CRB1-related retinopathy overlapping the ocular phenotype of S-adenosylhomocysteine hydrolase deficiency.

BACKGROUND: S-adenosylhomocysteine hydrolase deficiency due to pathologic variants in AHCY gene is a rare neurometabolic disease for which no eye phenotype has been documented. Pathologic variants in CRB1 gene are known to cause a wide spectrum of autosomal recessive retinal diseases with Leber's congenital amaurosis as a most common. The aim of this study is to report co-inheritance of neurometabolic disease and eye disease in a pedigree. MATERIALS AND METHODS: Comprehensive eye examination was performed in available family members together with color vision test, visual fields, fundus images, OCT, electroretinogram and visual evoked potentials. Genetic testing included whole-exome sequencing (WES), retinal dystrophy gene panel and segregation analysis. RESULTS: Two children from a family not known to be consanguineous were affected with neurometabolic disease and one of them presented with reduced vision due to maculopathy. The mother had symptoms of retinal degeneration of unspecified cause. Clinical WES revealed homozygous missense pathologic variants in AHCY gene c.148G>A, p.(Ala50Thr) as a cause of S-adenosylhomocysteine hydrolase deficiency. Retinal dystrophy gene panel sequencing revealed two heterozygous missense pathologic variants in CRB1 gene c.1831T>C, p.(Ser611Pro) and c.3955T>C, p.(Phe1319Leu) in the proband and her mother. These variants segregated with disease phenotype in family members. CONCLUSIONS: Establishing an ocular genetic diagnosis may be challenging with the co-existence of a rare systemic genetic disease with previously unknown eye involvement. Extensive phenotyping and genotyping of available family members showed that the proband and her mother shared a CRB1-related retinopathy at different stages while the brother did not.

S-Adenosylmethionine Promotes Oxidative Stress and Decreases Na+, K+-ATPase Activity in Cerebral Cortex Supernatants of Adolescent Rats: Implications for the Pathogenesis of S-Adenosylhomocysteine Hydrolase Deficiency.

S-Adenosylmethionine (AdoMet) concentrations are highly elevated in tissues and biological fluids of patients affected by S-adenosylhomocysteine hydrolase deficiency, who are clinically characterized by cerebral symptoms whose pathogenesis is still unknown. In the present work, we investigated the effects of AdoMet on redox homeostasis and on the activity of Na+, K+-ATPase in the cerebral cortex of young rats. AdoMet caused lipid peroxidation (increase of malondialdehyde concentrations) and protein oxidation (increase of carbonyl formation and decrease of sulfhydryl content). AdoMet also reduced the antioxidant defenses (reduced glutathione, GSH) and Na+, K+-ATPase activity. Furthermore, AdoMet-induced lipid peroxidation was fully prevented by the antioxidants trolox, melatonin, and resveratrol, and the decrease of GSH concentrations was abolished by trolox, suggesting the involvement of reactive oxygen species in these effects. In this context, AdoMet induced reactive oxygen (increase of 2',7'-dichloroflurescein-DCFH oxidation) but not nitrogen (nitrate and nitrite levels) species generation. Finally, the decrease of Na+, K+-ATPase activity provoked by AdoMet was totally prevented by trolox, implying a possible oxidation of cysteine groups of the enzyme that are critical for its function and highly susceptible to oxidative attack. It is also noted that adenosine and methionine did not alter the parameters evaluated, suggesting selective effects of AdoMet. Our data strongly indicate that disturbance of redox homeostasis caused by a major metabolite (AdoMet) accumulating in S-adenosylhomocysteine hydrolase deficiency may represent a deleterious mechanism of brain damage in this disease. Finally, reduction of Na+, K+-ATPase activity provoked by AdoMet may lead to impaired neurotransmission, but disturbance of this system should be better clarified in future studies.

Clinical picture of S-adenosylhomocysteine hydrolase deficiency resembles phosphomannomutase 2 deficiency.

We report on the seventh known patient with S-adenosylhomocysteine hydrolase (SAHH) deficiency presenting at birth with features resembling phosphomannomutase 2 (PMM2-CDG Ia) deficiency. Plasma methionine and total homocysteine levels were normal at 2 months and increased only after the 8th month of age. SAHH deficiency was confirmed at 4.5 years of age by showing decreased SAHH activity (11% in both erythrocytes and fibroblasts), and compound heterozygosity for a known mutation c.145C>T (p.R49C) and a novel variant c.211G>A (p.G71S) in the AHCY gene. Retrospective analysis of clinical features revealed striking similarities between SAHH deficiency and the PMM2-CDG Ia.

Plasma biomarker identification in S-adenosylhomocysteine hydrolase deficiency.

