Cloning of the human and murine ROM1 genes: genomic organization and sequence conservation

Rom-1 and peripherin are related membrane proteins of the photoreceptor outer segments. Both proteins are located at the rims of the photoreceptor disks, where they may act jointly in disk biogenesis. Mutations in the gene (RDS) encoding peripherin cause autosomal dominant retinitis pigmentosa, autosomal dominant punctata albescens and butterfly macular degeneration in man, and retinal degeneration slow in mice. To facilitate ROM1 mutation and linkage analysis in inherited retinal diseases, we cloned and characterized the human and murine ROM1 genes. In both species, the ROM1 coding region is contained within approximately 1.8 kb of genomic DNA and is interrupted by only two introns. The structures of the ROM1 and RDS genes are similar, with perfect conservation of the intron splice sites. Putative transcription regulatory regions of the ROM1 locus, 5' to an apparent transcription start site, were identified by cloning the mouse Rom-1 gene and comparing the sequence to the human homologue. Alignment of the human and murine rom-1 predicted protein sequences with the peripherin polypeptides of four species reveals a high degree of conservation (47% overall identity between the six proteins) in the central hydrophilic domain of the two family members. Despite this conservation of sequence, the predicted pI's of only this region of rom-1 and peripherin differ substantially, being 5.2 and 8.2, respectively. The charge difference in this region may mediate the non-covalent association of these two proteins in vivo. The conserved genomic structure and sequence of ROM1 and RDS indicates that these genes evolved from a common ancestor by duplication event.

Ornithine transcarbamylase deficiency presenting with strokelike episodes

A girl with ornithine transcarbamylase deficiency had a history of recurrent strokelike episodes. The differential diagnosis of unexplained stroke should include primary urea cycle defects.

"Reducing body"-like inclusions in skeletal muscle in childhood-onset acid maltase deficiency

Unusual inclusions with some of the features of "reducing bodies" were encountered in the skeletal muscle biopsy of a 2.5-year-old boy with childhood-onset acid maltase deficiency. The biopsy revealed a vacuolar myopathy with lysosomal storage of glycogen and eosinophilic refractile inclusions in myofibers, which appeared dark blue with the menadione-nitroblue tetrazolium reaction. The significance of the association of inclusions with reducing properties in the setting of acid maltase deficiency is discussed.

Localization of the photoreceptor gene ROM1 to human chromosome 11 and mouse chromosome 19: sublocalization to human 11q13 between PGA and PYGM

Rom-1 is a retinal integral membrane protein that, together with the product of the human retinal degeneration slow gene (RDS), defines a photoreceptor-specific protein family. The gene for rom-1 (HGM symbol: ROM1) has been assigned to human chromosome 11 and mouse chromosome 19 by Southern blot analysis of somatic cell hybrid DNAs. ROM1 was regionally sublocalized to human 11p13-11q13 by using three mouse-human somatic cell hybrids; in situ hybridization refined the sublocalization to human 11q13. Analysis of somatic cell hybrids suggested that the most likely localization of ROM1 is in the approximately 2-cM interval between human PGA (human pepsinogen A) and PYGM (muscle glycogen phosphorylase). ROM1 appears to be a new member of a conserved syntenic group whose members include such genes as CD5, CD20, and OSBP (oxysterol-binding protein), on human chromosome 11 and mouse chromosome 19. Localization of the ROM1 gene will permit the examination of its linkage to hereditary retinopathies in man and mouse.

The Lowe's oculocerebrorenal syndrome gene encodes a protein highly homologous to inositol polyphosphate-5-phosphatase

Lowe's oculocerebrorenal syndrome (OCRL) is a human X-linked developmental disorder of unknown pathogenesis and has a pleiotropic phenotype affecting the lens, brain and kidneys. The OCRL locus has been mapped to Xq25-q26 by linkage and by finding de novo X; autosome translocations at Xq25-q26 in two unrelated females with OCRL. Here we use yeast artificial chromosomes with inserts that span the X chromosomal breakpoint from a female OCRL patient in order to isolate complementary DNAs for a gene that is interrupted by the translocation. We show that the transcript is absent in both female OCRL patients with X; autosome translocations and that it is absent or abnormally sized in 9 of 13 unrelated male OCRL patients with no detectable genomic rearrangement. The open reading frame encodes a new protein with 71% similarity to human inositol polyphosphate-5-phosphatase. Our results suggest that OCRL may be an inborn error of inositol phosphate metabolism.

