Characterization and chromosomal mapping of a genomic clone encoding human alanine:glyoxylate aminotransferase

The AGT gene in Africa: a distinctive minor allele haplotype, a polymorphism (V326I), and a novel PH1 mutation (A112D) in Black Africans

We describe a novel missense mutation (A112D) and polymorphism (V326I) in the human AGT gene in two black African patients with primary hyperoxaluria type 1, an autosomal recessive disease resulting from a deficiency of the liver peroxisomal enzyme alanine:glyoxylate aminotransferase (AGT; EC 2.6.1.44). V326I was found in DNA from normal control Blacks with an allele frequency of 3%. Expression studies confirmed that A112D reduced AGT enzyme activity by 95% while V326I had no effect. Both A112D and V326I were homozygous in both patients and lie on a variant of the minor allele of the AGT gene. This variant haplotype, Mi(A), includes an intron 1 duplication and intron 4 VNTR (38 repeat) but lacks the P11L and I340M normally associated with the minor allele in Caucasians. Among the South African Blacks tested, the Mi(A) haplotype had an allele frequency of 12% compared to 3 % for the Caucasian-type minor allele haplotype.

Hyperoxaluric calcium nephrolithiasis

Hyperoxaluria leads to increased calcium oxalate supersaturation and calcium oxalate stone formation. Excess oxalate can arise from endogenous overproduction as in primary hyperoxaluria or from dietary sources. In the last 15 years great strides have been made in the diagnosis and treatment of primary hyperoxaluria. However options still seem limited in treating the mild hyperoxaluria found in many stone formers. Inadequate knowledge of food oxalate content, the effect of dietary oxalate precursors on oxalate excretion, and the factors affecting handling of oxalate by the intestine prevent development of rational therapies for treatment of hyperoxaluria. Recent studies of oxalate degrading bacteria and renewed interest in the role of diet calcium in oxalate absorption may lead to better therapeutic strategies for hyperoxaluric calcium nephrolithiasis.

Functional analysis of the 5'-flanking region of the human alanine:glyoxylate aminotransferase gene AGXT

Primer extension of human liver poly(A)(+) RNA revealed that the main transcription start site of the human alanine:glyoxylate aminotransferase gene (AGXT) is situated near 45 bp upstream from the translation start site. Deletion analysis using the 1203 bp 5'-flanking region of the AGXT gene and a luciferase reporter suggested that the promoter sequence is most likely located 2-325 bp upstream from the translation start site, possibly with enhancer elements 440-700 bp upstream. It was also suggested that the region -2 to -64 is important for the expression of the AGXT gene. The region -2 to -325 has two TATA boxes and some initiator elements.

Three novel deletions in the alanine:glyoxylate aminotransferase gene of three patients with type 1 hyperoxaluria

We describe three novel deletions in the human AGT gene in three patients with primary hyperoxaluria type 1, an autosomal recessive disease resulting from a deficiency of the liver peroxisomal enzyme, alanine glyoxylate aminotransferase (AGT; EC 2.6.1.44). A deletion of 4 nucleotides in the exon 6/intron 6 splice junction (679-IVS6+2delAAgt) is expected to cause missplicing. It would also code for a K227E missense alteration in any mRNA successfully spliced. A 2-bp deletion in exon 11 (1125-1126del CG, cDNA) results in a frameshift. A deletion of at least 5-6 kb, EX1 EX5del, spanned exons 1-5 and contiguous upstream sequence. All three deletions are heterozygous with previously documented missense mutations; the intron 6 deletion with F152I, the exon 11 deletion with G82E, and EX1 EX5del with the common mistargeting mutation, G170R.

Functional analysis of 14 genes that constitute the purine catabolic pathway in Bacillus subtilis and evidence for a novel regulon controlled by the PucR transcription activator

The soil bacterium Bacillus subtilis has developed a highly controlled system for the utilization of a diverse array of low-molecular-weight compounds as a nitrogen source when the preferred nitrogen sources, e.g., glutamate plus ammonia, are exhausted. We have identified such a system for the utilization of purines as nitrogen source in B. subtilis. Based on growth studies of strains with knockout mutations in genes, complemented with enzyme analysis, we could ascribe functions to 14 genes encoding enzymes or proteins of the purine degradation pathway. A functional xanthine dehydrogenase requires expression of five genes (pucA, pucB, pucC, pucD, and pucE). Uricase activity is encoded by the pucL and pucM genes, and a uric acid transport system is encoded by pucJ and pucK. Allantoinase is encoded by the pucH gene, and allantoin permease is encoded by the pucI gene. Allantoate amidohydrolase is encoded by pucF. In a pucR mutant, the level of expression was low for all genes tested, indicating that PucR is a positive regulator of puc gene expression. All 14 genes except pucI are located in a gene cluster at 284 to 285 degrees on the chromosome and are contained in six transcription units, which are expressed when cells are grown with glutamate as the nitrogen source (limiting conditions), but not when grown on glutamate plus ammonia (excess conditions). Our data suggest that the 14 genes and the gde gene, encoding guanine deaminase, constitute a regulon controlled by the pucR gene product. Allantoic acid, allantoin, and uric acid were all found to function as effector molecules for PucR-dependent regulation of puc gene expression. When cells were grown in the presence of glutamate plus allantoin, a 3- to 10-fold increase in expression was seen for most of the genes. However, expression of the pucABCDE unit was decreased 16-fold, while expression of pucR was decreased 4-fold in the presence of allantoin. We have identified genes of the purine degradation pathway in B. subtilis and showed that their expression is subject to both general nitrogen catabolite control and pathway-specific control.

Genetic disorders and urolithiasis

A recent analysis of the McKusick's On-Line Mendelian Inheritance in Man (OMIM) database revealed over 30 genetic or putatively genetic conditions in which urolithiasis contributes to the disease pathology at least to some extent. There is wide clinical, biochemical, and genetic heterogeneity in many of these conditions.

