Primary hyperoxaluria type 1 in the Canary Islands: a conformational disease due to I244T mutation in the P11L-containing alanine:glyoxylate aminotransferase

Evaluation of mutation screening as a first line test for the diagnosis of the primary hyperoxalurias11The nomenclature used in this paper is based on that recommended by Antonarakis SE, and the Nomenclature Working Group (1998): Recommendations for a nomenclature system for human gene mutations. Hum Mutat 11:1–3, 1998, where “c” denotes cDNA sequence and nucleotide numbering uses the “A” of the ATG translation initiation start site as +1

BACKGROUND

A definitive diagnosis of primary hyperoxaluria type 1 (PH1) and primary hyperoxaluria type 2 (PH2) requires the measurement of alanine:glyoxylate aminotransferase (AGT) and glyoxylate reductase (GR) activities, respectively, in a liver biopsy. We have evaluated a molecular genetic approach for the diagnosis of these autosomal-recessive diseases.

METHODS

Polymerase chain reaction (PCR) was used to detect three common mutations in the AGXT gene (c.33_34insC, c.508G>A, and c.731T>C) and one, c.103delG, in the GRHPR gene in DNA samples from 365 unrelated individuals referred for diagnosis of PH1 and/or PH2 by liver enzyme analysis.

RESULTS

One or more of these mutations was found in 183 (68.8%) biopsy proven cases of PH1 and PH2 with a test negative predictive value of 62% and 2%, respectively. 102 (34.1%) patients were homozygous or compound heterozygous, making a molecular diagnosis possible. Age of onset and presenting features were similar in patients homozygous for any of the four mutations. Of the AGXT homozygotes, only the c.508G>A mutant was associated with significant AGT catalytic activity and in two of these activity was in the low normal range, possibly reflecting variation in mitochondrial content of the biopsy as this particular mutation is associated with mitochondrial mistargeting.

CONCLUSION

Limited mutation analysis can provide a useful first line test for PH1 and PH2 in patients in whom primary hyperoxaluria is suspected and in whom secondary causes have been excluded. Those patients in whom a single mutation, or no mutation, is found can then be selectively targeted for liver biopsy.

Genetic heterogeneity in primary hyperoxaluria type 1: impact on diagnosis

Primary hyperoxaluria type 1 (PH1) is an autosomal recessive disease characterized by progressive kidney failure due to renal deposition of calcium oxalate. The disease is caused by a deficiency of alanine:glyoxylate aminotransferase (AGT) which catalyzes the conversion of glyoxylate to glycine. When AGT is absent, glyoxylate is converted to oxalate which forms insoluble calcium salts that accumulate in the kidney and other organs. In the most common phenotype there is a unique phenomenon wherein AGT is mis-targeted to the mitochondria instead of the peroxisomes. The diagnosis of PH1 is complicated by heterogeneity of clinical presentation, course of the disease, biochemical markers, AGT enzymatic activity and genotype. More than 50 mutations and polymorphisms have been reported in the AGT gene; three common mutations accounting for almost 50% of PH1 alleles. The mutations are of all types, with missense making up the largest fraction. There are some mutations with apparent ethnic associations and at least one that appears to be pan-ethnic. Although correlations can in some cases be made between biochemical phenotype and genotype, correlation with clinical phenotype is complicated by the involvement of other genetic and non-genetic factors that affect disease severity. A number of polymorphisms have been described in the AGT gene some of which cause missense changes and, in some cases, alter enzyme activity. As DNA testing becomes more commonly used for diagnosis it is important to correlate observed sequence changes with previously documented changes as an aid to assessing their potential significance.

The molecular basis of kidney stones

PURPOSE OF REVIEW

To emphasize an exploration of mechanisms of kidney stone disease based on a molecular understanding of excess urinary excretions of calcium, oxalate, cystine, and uric acid.

RECENT FINDINGS

Hypercalciuria is discussed relative to mutations in the renal chloride genes CLCN5 and CLCNKB, WNK kinases, ATPB61, and NPT2. Hyperoxaluria is discussed relative to mutations in AGXT and GRHPR. Cystinuria is discussed relative to mutations in SLC3A1 and SLC7A9. Hyperuricosuria is discussed with novel gene findings, and hyperxanthinuria with new findings in XDH.

SUMMARY

An enhanced understanding of the diagnosis, course, and prognosis for genetic causes of kidney stone diseases has been made available to the clinician caring for patients with kidney stones and to the scientist interested in their cause, as a result of molecular breakthroughs in the kidney handling of normal urinary constituents. We look forward to a new era of the therapeutics of kidney stones based on such advances.