Cloning of the cDNA for a novel photoreceptor membrane protein (rom-1) identifies a disk rim protein family implicated in human retinopathies

The molecules essential to the continual morphogenesis and shedding of the opsin-containing disks of vertebrate photoreceptors are largely unknown. We describe a 37 kd protein, rom-1, which is 35% identical and structurally similar to peripherin/retinal degeneration slow (rds). Like peripherin, rom-1 is a retina-specific integral membrane protein localized to the photoreceptor disk rim. The two proteins are similarly oriented in the membrane, and each has a highly conserved (15/16 residues) cysteine- and proline-rich domain in the disk lumen. Although both rom-1 and peripherin form disulfide-linked dimers, they do not form heterodimers with each other, but appear to associate noncovalently. These results suggest both that rom-1 and peripherin are functionally related members of a new photoreceptor-specific protein family and that rom-1, like peripherin, is likely to be important to outer segment morphogenesis. The association of mutations in RDS with retinitis pigmentosa indicates that ROM1 is a strong candidate gene for human retinopathies.

Histidase and histidinemia. Clinical and molecular considerations

Histidase (histidine ammonia-lyase, EC 4.3.1.3) catalyzes the deamination of L-histidine to trans-urocanic acid in the liver and skin of mammals. Histidase deficiency results in increased histidine and histamine in blood, and decreased urocanic acid in blood and skin. In this review we discuss current research on: (1) the mechanism of formation of an unusual residue, dehydroalanine, at the active site of histidase; and (2) the role of urocanic acid as an ultraviolet light-induced immunoregulator in the skin, and the implications of urocanic acid deficiency for human histidinemia. Genetic mechanisms that may account for the 1% of histidinemic patients with neurological impairments are considered briefly.

Localization of histidase to human chromosome region 12q22----q24.1 and mouse chromosome region 10C2----D1

The human gene for histidase (histidine ammonia-lyase; HAL), the enzyme deficient in histidinemia, was assigned to human chromosome 12 by Southern blot analysis of human X mouse somatic cell hybrid DNA. The gene was sublocalized to region 12q22----q24.1 by in situ hybridization, using a human histidase cDNA. The homologous locus in the mouse (Hal) was mapped to region 10C2----D1 by in situ hybridization, using a cell line from a mouse homozygous for a 1.10 Robertsonian translocation. These assignments extend the conserved syntenic region between human chromosome 12 and mouse chromosome 10 that includes the genes for phenylalanine hydroxylase, gamma interferon, peptidase, and citrate synthase. The localization of histidase to mouse chromosome 10 suggests that the histidase regulatory locus (Hsd) and the histidinemia mutation (his), which are both known to be on chromosome 10, may be alleles of the histidase structural gene locus.

Molecular analysis of human argininosuccinate lyase: mutant characterization and alternative splicing of the coding region

Argininosuccinic acid lyase (ASAL) deficiency is a clinically heterogeneous autosomal recessive urea cycle disorder. We previously established by complementation analysis that 28 ASAL-deficient patients have heterogeneous mutations in a single gene. To prove that the ASAL structural gene is the affected locus, we sequenced polymerase chain reaction-amplified ASAL cDNA of a representative mutant from the single complementation group. Fibroblast strain 944 (approximately 1% of residual ASAL activity), from a late-onset patient who was the product of a consanguineous mating, had only a single base-pair change in the coding region, a C-283----T transition at a CpG dinucleotide in exon 3. This substitution converts Arg-95 to Cys (R95C), occurs in a stretch of 13 residues that is identical in yeast and human ASAL, and was present in both of the patient's alleles but not in 14 other mutant or 10 normal alleles. Expression in COS cells demonstrated that the R95C mutation produces normal amounts of ASAL mRNA but little protein and less than 1% ASAL activity. We observed that amplified cDNA from mutant 944 and normal cells (liver, keratinocytes, lymphoblasts, and fibroblasts) contained, in addition to the expected 5' 513-base-pair band, a prominent 318-base-pair ASAL band formed by the splicing of exon 2 from the transcript. The short transcript maintains the ASAL reading frame but removes Lys-51, a residue that may be essential for catalysis, since it binds the argininosuccinate substrate. We conclude (i) that the identification of the R95C mutation in strain 944 demonstrates that virtually all ASAL deficiency results from defects in the ASAL structural gene and (ii) that minor alternative splicing of the coding region occurs at the ASAL locus.