Primary hyperoxaluria type I: a model for multiple mutations in a monogenic disease within a distinct ethnic group

Primary hyperoxaluria type 1 is an autosomal recessive inherited metabolic disease in which excessive oxalates are formed by the liver and excreted by the kidneys, causing a wide spectrum of phenotypes ranging from renal failure in infancy to mere renal stones in late adulthood. Mutations in the AGXT gene, encoding the liver-specific enzyme alanine:glyoxylate aminotransferase, are responsible for the disease. Seven mutations were detected in eight families in Israel. Four of these mutations are novel and three occur in children living in single-clan villages. The mutations are scattered along various exons (1, 4, 5, 7, 9, 10), and on different alleles comprising at least five different haplotypes. All but one of the mutations are in a homozygous pattern, reflecting the high rate of consanguinity in our patient population. Two affected brothers are homozygous for two different mutations expressed on the same allele. The patients comprise a distinct ethnic group (Israeli Arabs) residing in a confined geographic area. These results, which are supported by previous data, suggest for the first time that the phenomenon of multiple mutations in a relatively closed isolate is common and almost exclusive to the Israeli-Arab population. Potential mechanisms including selective advantage to heterozygotes, digenic inheritance, and the recent emergence of multiple mutations are discussed.

Evolution of alanine:glyoxylate aminotransferase intracellular targeting: structural and functional analysis of the guinea pig gene

The distribution of alanine:glyoxylate aminotransferase 1 (AGT) within liver cells has changed many times during mammalian evolution. Depending on the particular species, AGT can be found in mitochondria or peroxisomes, or mitochondria and peroxisomes. In some cases significant cytosolic AGT is also present. In the livers of most rodents, AGT has what is thought to be the more 'ancestral' distribution (i.e. mitochondrial and peroxisomal). However, AGT is distributed very differently in the guinea pig, being peroxisomal and cytosolic. In this study, we have attempted to determine the molecular basis for the loss of mitochondrial AGT targeting and the apparent inefficiency of peroxisomal targeting of AGT in the guinea pig. Our results show that the former is owing to the evolutionary loss of the more 5' of two potential transcription and translation initiation sites, resulting in the loss of the ancestral N-terminal mitochondrial targeting sequence from the open reading frame. Guinea pig AGT is targeted to peroxisomes via the peroxisomal targeting sequence type 1 (PTS1) peroxisomal import machinery, even though its C-terminal tripeptide, HRL, deviates from the standard consensus PTS1 motif. Although HRL appears to target AGT to peroxisomes less efficiently than the classical PTS1 SKL, the main reason for the low efficiency of AGT peroxisomal targeting in guinea pig cells (compared with cells from other species) lies not with guinea pig AGT but with some other, as yet undefined, part of the guinea pig peroxisomal import machinery.

A unique molecular basis for enzyme mistargeting in primary hyperoxaluria type 1

The intermediary metabolic enzyme alanine:glyoxylate aminotransferase (AGT) is normally targeted to the peroxisomes in human liver cells. However, in a third of patients suffering from the autosomal recessive disease primary hyperoxaluria type 1 (PH1), AGT is mistargeted to the mitochondria. Such organelle-to-organelle mistargeting is without parallel in human genetic disease. AGT mistargeting results from the combination of a common Pro11-->Leu polymorphism and a rare Gly170-->Arg mutation. The former generates a functionally weak mitochondrial targeting sequence (MTS) while the latter, in combination with the former, increases the efficiency of this MTS by slowing the rate at which AGT dimerises. The fact that the intracellular compartmentation of AGT can be determined, at least in part, by its oligomeric status highlights the fundamental differences in the molecular requirements for protein import into two intracellular organelles--the peroxisomes and mitochondria.

Primary hyperoxaluria type 1: diagnostic relevance of mutations and polymorphisms in the alanine:glyoxylate aminotransferase gene (AGXT)

Primary hyperoxaluria type 1 (PH1) is an autosomal recessive disorder of glyoxylate metabolism caused by deficiency of the hepatic peroxisomal enzyme alanine:glyoxylate aminotransferase (AGT). The disease shows considerable phenotypic, enzymatic and genetic heterogeneity. To date, 7 polymorphisms and 11 point mutations have been described in the gene encoding AGT. We report on the prevalence of these polymorphisms and mutations in 79 patients with PH1 with the aim of assessing their diagnostic relevance. A strong association of the C154T, intron 1 insertion and C386T polymorphisms is confirmed and this linkage extends to include the type 1 variant of a polymorphic tandem repeat in intron 4. Only 64 of 158 (40%) PH1 alleles have one of the defined mutations, with the G630A mutation accounting for 39 of these and T853C for 14. Overall only 20 (25%) of the patients studied had the genetic basis of their disease fully explained: 7 were homozygous for the G630A mutation, 5 were homozygous for the T853C mutation, 1 was homozygous for the C819T mutation, and 7 had two different mutations identified and were presumed to be compound heterozygotes. Only the two more frequent G630A and T853C mutations are of general diagnostic relevance for mutation screening. It seems likely that there are a significant number of other mutations, perhaps family-specific, still to be described. There was no apparent difference in the types of mutations in patients presenting in the first year of life (36%), suggesting that other factors, such as periods of dehydration or urinary tract infections, might contribute more to the clinical manifestation than genotype.

Primary hyperoxaluria in an adult with renal failure, livedo reticularis, retinopathy, and peripheral neuropathy

We present the case of a young woman who developed renal failure of unknown cause, and after 2 months of maintenance hemodialysis developed livedo reticularis, retinopathy, and peripheral sensory neuropathy. The patient was subsequently shown to have primary oxalosis type I, a rare autosomal recessive error of metabolism characterized by accumulation of calcium oxalate crystals in the kidneys, eyes, skin, and other organs. Intravascular obstruction, caused by deposition of calcium oxalate crystals in cutaneous arterioles, is thought to be responsible for the ischemic livedo reticularis lesions observed in this patient. A method is described for measuring serum glycolate by isotope dilution gas chromatography-mass spectrometry (GC-MS). An approach to the diagnosis and management is also briefly mentioned.

Primary hyperoxaluria type 1: a cluster of new mutations in exon 7 of the AGXT gene

Primary hyperoxaluria type 1 (PH1) is a severe autosomal recessive inborn error of glyoxylate metabolism caused by deficiency of the hepatic peroxisomal enzyme alanine:glyoxylate aminotransferase. This enzyme is encoded by the AGXT gene on chromosome 2q37.3. DNA samples from 79 PH1 patients were studied using single strand conformation polymorphism analysis to detect sequence variants, which were then characterised by direct sequencing and confirmed by restriction enzyme digestion. Four novel mutations were identified in exon 7 of AGXT: a point mutation T853C, which leads to a predicted Ile244Thr amino acid substitution, occurred in nine patients. Two other mutations in adjacent nucleotides, C819T and G820A, mutated the same codon at residue 233 from arginine to cysteine and histidine, respectively. The fourth mutation, G860A, introduced a stop codon at amino acid residue 246. Enzyme studies in these patients showed that AGT catalytic activity was either very low or absent and that little or no immunoreactive protein was present. Together with a new polymorphism in exon 11 (C1342A) these findings underline the genetic heterogeneity of the AGXT gene. The novel mutation T853C is the second most common mutation found to date with an allelic frequency of 9% and will therefore be of clinical importance for the diagnosis of PH1.

