21 Disorders of Sulfur Amino Acid Metabolism

Generoso Andria, Brian Fowler, Gianfranco Sebastio

21.1 Homocystinuria due to Cystathione β-Synthase Deficiency – 275 21.1.1 Clinical Presentation – 275 21.1.2 Metabolic Derangement – 276 21.1.3 Genetics – 276 21.1.4 Diagnostic Tests – 277 21.1.5 Treatment and Prognosis – 277

21.2 Methionine S-Adenosyltransferase Deficiency – 278 21.2.1 Clinical Presentation – 278 21.2.2 Metabolic Derangement – 278 21.2.3 Genetics – 279 21.2.4 Diagnostic Tests – 279 21.2.5 Treatment and Prognosis – 279

21.3 Glycine N-Methyltransferase Deficiency – 279 21.3.1 Clinical Presentation – 279 21.3.2 Metabolic Derangement – 279 21.3.3 Genetics – 279 21.3.4 Diagnostic Tests – 279 21.3.5 Treatment and Prognosis – 279

21.4 S-Adenosylhomocysteine Hydrolase Deficiency – 279 21.4.1 Clinical Presentation – 279 21.4.2 Metabolic Derangement – 279 21.4.3 Genetics – 280 21.4.4 Diagnostic Tests – 280 21.4.5 Treatment and Prognosis – 280

21.5 γ-Cystathionase Deficiency – 280 21.5.1 Clinical Presentation – 280 21.5.2 Metabolic Derangement – 280 21.5.3 Genetics – 280 21.5.4 Diagnostic Tests – 280 21.5.5 Treatment and Prognosis – 280

21.6 Isolated Sulfite Oxidase Deficiency – 280 21.6.1 Clinical Presentation – 280 21.6.2 Metabolic Derangement – 280 21.6.3 Genetics – 281 21.6.4 Diagnostic Tests – 281 21.6.5 Treatment and Prognosis – 281

References – 281 274 Chapter 21 · Disorders of Sulfur Amino Acid Metabolism

Metabolism of the Sulfur-Containing Amino Acids Methionine, homocysteine and cysteine are linked by 5-methyl-tetrahydrofolate (THF), catalyzed by cobal- the methylation cycle (. Fig. 21.1, left part) and the amin-requiring 5-methyl THF-homocysteine methyl- trans-sulfuration pathway (. Fig. 21.1, right part). Con- transferase (enzyme 2), or betaine, catalyzed by betaine- version of methionine into homocysteine proceeds homocysteine methyltransferase (enzyme 3). Homo- via methionine S-adenosyltransferase (enzyme 4). This cysteine can also be condensed with serine to form yields S-adenosylmethionine, the methyl-group donor cystathionine via a reaction catalyzed by pyridoxal- in a wide range of transmethylation reactions, a quanti- phosphate-requiring cystathionine E-synthase (enzy- IV tatively important one of which is glycine N-methyl- me 1). Cystathionine is cleaved to cysteine and D-keto- transferase (enzyme 5). These reactions also produce butyrate by another pyridoxal-phosphate-dependent S-adenosylhomocysteine, which is cleaved to adenosine enzyme, J-cystathionase (enzyme 7). The last step of and homocysteine by S-adenosylhomocysteine hydro- the trans-sulfuration pathway converts sulfite to sulfate lase (enzyme 6). Depending on a number of factors, and is catalyzed by sulfite oxidase (enzyme 8), which about 50% of available homocysteine is recycled into requires a molybdenum cofactor. methionine. This involves methyl transfer from either

. Fig. 21.1. Metabolism of the sulfur-containing amino acids. ferase; 4, methionine S-adenosyltransferase; 5, glycine N-methyl- 1, cystathionine E-synthase; 2, 5-methyltetrahydrofolate-homo- transferase; 6, S-adenosylhomocysteine hydrolase; 7, J-cystathio- cysteine methyltransferase; 3, betaine-homocysteine methyltrans- nase; 8, sulfite oxidase 275 21 21.1 · Homocystinuria due to Cystathione E-Synthase Deficiency