S-Adenosylhomocysteine hydrolase (AHCY) deficiency is a rare congenital disorder in methionine metabolism clinically characterized by white matter atrophy, delayed myelination, slowly progressive myopathy, retarded psychomotor development and mildly active chronic hepatitis. In the present study, we utilized a comparative proteomics strategy based on 2-DE/MALDI-MS and LC/ESI-MS to analyze plasma proteins from three AHCY-deficient patients prior to and after receiving dietary treatment designed to alleviate disease symptoms. Obtained results revealed candidate biomarkers for the detection of myopathy specifically associated with AHCY deficiency, such as carbonic anhydrase 3, creatine kinase, and thrombospondin 4. Several proteins mediating T-cell activation and function were identified as well, including attractin and diacylglycerol kinase α. Further validation and functional analysis of identified proteins with clinical value would ensure that these biomarkers make their way into routine diagnosis and management of AHCY deficiency.

Hypermethioninemias of genetic and non-genetic origin: A review.

This review covers briefly the major conditions, genetic and non-genetic, sometimes leading to abnormally elevated methionine, with emphasis on recent developments. A major aim is to assist in the differential diagnosis of hypermethioninemia. The genetic conditions are: (1) Homocystinuria due to cystathionine ß-synthase (CBS) deficiency. At least 150 different mutations in the CBS gene have been identified since this deficiency was established in 1964. Hypermethioninemia is due chiefly to remethylation of the accumulated homocysteine. (2) Deficient activity of methionine adenosyltransferases I and III (MAT I/III), the isoenzymes the catalytic subunit of which are encoded by MAT1A. Methionine accumulates because its conversion to S-adenosylmethionine (AdoMet) is impaired. (3) Glycine N-methyltrasferase (GNMT) deficiency. Disruption of a quantitatively major pathway for AdoMet disposal leads to AdoMet accumulation with secondary down-regulation of methionine flux into AdoMet. (4) S-adenosylhomocysteine (AdoHcy) hydrolase (AHCY) deficiency. Not being catabolized normally, AdoHcy accumulates and inhibits many AdoMet-dependent methyltransferases, producing accumulation of AdoMet and, thereby, hypermethioninemia. (5) Citrin deficiency, found chiefly in Asian countries. Lack of this mitochondrial aspartate-glutamate transporter may produce (usually transient) hypermethioninemia, the immediate cause of which remains uncertain. (6) Fumarylacetoacetate hydrolase (FAH) deficiency (tyrosinemia type I) may lead to hypermethioninemia secondary either to liver damage and/or to accumulation of fumarylacetoacetate, an inhibitor of the high K(m) MAT. Additional possible genetic causes of hypermethioninemia accompanied by elevations of plasma AdoMet include mitochondrial disorders (the specificity and frequency of which remain to be elucidated). Non-genetic conditions include: (a) Liver disease, which may cause hypermethioninemia, mild, or severe. (b) Low-birth-weight and/or prematurity which may cause transient hypermethioninemia. (c) Ingestion of relatively large amounts of methionine which, even in full-term, normal-birth-weight babies may cause hypermethioninemia.

S-adenosylhomocysteine hydrolase deficiency: two siblings with fetal hydrops and fatal outcomes.

This paper reports the clinical and metabolic findings in two sibling sisters born with fetal hydrops and eventually found to have deficient S-adenosylhomocysteine hydrolase (AHCY) activity due to compound heterozygosity for two novel mutations, c.145C>T; p.Arg49Cys and c.257A>G; p.Asp86Gly. Clinically, the major abnormalities in addition to fetal hydrops (very likely due to impaired synthetic liver function) were severe hypotonia/myopathy, feeding problems, and respiratory failure. Metabolic abnormalities included elevated plasma S-adenosylhomocysteine, S-adenosylmethionine, and methionine, with hypoalbuminemia, coagulopathies, and serum transaminase elevation. The older sister died at age 25 days, but the definitive diagnosis was made only retrospectively. The underlying genetic abnormality was diagnosed in the second sister, but treatment by means of dietary methionine restriction and supplementation with phosphatidylcholine and creatine did not prevent her death at age 122 days. These cases extend the experience with AHCY deficiency in humans, based until now on only the four patients previously identified, and suggest that the deficiency in question may be a cause of fetal hydrops and developmental abnormalities of the brain.

S-adenosylhomocysteine hydrolase (AHCY) deficiency: two novel mutations with lethal outcome.

