Investigation of gyrate atrophy using a cDNA clone for human ornithine aminotransferase

A possible ocular biomarker for response to hyperornithinemia in gyrate atrophy: the effect of pyridoxine, lysine, and arginine-restricted diet in a patient with advanced disease

BACKGROUND

Loss of function variants in the ornithine aminotransferase (OAT) gene cause accumulation of ornithine levels, leading to gyrate atrophy. The benefit of ornithine-lowering therapies has been documented in a mouse model and young patients, however, the effect in adults with advanced disease has not been well described.

MATERIALS AND METHODS

Case report of an adult patient with advanced gyrate atrophy, who underwent treatment with pyridoxine and an arginine-restricted diet for four years.

RESULTS

A 51-year-old female with advanced chorioretinal degeneration presented with hyperornithinemia (961 vs. normal 18-135 µmol/L) and compound heterozygous pathogenic variants in OAT (p.Tyr299* and p.Ala270Pro). Treatment with pyridoxine and arginine-diet restriction yielded a maximal reduction in ornithine levels by 71% (275 µmol/L). Optical coherence tomography (OCT) showed a reduction in ellipsoid zone (EZ) thickness that correlated with lower ornithine levels and reversed with higher ornithine levels. While her best-corrected visual acuity remained unchanged, the progressive decline in her visual fields appeared to stabilize during a one-year period when ornithine levels were below 500 µmol/L.

CONCLUSIONS

In this report, we demonstrate that chorioretinal degeneration appears to stabilize in an adult patient with gyrate atrophy in association with a partial reduction in ornithine levels. We also observed a correlation with reduced EZ thickness on OCT and propose this may be a novel biomarker for ornithine reduction therapies. Our case study characterizes the potential retinal structure-function benefits of ornithine-lowering treatments even in cases of advanced chorioretinal degeneration. Thus, we recommend a low threshold for treating all patients with gyrate atrophy.

"Eye genetics at the fork in the road" 2017 Franceschetti Lecture, Leeds UK

Inherited retinal diseases - a disparate group of eye disorders with over 200 known genetic causes - are now the leading cause of blindness in working-age adults in developed countries. Until recently there was no cure for genetic eye diseases. After over a century of defining inherited retinal diseases with their phenotypes, and then several decades of discovering associated genes and their mutations, we now have gene therapy, stem cell therapy, predictive DNA testing and a revolution in adaptive computer technology. With the explosion of expensive treatment options, we need to consider whether finite resources should go towards treatment, prevention or rehabilitation or an amalgamation of all three. In addition, although evidence-based medicine is the goal, how do we direct our desperate patients towards genuine clinical trials and away from quackery? How do we provide scientifically valid treatments for eye diseases too rare to run proper trials and then capture the results of "off label treatments"?

Ornithine-δ-Aminotransferase Inhibits Neurogenesis During Xenopus Embryonic Development

PURPOSE

In humans, deficiency of ornithine-δ-aminotransferase (OAT) results in progressive degeneration of the neural retina (gyrate atrophy) with blindness in the fourth decade. In this study, we used the Xenopus embryonic developmental model to study functions of the OAT gene on embryonic development.

METHODS

We cloned and sequenced full-length OAT cDNA from Xenopus oocytes (X-OAT) and determined X-OAT expression in various developmental stages of Xenopus embryos and in a variety of adult tissues. The phenotype, gene expression of neural developmental markers, and enzymatic activity were detected by gain-of-function and loss-of-function manipulations.

RESULTS

We showed that X-OAT is essential for Xenopus embryonic development, and overexpression of X-OAT produces a ventralized phenotype characterized by a small head, lack of axial structure, and defective expression of neural developmental markers. Using X-OAT mutants based on mutations identified in humans, we found that substitution of both Arg 180 and Leu 402 abrogated both X-OAT enzymatic activity and ability to modulate the developmental phenotype. Neurogenesis is inhibited by X-OAT during Xenopus embryonic development.