Cloning and expression of rat histidase. Homology to two bacterial histidases and four phenylalanine ammonia-lyases

Histidase (histidine ammonia-lyase, EC 4.3.1.3) catalyzes the deamination of histidine to urocanic acid. Apart from phenylalanine ammonia-lyase, which is not expressed in animals, histidase is the only enzyme known to have a dehydroalanine residue in its active site. The amino site precursor and the mechanism of formation of dehydroalanine are not known. As an initial step to determining the precursor of dehydroalanine in histidase, we have isolated a functional cDNA clone for histidase from a rat liver cDNA library using an affinity-purified antiserum. The 2.2-kilobase cDNA has a 1,971-base pair open reading frame coding for a 657-amino acid polypeptide with a predicted molecular mass of 72,165 Da. The cDNA has a rare polyadenylation signal (AAUACA) that appears to inefficiently direct polyadenylation in transfected COS monkey kidney cells. Conversion of this sequence to the consensus polyadenylation signal (AAUAAA) resulted in increased levels of stable mRNA. COS cells transfected with a histidase expression vector produce active histidase. The formation of active histidase in cells that have no endogenous histidase activity suggests either that the requisite modifying enzyme is present in these cells or that the dehydroalanine residue forms by an autocatalytic mechanism. Rat histidase was found to have 41 and 43% amino acid identity to Pseudomonas putida and Bacillus subtilis histidases, respectively. Phenylalanine ammonia-lyases from parsley, kidney bean, and two yeast strains were also found to have approximately 20% amino acid identity to rat histidase. On the basis of the similarity of function of histidase and phenylalanine ammonia-lyase, dehydroalanine at the active sites, and the sequence conservation over a large evolutionary distance (mammals, bacteria, yeast, and plants), we propose that the genes for histidase and phenylalanine ammonia-lyase have diverged from a common ancestral gene, of which the most conserved regions are likely to be involved in catalysis or dehydroalanine formation.

Common acute lymphocytic leukemia antigen is identical to neutral endopeptidase

We purified CALLA from human kidney and isolated a cDNA clone reactive with two oligonucleotide probes corresponding to two distinct peptides. The amino acid sequence translated from the CALLA cDNA revealed 100% identity with that of human neutral endopeptidase (NEP, enkephalinase). The distribution of CALLA antigen and NEP in normal tissues are similar.

Argininosuccinate lyase deficiency: evidence for heterogeneous structural gene mutations by immunoblotting

Argininosuccinate lyase (AS lyase) deficiency is an inborn error of the urea cycle with extensive clinical and genetic heterogeneity. We investigated the biochemical basis of the enzyme defect and the genetic heterogeneity in this disorder using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting of fibroblast extracts. The AS lyase monomer in control fibroblasts was present in two bands of approximately 51 and approximately 49 Kd. Each of 28 mutant strains had some cross-reactive material (CRM) of the lower (approximately 49 Kd) MW, in quantities ranging from trace to substantial levels. The approximately 51 Kd band was found in only six mutants with near-normal amounts of AS lyase CRM or high residual enzyme activity. The residual AS lyase enzyme activity in a mutant did not necessarily reflect the amount of the 49-51 Kd monomer in that strain. In contrast, there was a strong general correlation between the quantity of 49-51 Kd CRM in a mutant and the frequency of complementation by that mutant. In addition to the CRM of normal molecular weight (MW) (49-51 Kd), the majority of mutants (but not controls) had significant CRM present in one to five bands of MW less than 49 Kd. The immunoprecipitation of at least one of these low MW bands was inhibited by purified human AS lyase. Mutants indistinguishable by clinical, enzymatic, or complementation analysis have been shown to be heterogeneous in their content of AS lyase CRM, greatly extending the number of distinct mutant alleles identified at this locus. These data demonstrate that multiple unique mutations in the structural gene coding for the monomer cause AS lyase deficiency and that the AS lyase monomers made by these mutants may be unstable. Integration of these findings with enzymatic and complementation data has indicated the functional domain of the AS lyase monomer likely to be altered in certain mutants.