A vertical (pseudodominant) pattern of inheritance in the autosomal recessive disease primary hyperoxaluria type 1: lack of relationship between genotype, enzymic phenotype, and disease severity

Primary hyperoxaluria type 1 (PH1) is a rare autosomal recessive disease caused by a deficiency of alanine:glyoxylate aminotransferase (encoded by the AGXT gene). Primary hyperoxaluria type 1 is characterized by the elevated urinary excretion of oxalate and glycolate, and the deposition of insoluble calcium oxalate in the renal parenchyma and urinary tract. In the present study, we investigated an unusual family containing four affected individuals in two different generations. Based on our genetic, enzymic, metabolic, and clinical analyses, we have come to the following conclusions. First, although the pattern of inheritance of PH1 is usually horizontal (ie, all patients in the same generation), as expected for an autosomal recessive disease, it can sometimes show a vertical (pseudodominant) pattern of inheritance (ie, patients in more than one generation) due to the segregation within a family of three, rather than two, mutant AGXT alleles. Second, affected members of such a family can manifest very different clinical phenotypes both within and between generations. Although the clinical differences between generations might be at least partly due to differences in AGXT genotype, differences can equally occur within the same generation in individuals who possess the same AGXT genotype. Finally, individuals with PH1 at the level of the AGXT genotype might remain asymptomatic and undiagnosed for many years. The consequences of these findings for the clinical management and genetic counseling of families with PH1 are profound and wide-ranging.

Hyperoxaluria with hyperglycoluria not due to alanine:glyoxylate aminotransferase defect: a novel type of primary hyperoxaluria

Considering the clinical heterogeneity of primary hyperoxaluria type I (PH1) and the fact that in many instances this diagnosis was made without enzymatic and immunohistochemical investigation, other disturbances of oxalate metabolism than those presently known can be expected in PH1. Using a gaschromatographic/mass spectrometric method that allows quantification of these acids, hyperoxaluria and hyperglycoluria was found repeatedly in two unrelated patients. The hyperoxaluria was unresponsive to pyridoxine. There was no nephrocalcinosis or urolithiasis. In the liver biopsy normal AGT activity and normal localization of this enzyme in the peroxisome was found. In one patient abnormal Km and maximal activity and mozaicism of AGT were excluded. Hyperoxaluria and hyperglycoluria were also found in other family members, suggesting autosomal dominant transmission. Although the underlying defect leading to hyperoxaluria and hyperglycoluria could not be identified in these patients, it is probable that they represent a separate type of primary hyperoxaluria.

Molecular basis of the variable mitochondrial and peroxisomal localisation of alanine-glyoxylate aminotransferase

The molecular basis of the variable species-specific peroxisomal and/or mitochondrial targeting of the enzyme alanine-glyoxylate aminotransferase 1 (AGT) has been studied in human fibroblasts by confocal immunofluorescence microscopy after intranuclear microinjection of various human, rabbit, marmoset, and feline AGT cDNA constructs. The expression of full-length human and rabbit AGT cDNA led to an exclusively peroxisomal distribution of AGT. However, the distribution of feline and marmoset AGT depended on the cDNA construct injected. In both species, injection of the short cDNAs (from transcripts that occur naturally in marmoset liver but not in feline liver) led to an exclusively peroxisomal distribution. However, injection of the long cDNAs (from transcripts that occur naturally in both species) led to most of the AGT being targeted to the mitochondria and only a small, yet significant, fraction to the peroxisomes. Reintroduction of the 'ancestral' first potential translation initiation site into human AGT cDNA led to an 'ancestral' distribution of AGT (i.e. both mitochondrial and peroxisomal). Deletion of the second potential translation start site from the long feline cDNA led to a distribution that was almost entirely mitochondrial, which suggests that most peroxisomal AGT encoded by the long cDNA results from internal translation initiation from this site with the consequent loss of the N-terminal mitochondrial targeting sequence. Expression of rabbit cDNA and the short marmoset and feline cDNAs in cells selectively deficient in the import of peroxisomal matrix proteins showed that peroxisomal AGT in all these species is imported via the peroxisomal targeting sequence type 1 (PTS1) import pathway. The almost complete functional dominance of the N-terminal mitochondrial targeting sequence over the C-terminal PTS. which was not due to any direct interference of the former with peroxisomal import, was maintained even when the unusual PTS1 of AGT (KKL in human) was replaced by the prototypical PTS1 SKL. The results demonstrate that the major determinant of alanine-glyoxylate aminotransferase subcellular distribution in mammals is the presence or absence of the mitochondrial targeting sequence rather than the peroxisomal targeting sequence. Various strategies have arisen during the evolution of mammals to enable the exclusion of the mitochondrial targeting sequence from the newly synthesised polypeptide, all of which involve the use of alternative transcription and/or translation initiation sites.

[Infantile forms of primary hyperoxaluria type I: apropos of 4 cases]

BACKGROUND

Approximately 10% of the cases with primary hyperoxaluria type I present before age 1 and are usually classified as neonatal primary oxaluria.

CASE REPORTS

Four unrelated infants, aged 3 to 9 months, were admitted for severe renal failure due to primary hyperoxaluria type I. Other affected members were known in two of these four families but the disease was not present at the same age in each family. Echogenicity of kidneys was increased in all the patients and calcium oxalate crystals were seen in the collecting system and within the renal parenchyma. Urolithiasis was not present. Treatment of renal failure, ie, peritoneal dialysis, was uneffective in one patient and was interrupted in two others because organ transplantation was impossible.

CONCLUSIONS

The infantile forms of primary hyperoxaluria type I may be rapidly complicated by severe renal failure in the absence of urolithiasis.