CONCLUSIONS

Neurogenesis is inhibited by X-OAT during Xenopus embryonic development, but it is essential for Xenopus embryonic development. The Arg 180 and Leu 402 are crucial for these effects of the OAT molecule in development.

Gyrate atrophy of the choroid and retina diagnosed by ornithine-δ-aminotransferase gene analysis: a case report

A pair of 19-year-old female identical twins was referred to our hospital with progressive visual loss. They exhibited bilateral chorioretinal atrophy involving the midperiphery on fundoscopy and fluorescein angiography. Bilateral visual field constriction was noted on dynamic Goldmann perimetry, and a markedly impaired response was observed on both photopic and scotopic electroretinograms. Cystoid macular edema was identified in both eyes on optical coherence tomography. Plasma levels of ornithine were elevated. Based on these observations, the patients were diagnosed with gyrate atrophy of the choroid and retina. The clinical diagnosis was confirmed by mutation analysis of the ornithine-δ-aminotransferase (OAT) gene. Patients were treated with a pyridoxine supplement (300 mg/day) and an arginine-restricted diet to lower plasma levels of ornithine, which were successfully reduced without progression of chorioretinal atrophy for 15 months. Our report describes the first case of gyrate atrophy in the Korean population diagnosed by OAT gene analysis and treated with vitamin B6 dietary supplementation.

Redox regulation of Plasmodium falciparum ornithine δ-aminotransferase

Ornithine δ-aminotransferase (OAT) of the malaria parasite Plasmodium falciparum catalyzes the reversible conversion of ornithine into glutamate-5-semialdehyde and glutamate and is-in contrast to its human counterpart-activated by thioredoxin (Trx) by a factor of 10. Trx, glutaredoxin, and plasmoredoxin are redox-active proteins that play a crucial role in the maintenance and control of redox reactions, and were shown to interact with P. falciparum OAT. OAT, which is involved in ornithine homeostasis and proline biosynthesis, is essential for mitotic cell division in rapidly growing cells, thus representing a potential target for chemotherapeutic intervention. Here we report the three-dimensional crystal structure of P. falciparum OAT at 2.3 Å resolution. The overall structure is very similar to that of the human OAT. However, in plasmodial OAT, the loop involved in substrate binding contains two cysteine residues, which are lacking in human OAT. Site-directed mutagenesis of these cysteines and functional analysis demonstrated that Cys154 and Cys163 mediate the interaction with Trx. Interestingly, the Cys154→Ser mutant has a strongly reduced specific activity, most likely due to impaired binding of ornithine. Cys154 and Cys163 are highly conserved in Plasmodium but do not exist in other organisms, suggesting that redox regulation of OAT by Trx is specific for malaria parasites. Plasmodium might require a tight Trx-mediated control of OAT activity for coordinating ornithine homeostasis, polyamine synthesis, proline synthesis, and mitotic cell division.

A sensitive assay for ornithine amino transferase in rat brain mitochondria by ninhydrin method

To establish/develop an assay method for measuring Ornithine Aminotransferase (EC.2.6.1.13) activity using rat brain mitochondria as a source of enzyme in presence and absence of Pyridoxal Phosphate (PLP). The modified method, with the improved sensitivity, is adopted for the assay of ornithine amino transferase activity in rat brain mitochondria. The enzyme activity was measured at 620 nm, the study showed that reaction was optimum at 37°C for 30 minutes. The assay is sensitive enough to detect activity at the order of nanomoles pyrroline-5-carboxylate/mg protein/minute and can be compared as an alternative to the radio isotopic method which is more cumbersome and aminobenzaldehyde method which is less sensitive. The K(m) & V(max) shows maximum activity in the presence of Pyridoxal Phosphate (Coenzyme) concentration at 0.05mM when compared with absence of Pyridoxal Phosphate as higher the concentration of Pyridoxal Phosphate affects the affinity of the enzyme to substrate. The OAT activity in different tissues of the rat was also studied and highest activity was found in liver and kidney.