As in Marfan syndrome, homocystinuric patients tend to Several inherited defects are known in the conversion of be tall, with thinning and elongation (dolichostenomelia) the sulfur-containing amino acid methionine to cys teine of long bones near puberty, enlarged metaphyses and epi- and the ultimate oxidation of cysteine to inorganic sul- physes, especially at the knees, and arachnodactyly, present fate (. Fig. 21.1). Cystathionine E-synthase (CBS) defi- in about half the patients. Other bone deformities include ciency is the most important. It is associated with severe genu valgum with knobbly knees, pes cavus, and pectus abnormalities of four organs or organ systems: the eye carinatum or excavatum. Restricted joint mobility, partic- (dislocation of the lens), the skeleton (dolichosteno- ularly at the extremities, contrasts with the joint laxity melia and arachnodactyly), the vascular system (throm- observed in Marfan syndrome. Abnormal X-ray findings boembolism), and the central nervous system (mental include biconcavity and flattening of the intervertebral retardation, cerebro-vascular accidents). A low-methio- discs, growth arrest lines in the distal tibia, metaphyseal nine, high-cystine diet, pyridoxine, folate, and betaine spicules in the hands and feet, enlarged carpal bones, re- in various combinations, and antithrombotic treatment tarded lunate development, and shortening of the fourth may halt the otherwise unfavourable course of the di- metacarpal. sease. Methionine S-adenosyltransferase deficiency and J-cystathionase deficiency usually do not require Central Nervous System treatment. Isolated sulfite oxidase deficiency leads (in Developmental delay and mental retardation affect about its severe form) to refractory convulsions, lens disloca- 60% of patients to a variable degree of severity. Seizures, tion, and early death. No effective treatment exists. electroencephalogram abnormalities, and psychiatric dis- Combined deficiency of sulfite oxidase and xanthine turbances have also been reported in approximately half oxidase is discussed in Chap. 35. Deficiencies of glycine of cases. Focal neurologic signs may be a consequence of N-methyltransferase and S-adenosylhomocysteine cerebro-vascular accidents. hydrolase have been described in a few patients. Vascular System Thromboembolic complications, occurring in arteries and veins of all parts of the body, constitute the major cause of 21.1 Homocystinuria due to Cysta- morbidity and mortality. The prognosis is influenced by thione β-Synthase Deficiency the site and extent of the vascular occlusion. Thrombophle- bitis and pulmonary embolism are the most common vas- 21.1.1 Clinical Presentation cular accidents. Thrombosis of large- and medium-sized arteries, particularly carotid and renal arteries, is a frequent The eye, skeleton, central nervous system, and vascular sys- cause of death. Ischemic heart disease is a less prominent tem are all involved in the typical presentation. The patient feature of homocystinuria. Association with other geno- is normal at birth and, if not treated, progressively develops types linked to increased risk of vascular disease, such as the full clinical picture. the factor V Leiden R506Q mutation or the 677CoT muta- tion of the MTHFR gene, were reported to increase the risk Eye of thrombosis in homocystinuric patients [1]. Dislocation of the ocular lens (ectopia lentis), myopia, and glaucoma are frequent, severe and characteristic complica- Other Features tions. Retinal detachment and degeneration, optical atrophy, Spontaneous pneumothorax and pancreatitis were reported and cataracts may eventually appear. Myopia may precede to be rare findings in homocystinuric patients [2]. lens dislocation, and worsens afterwards. Ectopia lentis is detected in most untreated patients from 5–10 years of age Clinical Variability and Natural History and in nearly all untreated patients by the end of the fourth The spectrum of clinical abnormalities is wide, and mild decade and is often the clue to diagnosis. The dislocation cases may only be recognized by late complications, such as is generally downwards, whereas it is usually upwards in thromboembolic accidents. Time-to-event curves based on Marfan syndrome, a phenocopy of homocystinuria caused detailed information on 629 patients were calculated by by mutations of the fibrillin-1 gene. Once ectopia lentis Mudd et al. [3] for the main clinical manifestations and has occurred, a peculiar trembling of the iris (iridodonesis) mortality. Each abnormality occurred significantly earlier following eye or head movement may be evident. and at a higher rate in untreated pyridoxine-nonresponsive individuals than in untreated pyridoxine-responsive ones. Skeleton The risk of thromboembolic accidents in patients under- Osteoporosis is almost invariably detected, at least after going surgery was relatively small, complications, six of childhood. Frequent consequences are scoliosis and a ten- which were lethal, being recorded in only 25 patients fol- dency towards pathological fractures and vertebral collapse. lowing 586 operations. 276 Chapter 21 · Disorders of Sulfur Amino Acid Metabolism