Strategies for the prenatal diagnosis of primary hyperoxaluria type 1

Primary hyperoxaluria type 1 (PH1) is a potentially lethal autosomal recessive disorder of glyoxylate metabolism caused by a deficiency of the liver-specific peroxisomal enzyme alanine:glyoxylate aminotransferase (AGT). Over the past 13 years, various strategies have been adopted for its prenatal diagnosis, including (1) glyoxylate metabolite analysis of amniotic fluid in the second trimester; (2) AGT enzyme assay, immunoassay, and immuno-electron microscopy of fetal liver biopsies also in the second trimester; and (3) linkage and mutation analysis of DNA isolated from chorionic villus samples in the first trimester. These methods have evolved in parallel with our increased understanding of the molecular aetiology and pathogenesis of the disease. Although the usefulness of metabolite analysis remains unproven, all the other methods have been successfully applied to the prenatal diagnosis of PH1. In this review, examples of the use of the available methodologies are provided, and their pros and cons are discussed with reference to specific cases.

Mapping of human non-muscle type cofilin (CFL1) to chromosome 11q13 and muscle-type cofilin (CFL2) to chromosome 14

Cofilin is a widely-distributed, intracellular, actin binding protein which is involved in the translocation of actin-cofilin complex from cytoplasm to nucleus. We have cloned a non-muscle-type cofilin (CFL1) from a human promyelocytic cDNA library and mapped this to human chromosome 11 by PCR amplification of 3' untranslated sequence in a panel of rodent-human somatic cell hybrids, and to the interval 11q12-q13.2 in a chromosome 11 somatic cell hybrid mapping panel. Confirmation of regional localisation to 11q13 has been obtained by fluorescent in situ hybridisation of genomic cosmid clones, by demonstration of the presence of both SEA (the human homologue of avian retrovirus proviral tyrosine kinase, 11q13) and CFL1 in some of these clones and by close linkage of CFL1 to SEA in a panel of high-dose irradiation hybrids. We have identified human muscle-type cofilin sequences by comparison of human expressed sequence tags with M-type cofilins of other species and we have mapped the human M-type cofilin, CFL2, to chromosome 14.

Mammalian alanine/glyoxylate aminotransferase 1 is imported into peroxisomes via the PTS1 translocation pathway. Increased degeneracy and context specificity of the mammalian PTS1 motif and implications for the peroxisome-to-mitochondrion mistargeting of AGT in primary hyperoxaluria type 1

Alanine/glyoxylate aminotransferase 1 (AGT) is peroxisomal in most normal humans, but in some patients with the hereditary disease primary hyperoxaluria type 1 (PH1), AGT is mislocalized to the mitochondria. In an attempt to identify the sequences in AGT that mediate its targeting to peroxisomes, and to determine the mechanism by which AGT is mistargeted in PH1, we have studied the intracellular compartmentalization of various normal and mutant AGT polypeptides in normal human fibroblasts and cell lines with selective deficiencies of peroxisomal protein import, using immunofluorescence microscopy after intranuclear microinjection of AGT expression plasmids. The results show that AGT is imported into peroxisomes via the peroxisomal targeting sequence type 1 (PTS1) translocation pathway. Although the COOH-terminal KKL of human AGT was shown to be necessary for its peroxisomal import, this tripeptide was unable to direct the peroxisomal import of the bona fide peroxisomal protein firefly luciferase or the reporter protein bacterial chloramphenicol acetyltransferase. An ill-defined region immediately upstream of the COOH-terminal KKL was also found to be necessary for the peroxisomal import of AGT, but again this region was found to be insufficient to direct the peroxisomal import of chloramphenicol acetyltransferase. Substitution of the COOH-terminal KKL of human AGT by the COOH-terminal tripeptides found in the AGTs of other mammalian species (SQL, NKL), the prototypical PTS1 (SKL), or the glycosomal PTS1 (SSL) also allowed peroxisomal targeting, showing that the allowable PTS1 motif in AGT is considerably more degenerate than, or at least very different from, that acceptable in luciferase. AGT possessing the two amino acid substitutions responsible for its mistargeting in PH1 (i.e., Pro11-->Leu and Gly170-->Arg) was targeted mainly to the mitochondria. However, AGTs possessing each amino acid substitution on its own were targeted normally to the peroxisomes. This suggests that Gly170-->Arg-mediated increased functional efficiency of the otherwise weak mitochondrial targeting sequence (generated by the Pro11-->Leu polymorphism) is not due to interference with the peroxisomal targeting or import of AGT.

Large-scale cloning of human chromosome 2-specific yeast artificial chromosomes (YACs) using an interspersed repetitive sequences (IRS)-PCR approach

We report here an efficient approach to the establishment of extended YAC contigs on human chromosome 2 by using an interspersed repetitive sequences (IRS)-PCR-based screening strategy for YAC DNA pools. Genomic DNA was extracted from 1152 YAC pools comprised of 55,296 YACs mostly derived from the CEPH Mark I library. Alu-element-mediated PCR was performed for each pool, and amplification products were spotted on hybridization membranes (IRS filters). IRS probes for the screening of the IRS filters were obtained by Alu-element-mediated PCR. Of 708 distinct probes obtained from chromosome 2-specific somatic cell hybrids, 85% were successfully used for library screening. Similarly, 80% of 80 YAC walking probes were successfully used for library screening. Each probe detected an average of 6.6 YACs, which is in good agreement with the 7- to 7.5-fold genome coverage provided by the library. In a preliminary analysis, we have identified 188 YAC groups that are the basis for building contigs for chromosome 2. The coverage of the telomeric half of chromosome 2q was considered to be good since 31 of 34 microsatellites and 22 of 23 expressed sequence tags that were chosen from chromosome region 2q13-q37 were contained in a chromosome 2 YAC sublibrary generated by our experiments. We have identified a minimum of 1610 distinct chromosome 2-specific YACs, which will be a valuable asset for the physical mapping of the second largest human chromosome.

DNA diagnosis of X-linked adrenoleukodystrophy

The X-linked adrenoleukodystrophy (ALD) gene was identified recently and is predicted to encode a 745-amino-acid peroxisomal membrane protein. Strategies have been designed for the search for mutations in the ALD gene in patients. Several mutations have now been found and it seems that many different mutations are responsible for ALD. There is no straightforward correlation between genotype and phenotype since the same mutation can cause different ALD phenotypes in the same family. However, once a mutation has been found in a family, it can be traced in all at-risk individuals of that family, both post- and prenatally, without the need for very long-chain fatty acid (VLCFA) analysis. Segregation analysis with extragenic and intragenic polymorphisms may remain useful in families where mutation analysis is not possible for practical reasons; VLCFA analysis and measurement of the peroxisomal beta-oxidation with C26:0 fatty acid as a substrate will remain the alternative. We also briefly discuss the possibilities of DNA diagnosis for other peroxisomal disorders.