Correction of ornithine accumulation prevents retinal degeneration in a mouse model of gyrate atrophy of the choroid and retina

Deficiency of ornithine-delta-aminotransferase (OAT) in humans results in gyrate atrophy of the choroid and retina (GA), an autosomal recessive disorder characterized by ornithine accumulation and a progressive chorioretinal degeneration of unknown pathogenesis. To determine whether chronic, systemic reduction of ornithine can prevent this form of retinal degeneration, we used an arginine-restricted diet to maintain long term reduction of ornithine in a mouse model of OAT-deficiency (Oat(-/-)) produced by gene targeting. We evaluated the mice over a 12-month period by measurement of plasma amino acids, electroretinograms, and retinal histologic and ultrastructural studies. We found that an arginine-restricted diet substantially reduces plasma ornithine levels and completely prevents retinal degeneration in Oat(-/-). This result indicates that ornithine accumulation is a necessary factor in the pathophysiology of the retinal degeneration in GA and that restoration of OAT activity in retina is not required for effective treatment of GA.

Crystal structure of human recombinant ornithine aminotransferase

Ornithine aminotransferase (OAT), a pyridoxal-5'-phosphate dependent enzyme, catalyses the transfer of the delta-amino group of L-ornithine to 2-oxoglutarate, producing L-glutamate-gamma-semialdehyde, which spontaneously cyclizes to pyrroline-5-carboxylate, and L-glutamate. The crystal structure determination of human recombinant OAT is described in this paper. As a first step, the structure was determined at low resolution (6 A) by molecular replacement using the refined structure of dialkylglycine decarboxylase as a search model. Crystallographic phases were then refined and extended in a step-wise fashion to 2.5 A by cyclic averaging of the electron density corresponding to the three monomers within the asymmetric unit. Interpretation of the resulting map was straightforward and refinement of the model resulted in an R-factor of 17.1% (Rfree=24.3%). The success of the procedure demonstrates the power of real-space molecular averaging even with only threefold redundancy. The alpha6-hexameric molecule is a trimer of intimate dimers with a monomer-monomer interface of 5500 A2 per subunit. The three dimers are related by an approximate 3-fold screw axis with a translational component of 18 A. The monomer fold is that of a typical representative of subgroup 2 aminotransferases and very similar to those described for dialkylglycine decarboxylase from Pseudomonas cepacia and glutamate-1-semialdehyde aminomutase from Synechococcus. It consists of a large domain that contributes most to the subunit interface, a C-terminal small domain most distant to the 2-fold axis and an N-terminal region that contains a helix, a loop and a three stranded beta-meander embracing a protrusion in the large domain of the second subunit of the dimer. The large domain contains the characteristic central seven-stranded beta-sheet (agfedbc) covered by eight helices in a typical alpha/beta fold. The cofactor pyridoxal-5'-phosphate is bound through a Schiff base to Lys292, located in the loop between strands f and g. The C-terminal domain includes a four-stranded antiparallel beta-sheet in contact with the large domain and three further helices at the far end of the subunit. The active sites of the dimer lie, about 25 A apart, at the subunit and domain interfaces. The conical entrances are on opposite sides of the dimer. In the active site, R180, E235 and R413 are probable substrate binding residues. Structure-based sequence comparisons with related transaminases in this work support that view. In patients suffering from gyrate atrophy, a recessive hereditary genetic disorder that can cause blindness in humans, ornithine aminotransferase activity is lacking. A large number of frameshift and point mutations in the ornithine aminotransferase gene have been identified in such patients. Possible effects of the various point mutations on the structural stability or the catalytic competence of the enzyme are discussed in light of the three-dimensional structure.