This paper reports studies of two novel, allelic missense mutations found in the S-adenosylhomocysteine hydrolase (AHCY) gene from a new case of AHCY deficiency in an infant girl who died at age four months. The mutations lead to replacement of arginine with cysteine (p.Arg49Cys) and aspartic acid with glycine (p.Asp86Gly). Functional analysis of recombinant proteins containing the mutations detected showed that both dramatically reduce AHCY activity. The p.Arg49Cys mutant protein forms intermolecular disulphide bonds, leading to macromolecular structures that can be prevented by reducing agent DTT. The p.Asp86Gly protein tends to form enzymatically inactive aggregates and the loss of a single negative charge as a result of the mutation is involved in enzyme inactivation. We show that replacing Gly86 with negatively charged Glu86 in mutant protein restores enzymatic activity to 70% of wild-type, whereas changing Gly86 to positively charged Lys86 or uncharged Leu86 does not improve enzyme activity, indicating that the negative charge is important for maintenance of such activity. These studies significantly extend knowledge about the importance of residue 86 for AHCY activity. Residue 86 has not been implicated before in this way and the results suggest that the present model of S- adenosylhomocysteine (AdoHcy) hydrolysis may need refinement. Our functional studies provide novel insight into the molecular defect underlying AHCY deficiency and reveal that both low enzyme activity and protein stability of AHCY contribute to the clinical phenotype.

S-Adenosylhomocysteine hydrolase (AdoHcyase) deficiency: enzymatic capabilities of human AdoHcyase are highly effected by changes to codon 89 and its surrounding residues.

Recently, S-adenosylhomocysteine hydrolase deficiency was confirmed for the first time in an adult. Two missense mutations in codons 89 (A>V) and 143 (Y>C) in the AdoHcyase gene were identified [N.R.M. Buist, B. Glenn, O. Vugrek, C. Wagner, S. Stabler, R.H. Allen, I. Pogribny, A. Schulze, S.H. Zeisel, I. Baric, S.H. Mudd, S-Adenosylhomocysteine hydrolase deficiency in a 26-year-old man, J. Inh. Metab. Dis. 29 (2006) 538-545]. Accordingly, we have proven the Y143C mutation to be highly inactivating [R. Beluzic, M. Cuk, T. Pavkov, K. Fumic, I. Baric, S.H. Mudd, I. Jurak, O. Vugrek, A single mutation at tyrosine 143 of human S-adenosylhomocysteine hydrolase renders the enzyme thermosensitive and effects the oxidation state of bound co-factor NAD, Biochem. J. 400 (2006) 245-253]. Now we report that the A89V exchange leads to a 70% loss of enzymatic activity, respectively. Circular dichroism analysis of recombinant p.A89V protein shows a significantly reduced unfolding temperature by 5.5 degrees C compared to wild-type. Gel filtration of mutant protein is almost identical to wild-type indicating assembly of subunits into the tetrameric complex. However, electrophoretic mobility of p.A89V is notably faster as shown by native polyacrylamide gel electrophoresis implicating changes to the overall charge of the mutant complex. 'Bioinformatics' analysis indicates that Val(89) collides with Thr(84) causing sterical incompatibility. Performing site-directed mutagenesis changing Thr(84) to 'smaller' Ser(84) but preserving similar physico-chemical properties restores most of the catalytic capabilities of the mutant p.A89V enzyme. On the other hand, substitution of Thr(84) with Lys(84) or Gln(84), thereby introducing residues with higher volume in proximity to Ala(89) results in inactivation of wild-type protein. In view of our mutational analysis, we consider changes in charge and the sterical incompatibility in mutant p.A89V protein as main reason for enzyme malfunction with AdoHcyase deficiency as consequence.

A histone methylation-dependent DNA methylation pathway is uniquely impaired by deficiency in Arabidopsis S-adenosylhomocysteine hydrolase.

S-adenosylhomocysteine hydrolase (SAH) is a key enzyme in the maintenance of methylation homeostasis in eukaryotes because it is needed to metabolize the by-product of transmethylation reactions, S-adenosylhomocysteine (AdoHcy), which causes by-product inhibition of methyltransferases (MTase's). Complete loss of SAH function is lethal. Partial loss of SAH function causes pleiotropic effects including developmental abnormalities and reduced cytosine methylation. Here we describe a novel partial-function missense allele of the Arabidopsis SAH1 gene that causes loss of cytosine methylation specifically in non-CG contexts controlled by the CMT3 DNA MTase and transcriptional reactivation of a silenced reporter gene, without conferring developmental abnormalities. The CMT3 pathway depends on histone H3 lysine 9 methylation (H3 mK9) to guide DNA methylation. Our results suggest that this pathway is uniquely sensitive to SAH impairment because of its requirement for two transmethylation reactions that can both be inhibited by AdoHcy. Our results further suggest that gene silencing pathways involving an interplay between histone and DNA methylation in other eukaryotes can be selectively impaired by controlled SAH downregulation.

S-adenosylhomocysteine hydrolase deficiency in a 26-year-old man.