Molecular characterization and clinical use of a polymorphic tandem repeat in an intron of the human alanine:glyoxylate aminotransferase gene

The autosomal recessive disease primary hyperoxaluria type 1 (PH1) is caused by a deficiency of the liver-specific peroxisomal enzyme alanine:glyoxylate amino-transferase (AGT). This paper concerns the identification, characterization and clinical use of an unusual discretely polymorphic tandem repeat sequence in the fourth intron of the human AGT gene (gene locus designation AGXT). In a random Caucasian population, three alleles could be clearly recognized that consisted of either 12 (type III), 17 (type II) or approximately 38 (type I) tandemly repeated copies of a highly conserved 29/32-bp sequence with frequencies of 33%, 7% and 60%, respectively. In a random Japanese population, the allelic frequencies were markedly different (i.e. 31%, 45% and 19%, respectively). In addition, a fourth allele was identified, consisting of approximately 32 repeats (type IV), with an allelic frequency of approximately 5% in Japanese. The repetitive sequence was similar to previously identified mammalian sequences with homology to the Epstein-Barr virus IR3 repetitive element involving a 12/15-bp region GCA(GGN)GGAGGAGGG within the repeat unit. This IR3-like sequence was interspersed with a 17-bp sequence with no similarity to any currently known repetitive element. The type I and type III alleles were judged to be equivalent to a previously identified TaqI polymorphism. Two polymorphisms previously shown to be associated with the peroxisome-to-mitochondrion mistargeting of AGT in PH1 (a C154-->T point substitution in exon 1 and a 74-bp duplication in intron 1) were found to segregate exclusively with the type I intron 4 polymorphism in Caucasians, but not in Japanese. The polymorphic nature of the intron 4 tandem repeats makes them of potential use in the prenatal diagnosis of PH1, especially when coupled with the exon 1 C154-->T substitution or intron 1 duplication polymorphisms. A PH1 family, in which a fetus had been predicted previously to be either normal or a carrier by AGT enzymic analysis of a fetal liver biopsy, but who had been shown to be only partially informative with respect to the C154-->T/intron 1 polymorphisms, was analysed retrospectively. The family was completely informative for the intron 4 tandem repeat polymorphism and the carrier status of the fetus was confirmed.

Adrenoleukodystrophy and other peroxisomal diseases

The gene predisposing for X-linked adrenoleukodystrophy (ALD), the most common peroxisomal disorder, has been identified recently by positional cloning. The ALD protein is a 75 kDa peroxisomal membrane protein belonging to the family of ATP-binding cassette transporter proteins. With the combination of genetic complementation and candidate gene approaches, two genes responsible for Zellweger syndrome, a group of genetically heterogeneous disorders affecting peroxisome biogenesis, have also been identified.

Molecular evolution of alanine/glyoxylate aminotransferase 1 intracellular targeting. Analysis of the feline gene

The subcellular distribution of hepatic alanine:glyoxylate aminotransferase 1 (AGT) has changed, under the influence of dietary selection pressure, on several o occasions during the evolution of mammals. In some species (e.g. human and rabbit) AGT is entirely peroxisomal; in other species (e.g. marmoset and rat) this enzyme is found in similar amounts in peroxisomes and mitochondria; in yet other species (e.g. cat) it is mainly mitochondrial. The molecular basis of the species-specific dual intracellular targeting of AGT has been partially elucidated in the human and rabbit (as examples of the first group), and in the rat and marmoset (as examples of the second group). As part of a wider study on the molecular evolution of AGT intracellular targeting, we report in the present paper the results of an investigation into the molecular basis of the subcellular distribution of AGT in the cat (as an example of the third group). Cat liver AGT cDNA has been cloned and sequenced, and shown to have a high degree of similarity to AGT from human, rabbit, marmoset and rat. Southern-blotting analysis showed that AGT in the cat is probably encoded by a single gene, as it is in other species. Transcript analysis by RNase protection indicated that almost all of the AGT mRNA would possess an open reading frame encoding a polypeptide of 414 amino acids and a molecular mass of 45,508 Da. The N-terminal 22 amino acids comprised the putative mitochondrial-targeting sequence (by analogy with the equivalent sequence in marmoset and rat pre-mitochondrial AGT). The very low level of peroxisomal AGT in cat liver is compatible with the absence of any RNase-protected transcripts initiating downstream of the first putative translation initiation codon (i.e. absence of any transcripts in which the mitochondrial-targeting sequence is excluded from the open reading frame). In vitro studies showed that the 45 kDa polypeptide was imported into rat liver mitochondria and processed to a mature protein of approximately 43 kDa, compatible with the cleavage of the N-terminal 22 amino acids, as is also the case in rat and marmoset. A polypeptide in which the N-terminal 22 amino acids was absent could not be imported into mitochondria in vitro.

Cytosolic compartmentalization of hepatic alanine:glyoxylate aminotransferase in patients with aberrant peroxisomal biogenesis and its effect on oxalate metabolism

Two patients with atypical manifestations of aberrant peroxisomal biogenesis are described. Contrary to previous studies, which had shown that Zellweger syndrome patients usually have normal levels of urinary oxalate excretion, the patients in the present study had evidence of abnormal oxalate metabolism in the form of hyperoxaluria and, in one of the patients, calcium oxalate urolithiasis. Activity of the liver-specific peroxisomal enzyme alanine:-glyoxylate aminotransferase (AGT), which is a major determinant of the level of endogenous oxalate synthesis in humans, was normal in one patient and markedly supranormal in the other. Using the technique of post-embedding protein A-colloidal gold immunoelectron microscopy, AGT was found to be mainly cytosolic in the livers of both patients, with significant amounts also localized in the nuclei. In a small minority of the hepatocytes of one patient, who was homozygous for the more common (major) AGT allele, large numbers of unidentified fibrillar arrays were found in the cytosol, which labelled heavily for immunoreactive AGT. The background cytosolic AGT labelling was markedly reduced in such cells when compared to the majority of cells that did not contain fibrils. In the other patient, who was heterozygous for the major and minor AGT alleles, there appeared to be low levels of mitochondrial AGT labelling. In the light of these data, the possible metabolic function of cytosolic AGT in the livers of panperoxisomal disease patients is discussed.