Molecular cloning and characterization of Drosophila ornithine aminotransferase gene

The cDNA encoding the Drosophila ananassae ornithine aminotransferase (OAT, EC 2.6.1.13) precursor has been cloned and characterized. The predicted OAT protein sequence is 433 amino acids long with a molecular mass of 47,352 Da and is highly homologous to a mammalian OAT, which is a mitochondrial matrix enzyme and is matured by processing of its amino terminal presequence peptide. The Drosophila OAT has characteristics of leader peptides present in mitochondrial proteins. Immunoblotting experiments using polyclonal antibodies against the partial sequence of the OAT protein revealed that the OAT monomer has a molecular mass of 44 kDa. These results suggest that the Drosophila OAT is also processed and localized in the mitochondria. Quantitation of the OAT mRNA and measurement of the OAT activity during fly development show that OAT is expressed at high levels in the fat body of the third instar larvae in both D. ananassae and D. melanogaster.

A mouse model of gyrate atrophy of the choroid and retina. Early retinal pigment epithelium damage and progressive retinal degeneration

Gyrate atrophy (GA) of the choroid and retina is a blinding chorioretinal degeneration caused by deficiency of ornithine delta-aminotransferase (OAT). The phenotype of GA is characterized by progressive concentric reduction of the visual fields and ornithine accumulation. To understand better the pathogenesis of GA and to develop a model to test therapeutic strategies, we produced an OAT-deficient mouse by gene targeting. Like human GA patients, adult OAT-deficient mice exhibit chronic hyperornithinemia to levels 10-15-fold above normal and massive ornithinuria. Slowly progressive retinal degeneration is reflected by a gradual decline in electroretinogram amplitudes over the first 12 mo of life. At 2 mo, the retinal pigment epithelium is histologically normal, but electron microscopy reveals sporadic degeneration of scattered pigment epithelial cells. By 6 mo there are more diffuse abnormalities of the pigment epithelium with accumulation of large phagosomes and crystalloid inclusions. Although morphologically normal at 2 mo, the photo-receptor outer segments become highly disorganized and shortened to 60% of control length by 10 mo. Additionally, there is cumulative loss of the photoreceptor cells, which reaches 33% by 10 mo and is most pronounced in the central region of the retina. Our results indicate that retinal pigment epithelial cells are the initial site of insult in GA and that the OAT-deficient mouse is an excellent animal model of GA in human patients.

Mapping of ornithine aminotransferase gene sequences to mouse chromosomes 7, X, and 3

Ornithine aminotransferase (OAT), a mitochondrial matrix enzyme, is deficient in patients with gyrate atrophy of the choroid and retina. In human, the OAT structural gene maps to Chromosome (Chr) 10q26 and several OAT-related sequences, some of which are known to be processed pseudogenes, which map to Xp11.3-11.21. Here, we report chromosomal localization in the mouse of the OAT gene and related sequences. Genomic DNA blot analysis of a well-characterized panel of Chinese hamster x mouse somatic cell hybrids using a human OAT probe revealed two murine loci, one on mouse Chr 7 and the other on Chr X. In addition, segregation of restriction fragment length polymorphisms (RFLPs) detected by the OAT probe in recombinant inbred (RI) strains detected a third locus on Chr 3 and positioned the X locus near Cf-8 and Rsvp. Progeny of an intersubspecific backcross were used to map the Chr 7 locus between Tyr and Int-2, near Cyp2e-1.

The Wiskott-Aldrich syndrome: refinement of the localization on Xp and identification of another closely linked marker locus, OATL1

The Wiskott-Aldrich syndrome (WAS) has previously been mapped to the proximal short arm of the X chromosome between the DXS14 and DXS7 loci. In this study, further segregation analysis has been performed using a newly identified WAS family as well as an additional marker probe, HOATL1. The results indicate close linkage between the WAS and OATL1 loci (Z = 6.08 at theta = 0.00) and localize the TIMP, OATL1, DXS255, and WAS loci distal to DXS146 and the OATL1 and WAS loci proximal to TIMP. These linkage data narrow the boundaries within which the WAS locus maps to the chromosomal region bracketed by TIMP and DXS146 and support the loci order Xpter-DXS7-TIMP-(OATL1, WAS, DXS255)-DXS146.

