hybridization temperature and competitive oligonucleotides, indicated that the latter was superior to the former.

Detection of the most common mutation of adenine phosphoribosyltransferase deficiency among Japanese by a non-radioactive method

About 79% of all the Japanese patients with adenine phosphoribosyltransferase (APRT) deficiency have been estimated to possess at least one APRT*J allele with a substitution of ACG for ATG at codon 136. We developed a non-radioactive method for diagnosing genotypes of this disease. Part of the genomic DNA including the mutation site of the APRT*J allele was amplified using polymerase chain reaction and the amplified product was dot-blotted onto nylon membranes and then hybridized with either APRT*J-specific or non-APRT*J-specific synthetic oligonucleotides labelled at the 5' termini with biotin in the presence of non-labelled competitive synthetic sequences. The temperature was gradually decreased during the hybridization. When competitive sequences were omitted, difference in the intensity of the hybridization between APRT*J-containing and non-containing samples was not sufficiently clear to differentiate the genotypes. When an excess amount of competitive sequences was added in addition to biotin-labelled oligonucleotides, this method effectively differentiated samples containing only APRT*J alleles from those containing only non-APRT*J alleles. The present method was also useful to differentiate samples with both APRT*J and non-APRT*J alleles from those having only either of the alleles. An equivalent procedure using competitive sequence for hybridization and gradually decreasing the temperature will be useful for detecting point mutations in other genes.

Identification of a compound heterozygote for adenine phosphoribosyltransferase deficiency (APRT*J/APART*Q0) leading to 2,8-dihydroxyadenine urolithiasis

Homozygous deficiency of a purine salvage enzyme, adenine phosphoribosyltransferase (APRT), causes urolithiasis and renal failure. There are two known types of homozygous APRT deficiencies; type I patients completely lack APRT activity while type II patients only partially lack such activity. All type II patients possess at least one APRT*J allele with a substitution from ATG (Met) to ACG (Thr) at codon 136. Type I patients are considered to possess two alleles (APRT*Q0) both of which code for complete deficiencies. Thus, some patients with type II APRT deficiencies may have a genotype of APRT*J/APRT*Q0. As no individuals with such a genotype have previously been identified, we performed extensive analysis on four members of a family by (1) the T-cell method for the identification of a homozygote, (2) the B-cell method for the identification of heterozygotes, and (3) oligonucleotide hybridization after in vitro amplification of a part of genomic APRT sequence for the identification of APRT*J and non-APRT*J alleles. We report here the first evidence that 2,8-dihydroxyadenine urolithiasis developed in a boy aged 2 years with a genotype of APRT*J/APRT*Q0.