Primary hyperoxaluria type 1: genotypic and phenotypic heterogeneity

Primary hyperoxaluria type 1 (PH1) is an autosomal recessive disease caused by a deficiency of the liver-specific peroxisomal enzyme alanine: glyoxylate aminotransferase (AGT). The disease is notable for its extensive heterogeneity at the clinical, biochemical, enzymic and molecular genetic levels. A study of 116 PH1 patients over the past 8 years has revealed four main enzymic phenotypes: (1) absence of both AGT catalytic activity and immunoreactive AGT protein (approximately 40% of patients); (2) absence of AGT catalytic activity but presence of immunoreactive protein (approximately 16% of patients); (3) presence of both AGT catalytic activity and immunoreactive protein (approximately 41% of patients), in most of which cases the AGT is mistargeted to the mitochondria instead of the peroxisomes; and (4) a variation of the mistargeting phenotype in which AGT is equally distributed between peroxisomes and mitochondria, but in which that in the peroxisomes is aggregated into matrical core-like structures (approximately 3% of patients). Various point mutations, all occurring at conserved positions in the coding regions of the AGT gene, have been identified in these patients. The five mutations discussed in the present study, which have been found in individuals manifesting all of the four major enzymic phenotypes, account for the expressed alleles in about half of all Caucasian PH1 patients. The most common mutation found so far leads to a Gly170-->Arg amino acid substitution. This mutation, in combination with a normally occurring Pro11-->Leu polymorphism, appears to be responsible for the unprecedented peroxisome-to-mitochondrion mistargeting phenotype.

Localization of the Tamm-Horsfall glycoprotein (uromodulin) gene to chromosome 16p12.3-16p13.11

Mapping studies using a panel of 22 rodent-human somatic cell hybrids have helped to localize the Tamm-Horsfall glycoprotein (uromodulin) gene (UMOD), which has previously been reported to map to 16p13.11, to the region 16p12.3-qter. The combined results indicate that UMOD is located distal to D16S295 and proximal to D16S287 and in the region 16p12.3-16p13.11. Uromodulin is known to affect the formation of calcium-containing kidney stones, and this localization of UMOD will help in studies of families with autosomal forms of nephrolithiasis.

Regional localization of the lactase-phlorizin hydrolase gene, LCT, to chromosome 2q21

The gene LCT which codes for the intestinal disaccharidase lactase-phlorizin hydrolase has previously been mapped, using somatic cell hybrids and linkage analysis, using the CEPH families, to chromosome 2. We describe here the regional localization of LCT to chromosome 2q21 by polymerase chain reaction (PCR) analysis of somatic cell hybrids and in situ hybridization. LCT is closely linked to D2S44, with a lod score of 30.6 at theta = 0.10.

Mapping of RXRB to human chromosome 6p21.3

Retinoid X Receptor beta (RXRB) is a member of the retinoid X receptor (RXR) family of nuclear receptors which are involved in mediating the effects of retinoic acid (RA). We have confirmed the localization of RXRB to chromosome 6 and we have mapped the gene to chromosome 6p21.3-p21.1 by PCR amplification of 5' untranslated sequence in panels of rodent-human somatic cell hybrids and to 6p21.3 by fluorescent in situ hybridization.

Description of a dinucleotide repeat polymorphism in the human elastin gene and its use to confirm assignment of the gene to chromosome 7

Informative polymorphisms have been very difficult to detect in the elastin gene, and this has hampered the analysis of heritable connective tissue disorders, notably the Marfan syndrome. We have recently detected a dinucleotide repeat polymorphism in intron 17 of the human elastin gene consisting of 8 alleles with sizes between 161 and 175 bp. Analysis of 540 chromosomes from unrelated Caucasian individuals revealed a bimodal frequency distribution typical of (dC-dA)n.(dG-dT)n repeat polymorphisms, with allele frequencies ranging from 0.004 (161 bp) to 0.574 (163 bp). As the elastin gene was originally assigned to chromosome 2q31-ter and because more recent data have suggested an assignment to 7q11.1-21.1, we have genotyped a sub-set of the CEPH pedigrees and carried out pairwise linkage analysis with markers on chromosomes 7 and 2. Lod-scores of between +3.70 and +13.69 were obtained with markers spanning 7p13-q22.1, whilst negative lod-scores were observed with the chromosome 2 markers. Analysis of type II human ovarian teratomas placed the elastin gene within 11 cM of the centromere on chromosome 7. Additionally, we detected the dinucleotide repeat in human-rodent cell hybrids containing chromosome 7, but not those containing chromosome 2. These data confirm the assignment of elastin to chromosome 7 and provide a new, highly informative marker for the analysis of heritable disorders of connective tissue for which elastin is a candidate gene.

A hamster-human subchromosomal hybrid cell panel for chromosome 2

We have constructed hamster-human hybrid cell lines containing fragments of human chromosome 2 as their only source of human DNA. Microcell-mediated chromosome transfer was used to transfer human chromosome 2 from a monochromosomal mouse-human hybrid line to a radiation-sensitive hamster mutant (XR-V15B) defective in double-strand break rejoining. The human chromosome 2 carried the Ecogpt gene and hybrids were selected using this marker. The transferred human chromosome was frequently broken, and the resulting microcell hybrids contained different sized segments of the q arm of chromosome 2. Two microcell hybrids were irradiated and fused to XR-V15B to generate additional hybrids bearing reduced amounts of human DNA. All hybrids were analyzed by PCR using primers specific for 27 human genes located on chromosome 2. From these data we have localized the integrated gpt gene on the human chromosome 2 to the region q36-37 and present a gene order for chromosome 2 markers.

Human regeneration protein/lithostathine genes map to chromosome 2p12

The pancreatic stone protein (lithostathine) secreted by the exocrine pancreas is an inhibitor of CaCO3 crystal growth. This protein, which is also present in endocrine pancreas, has also been called the regeneration protein (reg). Here we report the mapping of the REG gene to chromosome 2 using the polymerase chain reaction for the specific amplification of human reg sequences in rodent/human somatic cell hybrid DNA. A regional assignment has been made by in situ hybridization to metaphase chromosomes using two different fluorescently labelled genomic probes corresponding to the REG gene and a related gene REGL. Both probes hybridized to chromosome 2p12 suggesting the tandem organization of these genes.