A 15-bp deletion in exon 5 of the ornithine aminotransferase (OAT) locus associated with gyrate atrophy

Gyrate atrophy of the choroid and retina (GA) is an autosomal recessive disorder in which a deficiency of the mitochondrial matrix enzyme ornithine aminotransferase (OAT) leads to progressive blindness. Previously, we and others have reported a number of missense mutations and splice defects in the OAT gene associated with GA. In the present case, through sequencing of the PCR amplified cDNA products, we have detected a novel 15-bp deletion within exon 5 of the OAT gene which retains the original reading frame. The deleted PCR product is the only one produced from the patient's mRNA, while mRNA from the patient's mother yields both deleted and normal length PCR products. The alternate, apparently nonexpressing OAT allele in this patient was inherited from the father, who displays only the normal length PCR product. The codon at the deletion joint remains unaltered, predicting the loss of the pentapeptide Tyr-Thr-Val-Lys-Gly without any other amino acid change. The breakpoints are adjacent to or within two copies of a 4-bp direct repeat, which may have implications for the mechanism of deletion.

Genes on the short arm of the human X chromosome are not shared with the marsupial X

Eight genes located on the short arm of the human X chromosome (MAOA, SYN1, OAT, OTC, CYBB, DMD, ZFX, POLA) have been mapped in several marsupial species by cell hybrid analysis and/or in situ hybridization using probes derived from human cDNA. Seven appear to be autosomal in all marsupial species examined. The eighth, CYBB, detected a site on the X, as well as major autosomal sites. Although these genes are not conserved on the X chromosome in marsupials, at least some of them are arranged together in autosomal clusters. The autosomal location of human Xp genes in marsupials could mean that this region either was lost from a large ancestral X chromosome in the marsupial lineage or was acquired by a small ancestral X (and perhaps Y) in the eutherian lineage. Either explanation demands that the region was not subject to X chromosome inactivation in a common ancestor 120-150 MyrBP.

The ornithine aminotransferase-encoding gene family of rat: cloning, characterization, and evolutionary relationships between a single expressed gene and three pseudogenes

As a first step towards understanding the molecular mechanisms through which the expression of the gene (OAT) encoding ornithine aminotransferase (OAT) is regulated in a tissue-specific manner, we have used a near full length OAT cDNA to isolate related sequences from a rat genomic DNA library. Twenty-one unique clones representing five contigs and spanning approximately 140 kb of genomic DNA were isolated and characterized. From these clones we have identified a single expressed OAT gene and three processed pseudogenes. The comparison of the EcoRI, BamHI, and HindIII fragments contained within these genomic clones with those detected in total genomic DNA by the cDNA probe suggests that essentially all of the OAT-related sequences in the rat genome have been isolated. Thus, the tissue-specific regulation of OAT gene expression appears to be effected through a single expressed gene. Data are presented which suggest that the OAT-1, OAT-2, and OAT-3 pseudogenes arose approximately 28.5, 7.3, and 25.1 Myr ago, respectively. Mutation rates are presented for each codon position of the expressed rat and human OAT genes. The region of the rat genome flanking the boundary of the OAT-3 pseudogene is of additional interest as it shares considerable identity to sequences contained within expressed genes and flanking other processed pseudogenes.

Changes in distribution of mitochondria in the developing chick retina

The distribution of mitochondria in the developing chick retina was examined by enzyme histochemistry, immunohistochemistry, and electron microscopy. Two distinct phenomena were observed: (1) progressive segregation of mitochondria in specific locations in the developing tissue; and (2) progressive loss of mitochondrial activity from the inner retina as it matures. Densitometric scans of stained tissue sections were used to quantitate the relative amounts of mitochondrial activity in the retinal layers. Mitochondria were localized to tissue regions by transmission electron microscopy, enzyme histochemistry for the inner membrane bound mitochondrial enzymes succinic dehydrogenase and cytochrome oxidase, a combined histochemical and ultrastructural method for cytochrome oxidase, and immunolocalization of the mitochondrial matrix enzyme glutamate dehydrogenase. Seven-day embryonic chick retina has a high number of mitochondrial structures and a high level of activity. As development proceeds, the mitochondria organize into layers within the tissue. However, the relative activity of mitochondria in much of the inner retina decreases. In the post-hatch retina, 50% of the mitochondrial activity is found in less than 10% of the tissue area, in the inner segments of the photoreceptor cells.