Molecular cloning of cDNA for rat argininosuccinate lyase and its expression in rat hepatoma cell lines

Using antibody and plaque hybridization screening, we isolated rat argininosuccinate lyase (AS lyase) cDNA clones from a liver cDNA library prepared in the phage expression vector lambda gt11. Five overlapping cDNAs covering 1.7 kilobases of the estimated 2.0-kilobase AS lyase mRNA were characterized and confirmed as AS lyase sequences by hybrid selection. We examined the differential expression of AS lyase in rat liver and four rat hepatoma cell lines (7800C1, H4, HTC, and MH1C1). These cells exhibited a 60-fold range of AS lyase enzyme activity, with a direct correlation between activity, amount of AS lyase immunoreactive protein, and quantity of specific AS lyase mRNA. These observations suggest that the differences in AS lyase expression between rat liver and the hepatoma cell lines result from variations in AS lyase transcriptional activity or alterations in nuclear processing of AS lyase RNA.

Interallelic complementation in an inborn error of metabolism: genetic heterogeneity in argininosuccinate lyase deficiency

We used complementation analysis as a probe for the detection of genetic heterogeneity within a single locus affected in a human disease, argininosuccinate lyase (L-argininosuccinate arginine-lyase, EC 4.3.2.1) deficiency. Fibroblasts cultured from 28 unrelated patients were fused in all possible pairwise combinations, and the argininosuccinate lyase activity in heterokaryons was assayed by measuring the incorporation of 14C from L-[ureido-14C]citrulline into acid-precipitable material. Partial complementation was observed in fusions involving 20 of the 28 strains, with the lyase activity increasing from 2- to 10-fold. Thirteen of the mutants were identified by the complementation analysis as being phenotypically unique. Of the 20 complementing strains, 3 were remarkable because they participated in all but 2 of the 32 positive complementation tests; 2 others constituted a unique subgroup that produced the highest increases in argininosuccinate lyase activity of all fusions. The 8 strains that did not complement any others consisted of two types: 3 mutants with the highest residual argininosuccinate lyase activity of all strains and 5 mutants with low residual activity. All of the mutants mapped to a single major complementation group. The data could be summarized as a circular complementation map with an attached linear tail, the mutants being distributed among 12 subgroups in a complex pattern. We conclude that all of these mutants are affected at a single locus, that extensive genetic heterogeneity is present in the mutant population, and that the affected locus in argininosuccinate lyase deficiency is likely to be the structural gene coding for that enzyme.

Differential diagnosis of cerebral palsy: Lesch-Nyhan syndrome without self-mutilation

Athetotic cerebral palsy was diagnosed in a 6-month-old boy with no history of perinatal trauma. Lesch-Nyhan syndrome (i.e., complete deficiency of hypoxanthine-guanine phosphoribosyltransferase [HGPRT] ) was diagnosed only when the boy began biting his lower lip at the age of 10 years. It is suggested, on the basis of this case and others like it in the literature, that the delayed onset or absence of self-mutilation in patients with Lesch-Nyhan syndrome may be more common than has been previously suspected. In all males said to have cerebral palsy, HGPRT deficiency must be ruled out, preferably by measuring the ratio of uric acid to creatinine in a random urine specimen.

Biopterin synthesis defect. Treatment with L-dopa and 5-hydroxytryptophan compared with therapy with a tetrahydropterin