Primary hyperoxaluria type 1 and peroxisome-to-mitochondrion mistargeting of alanine:glyoxylate aminotransferase

Under the influence of dietary selection pressure, the intracellular compartmentalization of alanine:glyoxylate aminotransferase (AGT) has changed on many occasions during the evolution of mammals. In some mammals, AGT is peroxisomal in others it is mainly mitochondrial while in yet others it is more-or-less equally divided between both organelles. Although in normal human liver AGT is usually found exclusively within the peroxisomes, in some individuals a small proportion (approximately 5%) is found also in the mitochondria. This apparently trivial intracellular redistribution of AGT is caused by the presence of a Pro11Leu polymorphism which allows the N-terminus of AGT to fold into a conformation (ie a positively-charged amphiphilic alpha-helix) which functions as a mitochondrial targeting sequence. In one third of patients with the autosomal recessive disease primary hyperoxaluria type 1, there is a further redistribution of AGT so that the great majority (approximately 90%) is located in the mitochondria and only a small minority (10%) in the peroxisomes. AGT cannot fulfil its proper metabolic role in human liver (ie glyoxylate detoxification) when located in the mitochondria. The erroneous compartmentalization is due to the presence of a Gly170Arg mutation superimposed upon the Pro11Leu polymorphism. The Gly170Arg mutation appears to have no direct effect on mitochondrial targeting and is predicted to enhance mitochondrial import of AGT by interfering with its peroxisomal targeting and/or import. The mitochondrial targeting sequence generated by the Pro11Leu polymorphism is not homologous to that found in the AGT of other mammals which localise AGT within the mitochondria normally. The identity of the peroxisomal targeting sequence in AGT is unknown, but the Gly170Arg mutation is found in a highly conserved region of the protein which might be involved in some aspects of the peroxisomal import pathway for AGT.

Localization of the t(2;13) breakpoint of alveolar rhabdomyosarcoma on a physical map of chromosome 2

A characteristic translocation t(2;13)(q35;q14) has been previously identified in the pediatric soft tissue tumor alveolar rhabdomyosarcoma. We have assembled a panel of lymphoblast, fibroblast, and somatic cell hybrid cell lines with deletions and unbalanced translocations involving chromosome 2 to develop a physical map of the distal 2q region. Twenty-two probes were localized on this physical map by Southern blot analysis of the mapping panel. The position of these probes with respect to the t(2;13) rhabdomyosarcoma breakpoint was then determined by quantitative Southern blot analysis of an alveolar rhabdomyosarcoma cell line with two copies of the derivative chromosome 13 and one copy of the derivative chromosome 2 and by analysis of somatic cell hybrid clones derived from an alveolar rhabdomyosarcoma cell line. We demonstrate that the t(2;13) breakpoint is situated within a map interval delimited by the distal deletion breakpoint in fibroblast line GM09892 and the t(X;2) breakpoint in somatic cell hybrid GM11022. Furthermore, from a comparison of our data with the linkage map of the syntenic region on mouse chromosome 1, we conclude that the t(2;13) breakpoint is most closely flanked by loci INHA and ALPI within this map interval.

Molecular evolution of alanine/glyoxylate aminotransferase 1 intracellular targeting. Analysis of the marmoset and rabbit genes

In mammals, the subcellular distribution of alanine:glyoxylate aminotransferase 1 (AGT) is species dependent, with the proportion of AGT targeted to mitochondria varying between 0% and greater than 90%, the remainder being located in the peroxisome. In order to extend our studies on the molecular evolution of intracellular targeting of AGT, we have investigated the organization and expression of the AGT genes of rabbit, which has all of its AGT located in the peroxisome, and marmoset, which has approximately 50% of its AGT located in the peroxisome and 50% in the mitochondrion. Southern-blot analysis indicates that, in both of these species, AGT is encoded by a single-copy gene, as has previously been shown for human (all AGT in the peroxisome) and rat (50% AGT in the peroxisome and 50% in the mitochondrion). Comparison of the cDNA sequences encoding marmoset, rabbit, human and rat AGT, combined with transcript mapping and in vitro mitochondrial protein-import analysis, has provided a molecular explanation for the differential targeting of AGT in these species. As in the rat, marmoset AGT is synthesized in two forms, via the use of alternative transcription and translation-initiation sites. These two forms of AGT differ only in the presence or absence of a 22-amino-acid amino-terminal peptide, which acts as a cleavable mitochondrial-targeting sequence, directing the longer form of AGT to mitochondria. The shorter form of AGT, lacking the mitochondrial-targeting sequence, is presumed to be localized in the peroxisomes. In humans and rabbits, similar but distinct evolutionary mutational events within the AGT gene have resulted in exclusion of the region encoding the mitochondrial-targeting sequence from the open reading frame, explaining the exclusive peroxisomal localization of AGT in these species. We discuss the impact of these results on our understanding of both the evolution of species dependence of AGT subcellular distribution and the recent identification of amino acid changes in human AGT which result in mistargeting of this protein to mitochondria.

Nucleotide sequence and chromosomal assignment of a cDNA encoding the large isoform of human glutamate decarboxylase

Glutamic acid decarboxylase (GAD) catalyses the conversion of L-glutamic acid to the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). Two forms of human GAD, GAD65 and GAD67, are encoded by two separate genes. A full length human GAD67 cDNA has been isolated from a human frontal cortex cDNA library and the nucleotide sequence determined. The GAD67 gene has been mapped to chromosome 2 using the polymerase chain reaction to amplify specifically the human sequence in rodent/human somatic cell hybrid DNA. This confirms that human GAD67 is not syntenic with the smaller GAD isoform GAD65 which has been assigned to chromosome 10. Production of polyclonal antiserum to a baculovirus-expressed GAD67 enabled immunocytological detection of GAD in the rat brain.

A single serine:pyruvate aminotransferase gene on rat chromosome 9q34-q36

It was found in our previous study (Oda et al., 1990. J. Biol. Chem. 265: 7513-7519) that in the rat two mRNAs encoding mitochondrial and peroxisomal serine:pyruvate aminotransferase (SPT/AGT) are formed from a single SPT/AGT gene through alternative transcription initiation in exon 1. In an attempt to analyze the mechanisms underlying this unique phenomenon, we have isolated genomic clones harboring the entire rat SPT/AGT gene. In the present study, the location of the rat SPT/AGT gene was determined to be in the q34-q36 region of chromosome 9 by fluorescence in situ hybridization. Southern blot analysis of rat genomic DNA revealed an allelic BamHI restriction fragment length polymorphism among three different inbred rat strains. These results indicated that a single copy SPT/AGT gene is located on chromosome 9q34-q36 in the rat genome. This locus has been assigned the gene symbol Spat.