A polymorphic synonymous mutation in human ornithine-delta-aminotransferase (N378N)

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Detection of point mutations associated with genetic diseases by an exon scanning technique

A major challenge in genetics is identifying the basis of human heritable disease. We describe an "exon scanning" technique which surveys exons in genomic DNA for sequence alterations. By hybridizing genomic DNA to RNA probes derived from cDNAs, we can use RNase A to survey entire coding regions, comprising exons spread across extensive regions of genomic DNA, for mutations associated with genetic disease. Exon scanning of the beta-globin locus in the DNA of patients with 12 different hemoglobinopathies detected all of the culpable single base substitutions and deletions, but not single base insertions. Our analysis also revealed unsuspected polymorphisms and corrected a diagnosis originally based on hemoglobin electrophoresis. Exon scanning of the ornithine aminotransferase gene in a gyrate atrophy patient detected and localized a mutation in the sixth exon. Subsequent PCR amplification and sequencing characterized this as a missense mutation (proline----glutamine). Exon scanning of genomic DNA for sequence alterations, in combination with PCR amplification and sequencing, should be a generally useful strategy for evaluating suspect genes in disorders of unknown etiology, as well as for clinical diagnosis.

The ornithine aminotransferase gene in gyrate atrophy of the retina: analysis of expression and gross structure of this gene in cultured fibroblasts

Gyrate atrophy (GA), a degenerative disease of the human chorioretina, is associated with a deficiency of ornithine aminotransferase (OAT) activity, hyperornithinemia, and ornithinuria. We have characterized a cDNA clone for OAT (HLOAT) that was isolated from a cDNA library constructed from mRNA prepared from Hep G2 cells, a human hepatoma cell line. We have used HLOAT and a nearly full length OAT cDNA clone isolated from a rat liver library (RLOAT) to examine in cultured fibroblasts from individuals with GA and control individuals, the expression of OAT mRNA and the gross structure of the OAT gene. Northern blot analyses of total cellular RNA indicated that 3 of 3 control cell lines and 5 of 6 GA cell lines are capable of expressing an OAT related mRNA of approximately 2100 bases, the size of OAT mRNA. To date, this is the only case of GA in which a complete lack of OAT mRNA has been observed. Southern blot analyses of DNA isolated from these cell lines indicated that the gross structure of the OAT gene is usually not detectably altered in individuals with GA. However, a unique pattern of restriction fragments was observed upon digestion with Eco RI or Hind III of DNA from the GA cell line that does not express OAT mRNA. These unique Eco RI and Hind III fragments arise from the OAT structural gene and will serve as useful molecular markers that allow this particular defective OAT allele to be identified. When the cellular DNAs were digested with Hinf I and examined with a probe that corresponds to at least a portion of the active site of the enzyme, i.e., the pyridoxal phosphate binding site, identical patterns of fragments were detected in all samples. Therefore, it appears unlikely that the loss of OAT activity associated with these GA cases, 4 of which are pyridoxal phosphate responders, is the result of insertions or deletions in this region of the OAT gene. This study indicates that the lack of OAT enzyme activity associated with GA is the result of a variety of different molecular defects within the OAT gene.

Molecular cloning and nucleotide sequence analysis of mRNA for human kidney ornithine aminotransferase. An examination of ornithine aminotransferase isozymes between liver and kidney

The cDNA encoding ornithine aminotransferase (EC 2.6.1.13; OAT) was isolated from a human kidney cDNA library. The isolated cDNA contained the entire protein coding region and partial 3'- and 5'-untranslated regions. The nucleotide sequences of human kidney OAT cDNA were absolutely homologous with those of human liver OAT cDNA, and human kidney and liver OAT fused completely against the antibody to human kidney OAT in an Ouchterlony double diffusion test. These findings settled the controversy as to which characteristics of liver and kidney OAT isozymes are different. An N-terminal sequence analysis of purified mature human kidney OAT clarified that the leader peptide was cleaved between Gln-35 and Gly-36.