This paper reports the third proven human case of deficient S-adenosylhomocysteine (AdoHcy) hydrolase activity. The patient is similar to the only two previously reported cases with this disorder in having severe myopathy, developmental delay, elevated serum creatine kinase (CK) concentrations, and hypermethioninaemia. Although he has been followed from infancy, the basic enzyme deficiency was established only at age 26 years. The diagnosis was based on markedly elevated plasma concentrations of both AdoHcy and S-adenosylmethionine, some 20% of the mean control activity of AdoHcy hydrolase activity in haemolysates of his red-blood cells, and two missense mutations in his gene encoding AdoHcy hydrolase. He had low values of erythrocyte phosphatidylcholine and plasma free choline and marginally elevated excretion of guanidinoacetate, suggesting that the elevated AdoHcy may have been inhibiting methylation of phosphatidylethanolamine and guanidinoacetate. His leukocyte DNA was globally more methylated than the DNA's of his parents or the mean extent of methylation measured in age-matched control subjects.

Inborn errors of sulfur-containing amino acid metabolism.

Two superimposed metabolic sequences, transsulfuration and the methionine/homocysteine cycle, form the pathway for methionine metabolism in mammalian liver. This combined pathway was formulated first to explain observations in subjects with homocystinuria caused by cystathionine synthase deficiency. Since that time additional inborn errors have been discovered, and currently we know of human subjects with isolated defects in all of the reactions of the combined pathway with only one exception: betaine homocysteine methyltransferase. Studies of these inborn errors have contributed significantly to our knowledge of human methionine metabolism and to the clinical consequences of impaired metabolism. Transsulfuration appears to function primarily for the metabolism of excess methionine, and each of the 5 defects in this pathway results in the accumulation of 1 or more of the normal metabolites. Thus, studies of these disorders may provide insight into both the potential pathological sequelae of nutritional methionine excess as well as whether laboratory testing allows the detection of excess.

S-Adenosylhomocysteine hydrolase deficiency: a second patient, the younger brother of the index patient, and outcomes during therapy.

S-Adenosylhomocysteine (AdoHcy) hydrolase deficiency has been proven in a human only once, in a recently described Croatian boy. Here we report the clinical course and biochemical abnormalities of the younger brother of this proband. This younger brother has the same two mutations in the gene encoding AdoHcy hydrolase, and has been monitored since birth. We report, as well, outcomes during therapy for both patients. The information obtained suggests that the disease starts in utero and is characterized primarily by neuromuscular symptomatology (hypotonia, sluggishness, psychomotor delay, absent tendon reflexes, delayed myelination). The laboratory abnormalities are markedly increased creatine kinase and elevated aminotransferases, as well as specific amino acid aberrations that pinpoint the aetiology. The latter include, most importantly, markedly elevated plasma AdoHcy. Plasma S-adenosylmethionine (AdoMet) is also elevated, as is methionine (although the hypermethioninaemia may be absent or nonsignificant in the first weeks of life). The disease seems to be at least to some extent treatable, as shown by improved myelination and psychomotor development during dietary methionine restriction and supplementation with creatine and phosphatidylcholine.

S-adenosylhomocysteine hydrolase deficiency in a human: a genetic disorder of methionine metabolism.

We report studies of a Croatian boy, a proven case of human S-adenosylhomocysteine (AdoHcy) hydrolase deficiency. Psychomotor development was slow until his fifth month; thereafter, virtually absent until treatment was started. He had marked hypotonia with elevated serum creatine kinase and transaminases, prolonged prothrombin time and low albumin. Electron microscopy of muscle showed numerous abnormal myelin figures; liver biopsy showed mild hepatitis with sparse rough endoplasmic reticulum. Brain MRI at 12.7 months revealed white matter atrophy and abnormally slow myelination. Hypermethioninemia was present in the initial metabolic study at age 8 months, and persisted (up to 784 microM) without tyrosine elevation. Plasma total homocysteine was very slightly elevated for an infant to 14.5-15.9 microM. In plasma, S-adenosylmethionine was 30-fold and AdoHcy 150-fold elevated. Activity of AdoHcy hydrolase was approximately equal to 3% of control in liver and was 5-10% of the control values in red blood cells and cultured fibroblasts. We found no evidence of a soluble inhibitor of the enzyme in extracts of the patient's cultured fibroblasts. Additional pretreatment abnormalities in plasma included low concentrations of phosphatidylcholine and choline, with elevations of guanidinoacetate, betaine, dimethylglycine, and cystathionine. Leukocyte DNA was hypermethylated. Gene analysis revealed two mutations in exon 4: a maternally derived stop codon, and a paternally derived missense mutation. We discuss reasons for biochemical abnormalities and pathophysiological aspects of AdoHcy hydrolase deficiency.