A glycine-to-glutamate substitution abolishes alanine:glyoxylate aminotransferase catalytic activity in a subset of patients with primary hyperoxaluria type 1

We have synthesized and sequenced alanine:glyoxylate aminotransferase (AGT; HGMW-approved symbol for the gene--AGXT) cDNA from the liver of a primary hyperoxaluria type 1 (PH1) patient who had normal levels of hepatic peroxisomal immunoreactive AGT protein, but no AGT catalytic activity. This revealed the presence of a single point mutation (G----A at cDNA nucleotide 367), which is predicted to cause a glycine-to-glutamate substitution at residue 82 of the AGT protein. This mutation is located in exon 2 of the AGT gene and leads to the loss of an AvaI restriction site. Exon 2-specific PCR followed by AvaI digestion showed that this patient was homozygous for this mutation. In addition, three other PH1 patients, one related to and two unrelated to, but with enzymological phenotype similar to that of the first patient, were also shown to be homozygous for the mutation. However, one other phenotypically similar PH1 patient was shown to lack this mutation. The mechanism by which the glycine-to-glutamate substitution at residue 82 causes loss of catalytic activity remains to be resolved. However, the protein sequence in this region is highly conserved between different mammals, and the substitution at residue 82 is predicted to cause significant local structural alterations.

Chromosomal assignment of human phenol and bilirubin UDP-glucuronosyltransferase genes (UGT1A-subfamily)

DNA probes were prepared from the 5'-terminal portion of four cDNA clones encoding human phenol and bilirubin UDP-glucuronosyltransferases (UGTs). An additional sequence common to all four clones was isolated from the 3'-terminal portion of one of the clones (UGT1A1). The four specific and the one common DNA sequences were used as probes on a panel of 16 human--rodent somatic cell hybrid DNAs by Southern-blot analysis. The results obtained indicate that all four cDNA clones are encoded by gene(s) located on human chromosome 2.

Molecular cloning of the human Goodpasture antigen demonstrates it to be the alpha 3 chain of type IV collagen

To characterize the autoantigen of Goodpasture's (anti-glomerular basement membrane) disease, a molecule of 26-kD reactive with autoantibodies from patients' sera was purified from collagenase digests of sheep glomerular basement membrane. Short internal amino acid sequences were obtained after tryptic or cyanogen bromide cleavage, and used to deduce redundant oligonucleotides for use in the polymerase chain reaction on cDNA derived from sheep renal cortex. Molecules of 175 bp were amplified and found to come from two cDNA sequences. One was identical to that of a type IV collagen chain (alpha 5) cloned from human placenta and shown to be expressed in human kidney. The other was from a type IV collagen chain with close similarities to alpha 1 and alpha 5 chains, and was used to obtain human cDNA sequences by cDNA library screening and by further polymerase chain reaction amplifications. The correspondence of the derived amino acid sequence of the new chain with published protein and cDNA sequences shows it to be the alpha 3 chain of type IV collagen. Its gene, COL4A3, maps to 2q36-2q37. The primary sequence and other characteristics of this chain confirm that it carries the Goodpasture antigen.

Alanine glyoxylate aminotransferase deficiency: biochemical and molecular genetic lessons from the study of a human disease

The decision to treat a patient with primary hyperoxaluria type 1 (PHI) by combined liver and kidney transplantation, the former to correct the metabolic lesion which was then thought to be deficiency of cytoplasmic 2-oxoglutarate:glyoxylate carboligase, and the latter to replace the organ which is destroyed, provided an opportunity to investigate the disease by modern biochemical methods. It was shown that 2-oxoglutarate:glyoxylate carboligase (the first decarboxylating component of 2-oxoglutarate dehydrogenase) is entirely mitochondrial so that deficiency of a cytoplasmic form of this enzyme could not be the cause of PHI. The deficient enzyme proved to be hepatic peroxisomal alanine:glyoxylate aminotransferase (AGT). The disease can be diagnosed enzymologically on percutaneous liver biopsies and this is possible for the fetus in utero. There are four types of genetically determined heterogeneity in PHI:(1) responsiveness and non-responsiveness to pharmacological doses of pyridoxine, in terms of an effect on the rate of oxalate production; (2) the presence or absence of residual catalytic AGT activity; (3) CRM+ and CRM-variants; (4) locational variation by virtue of which the enzyme (AGT) is mitochondrial and not peroxisomal. About one third of patients with PHI have residual AGT activity and at least a large proportion of these have mitochondrial and not peroxisomal AGT. The molecular features which guide peroxisomal and mitochondrial enzymes from their sites of synthesis into the appropriate organelle are reviewed and the possibilities for genetic variation in the relevant parts of the AGT molecule are discussed. The gene directing the synthesis of AGT has been cloned and sequenced, as has the AGT cDNA from a patient with mitochondrial AGT. Three point mutations causing amino acid substitution in the predicted AGT protein sequence have been identified: proline----leucine at residue 11, glycine----arginine at residue 170 and isoleucine----methionine at residue 340. The present evidence based on screening PHI patients and control subjects suggest that the substitution at residue 11, which cosegregates with that at residue 340, generates an amphiphilic alpha-helix which resembles mitochondrial targeting sequences but that misrouting of all the newly synthesized AGT into mitochondria requires the substitution at residue 170 which may act by impeding the entry of the enzyme into peroxisomes. The recognition of enzyme locational heterogeneity in PHI due to mutations affecting leader sequences should encourage a search for similar metabolic lesions in other inborn errors of metabolism affecting peroxisomal and/or mitochondrial enzymes.

The gene for human neurone specific ubiquitin C-terminal hydrolase (UCHL1, PGP9.5) maps to chromosome 4p14

Ubiquitin carboxy terminal hydrolase 1, UCHL1, is a neurone-specific protein involved in the ubiquitin-mediated proteolytic pathway. The gene for human UCHL1 has been mapped to chromosome 4 using the polymerase chain reaction to amplify specifically the human UCHL1 sequences in rodent/human somatic cell hybrid DNA. A regional assignment of this locus to 4p14 has been made by in situ hybridization to metaphase chromosomes using both tritium and fluorescently labelled probes.