We have identified a generalized deficiency of monoamine neurotransmitters in a patient with a defect in biopterin synthesis. Neurotransmitter precursors (L-3,4-dihydroxyphenylalanine [L-dopa]; 5-hydroxytryptophan [5-HTP] and a tetrahydropterin [6-methyltetrahydropterin (6MPH4)] were investigated for their ability to normalize monoamine neurotransmitter metabolism. Before treatment, the concentrations of dopamine (DA), norepinephrine, epinephrine, and six monoamine metabolites were very low or undetectable in plasma, cerebrospinal fluid, or urine. L-Dopa and 5-HTP replacement was begun at age 7 mo. This therapy generally corrected the deficiency of monoamines and their metabolites, and improved neurological development until the age of 25 mo. Despite these benefits, the intermittent administration of L-dopa could not produce a stable improvement of acute neurological function or DA metabolism. In the 3 h after L-dopa administration, plasma DA and the motor activity and alertness of the patient rose and fell in parallel. Doses of L-dopa that were clinically optimal produced normal plasma levels of norepinephrine and epinephrine, but excessive concentrations of DA and its metabolites. Furthermore, the clinical and biochemical effects of L-dopa were inhibited by phenylalanine and 5-HTP, respectively, demonstrating that these amino acids have antagonistic pharmacological effects. Physiological correction of the monoamine deficit and the hyperphenylalaninemia of this disorder was attempted at age 35 mo using high doses (8-38 mg/kg per d) of 6MPH4. 6MPH4, a synthetic analogue of tetrahydrobiopterin, controlled the hyperphenylalaninemia. Significant concentrations of 6MPH4 were obtained in the cerebrospinal fluid; no neurological improvement or stimulation of monoamine synthesis in the central nervous system was detected. These findings indicate the complexity in replacement therapy with L-dopa and 5-HTP, but suggest that this treatment may be partially effective in biopterin-deficient patients who are unresponsive to high doses of tetrahydropterins.

Use and design of low protein diets for children with inborn metabolic disorders

Low protein diets are used to treat infants and children with hyperammonemia due to urea cycle and other metabolic disorders as well as a number of amino and organic acidopathies. The incidence of these disorders is small and many are life-threatening. As a result, there is little in the literature on the dietary management of these patients. This paper draws on 10 years of clinical experience at the Hospital for Sick Children in Toronto, Ontario and presents a guide to the preparation of infant formulas providing levels of protein intake from 0.5 to 2.0 g per kg. Also described is a low protein equivalency system that is a useful guide for measuring both baby foods and table foods for affected children up to about six years of age. This dietary information is accompanied by a description of the disorders amenable to low protein diets, some of the adjunctive therapies employed and the nutritional concerns associated with severe restriction of protein.

Use of tetrahydropterins in the treatment of hyperphenylalaninemia due to defective synthesis of tetrahydrobiopterin: evidence that peripherally administered tetrahydropterins enter the brain

Substantial amounts of tetrahydrobiopterin and 6-methyltetrahydropterin can be detected in CSF when these pterins are given peripherally to patients with hyperphenylalaninemia due to defective biopterin synthesis. Results of this study suggest that administration of either of these pterins in proper doses may prove to be a treatment not only for the impaired peripheral phenylalanine metabolism, but also for the neurologic disorders that are characteristic of the variant forms of hyperphenylalaninemia due to defective tetrahydrobiopterin synthesis or metabolism.

Hyperornithinaemia and gyrate atrophy of the retina: improvement of vision during treatment with a low-arginine diet

A 15-year-old patient with hyperornithinaemia (0.6--1.2 mmol/l) and gyrate atrophy of the retina was given a low-arginine diet to reduce plasma ornithine to a concentration (0.15--0.25 mmol/l) near the normal range. After five weeks of treatment, there was improvement in the visual function of one eye which had been severely impaired without improvement for 3 years. This improved visual function was maintained until compliance with the diet deteriorated at eight months, after which visual function regressed towards pretreatment status. Overrestriction of dietary arginine produced hyperammonaemia, indicating that arginine is an essential aminoacid in ornithine transaminase deficiency. These results suggest that reduction of plasma ornithine may reverse a metabolic neuroretinopathy in this disease, and offer hope that progression of the retinal atrophy can be arrested as well.

Metabolic abnormalities in the idiopathic Fanconi syndrome: studies of carbohydrate metabolism in two patients

Two patients with idiopathic Fanconi syndrome and glucose intolerance were studied from a metabolic perspective. They had fasting hyperglycemia, massive glucosuria, insulinopenia, ketosis, and elevated serum free fatty acids. There was a markedly blunted insulin secretory response to glucagon, tolbutamide, glucose, and arginine. One patient had the findings of diabetic retinopathy and a sensory neuropathy. Neither patient could convert galactose to glucose, but they did not have galactosemia. As a result of these studies, and previous reports in which similar changes were noted, we conclude that diabetes mellitus may occur in patients who have had idiopathic Fanconi syndrome for many years.