Molecular basis of ornithine aminotransferase deficiency in B-6-responsive and -nonresponsive forms of gyrate atrophy

Gyrate atrophy (GA), a recessive eye disease involving progressive loss of vision due to chorioretinal degeneration, is associated with a deficiency of the mitochondrial enzyme ornithine aminotransferase (OATase; ornithine-oxo-acid aminotransferase; L-ornithine:2-oxo-acid aminotransferase, EC 2.6.1.13) with consequent hyperornithinemia. Genetic heterogeneity of GA has been suggested by the demonstration that administration of pyridoxine to increase the level of pyridoxal phosphate, a cofactor of OATase, reduces hyperornithinemia in a subset of patients. We have cloned and sequenced cDNAs for OATase from two GA patients, one responsive and one nonresponsive to pyridoxine treatment. The respective cDNAs contained different single missense mutations, which were sufficient to eliminate OATase activity when each cDNA was tested in a eukaryotic expression system. However, like the enzyme in fibroblasts from the pyridoxine-responsive patient, OATase encoded by the corresponding cDNA from this individual showed a significant increase in activity when assayed in the presence of an increased pyridoxal phosphate concentration. These data firmly establish that both pyridoxine responsive and nonresponsive forms of GA result from mutations in the OATase structural gene. Moreover, they provide a molecular characterization of the primary lesion in a pyridoxine-responsive genetic disorder.

An initiator codon mutation in ornithine-delta-aminotransferase causing gyrate atrophy of the choroid and retina

Gyrate atrophy of the choroid and retina (GA) is an autosomal recessive chorioretinal degeneration caused by deficiency of the mitochondrial matrix enzyme, ornithine-delta-aminotransferase (OAT). To study the molecular basis of the mutations causing GA, we cloned and sequenced the human OAT cDNA and determined the intron-exon arrangement of the structural gene. Using the cDNA template, we synthesized antisense RNA probes and performed RNase A protection experiments with RNA from four Lebanese GA patients. We found a probe-target mismatch at the 5' end of the first coding exon and amplified this region of the patients' genomic DNA using the polymerase chain reaction. Sequence analysis showed a G----A transition, changing the initiator ATG (methionine) codon to ATA. This mutation segregates with the GA allele in both pedigrees. Initiation of translation at the closest in-frame methionine codon would truncate OAT by 138 amino acids, eliminating the entire mitochondrial leader sequence and 113 amino acids of the mature peptide.

Molecular genetic approaches to the analysis of human ophthalmic disease

In this review of the recent literature, the contribution that the new techniques of molecular genetics has made in the analysis and diagnosis of human ophthalmic conditions is presented and discussed. Among the disorders reviewed are X-linked retinitis pigmentosa, Norrie's disease, gyrate atrophy and retinoblastoma, and there are also sections on crystallins and visual pigments.

Localization of the ornithine aminotransferase gene and related sequences on two human chromosomes

We have used a full length cDNA clone to determine the chromosomal location of the gene encoding human ornithine aminotransferase (OAT), a mitochondrial matrix enzyme. Southern blot analysis of Sca I-digested DNA from 34 human-mouse somatic cell hybrids revealed 11 human fragments. Three fragments mapped to chromosome 10q23-10qter, confirming the previous provisional assignment of the functional gene to this autosome by analysis of OAT expression in somatic cell hybrids (O'Donnell et al. 1985). The remaining eight fragments were assigned to the X chromosome, and regionally assigned to Xp21-Xp11 by use of an X-chromosome mapping panel. These X chromosome sequences could represent pseudogenes, or related members of a multigene family. Two of the X chromosome fragments are alternate alleles of a restriction fragment length polymorphism (RFLP) making this OAT-related locus an excellent genetic marker. The RFLP may now be used to determine any possible relationship between this locus and several X-linked eye defects.