An Italian multicenter survey [2] revealed a strong cor- vascular damage and thromboembolic complications. respondence of ectopia lentis, mental retardation, seizures, Thromboembolism has been suggested to be the end-point dolichostenomelia, and thrombotic accidents among af- of homocysteine-induced abnormalities of platelets, endo- fected sib-pairs, supporting a prominent role of genetic thelial cells, and coagulation factors. Many underlying me- factors in determining the phenotype. Nevertheless, rare chanisms have been investigated with, so far, no unifying cases of intrafamilial variability have been reported. Prob- theory proven. For example, a recent study in homocystin- ably, both early diagnosis and strict compliance to treat- uric patients points to enhanced peroxidation of arachid- ment will change the natural history of cardiovascular and onic acid as an important mechanism linking hyperhomo- mental efficiency even in pyridoxine-nonresponsive indi- cysteinemia and platelet activation in CBS patients suggest- IV viduals. ing the possible value of vitamin E treatment [6]. Among other deleterious effects homocysteine may Outcome of Pregnancies cause abnormal cross-linking of collagen, leading to ab- Pyridoxine-responsive women are able to undergo preg- normalities of the skin, joints, and skeleton in patients. This nancies without a significant risk of malformations in the mechanism seems unlikely to cause damage of the non- offspring. There is much less experience of outcome of preg- collagenous zonular fibers of the lens which is more likely nancies in non-responsive women. Recently, more details of to be due to disturbed fibrillin structure. 15 pregnancies in 11 women, 5 of whom were pyridoxine- nonresponsive, were reported [4]. Complications of preg- nancy included preeclampsia in 2 pregnancies and superfi- 21.1.3 Genetics cial venous thrombosis in a third pregnancy. First-trimester spontaneous abortion was observed in 2 pregnancies. Ten Homocystinuria due to CBS deficiency is inherited as an pregnancies produced normal live born infants while one autosomal recessive trait. Clinical and biochemical varia- offspring had multiple congenital anomalies and another tions, such as pyridoxine responsiveness, are also geneti- had Beckwith-Wiedemann syndrome. No relationship cally determined and related to specific mutations. could be established between the severity of biochemical The worldwide frequency of homocystinuria has been abnormalities during pregnancy and either pregnancy reported to be 1 in 344 000, while that in Ireland is much complications or offspring outcome. The results of this higher at 1 in 65 000. However, results of screening of a study suggest that pregnancy complications and offspring small newborn population for the most common CBS abnormalities are infrequent events. Nevertheless careful mutation (I278T, 7 below) suggest that the incidence in monitoring of these pregnancies is mandatory. Denmark might be higher although this needs to be con- firmed on a larger scale [7]. 21.1.2 Metabolic Derangement The CBS gene is located on chromosome 21 (21q22.3). Molecular studies on CBS patients have led to the cha- Cystathione E-synthase (CBS) activity can be found in racterization of more than 130 mutations most of which many tissues, including liver, brain, pancreas, and cultured are private [9; and website maintained by Kraus J.P. fibroblasts. In addition to the coenzyme pyridoxal phos- http://uchsc.edu/sm/cbs/cbsdata/cbsmain.htm]. Only a phate, CBS also binds two other ligands, the activator few mutations appear to be of epidemiologic relevance. S-adenosylmethionine and a heme moiety of unclear func- I278T which is found in some 25% of homocystinuric tion. In vivo responsiveness to pharmacological doses of alleles and A114V are both associated with a pyridoxine pyridoxine, present in approximately 50% of homocystin- responsive form of the disease. G307S is mostly found in uric patients, is generally associated with the presence of CBS patients of Irish origin and is not linked to response to a small amount of residual enzymatic activity, although pyridoxine. Compound heterozygotes show a variable exceptions to this rule are known [5]. response to pyridoxine although the presence of the I278T Deficiency of CBS leads to tissue accumulation of mutation seems to confer pyridoxine responsiveness even methionine, homocysteine, and their S-adenosyl deriva- in compound heterozygotes. tives, with lack of cystathionine and low levels of cysteine. In at least 5% of Caucasian alleles, exon 8 displays both The -SH group of homocysteine readily reacts with the -SH a 68-bp duplication of the intron-exon junction and the group of a second homocysteine molecule or of other mole- I278T mutation [10]. Recently, it has been shown that a cules, leading to the formation of a number of disulfide peculiar nucleotidic structure generated by the 68-bp compounds, such as homocystine, homocysteine-cysteine duplication allows the rescue of the wild-type sequence, mixed disulfide or protein-bound homocysteine. so preserving the protein function [11]. The role of this The pathophysiology of CBS deficiency has not yet been poly morphic mutation as a risk factor in mild hyper- completely elucidated, but accumulation of homocysteine homocysteinemia and in multifactorial diseases related to probably plays a major role in determining some of the this condition was also investigated with no clear conclu- most relevant clinical manifestations, including generalized sions. 277 21 21.1 · Homocystinuria due to Cystathione E-Synthase Deficiency

As with many other inherited metabolic disorders the Definitive diagnosis requires demonstration of greatly molecular approach for the diagnosis of CBS deficiency is a reduced CBS activity, usually assayed in cultured skin fibro- powerful tool but is limited due to the high proportion of blasts but also possible in phytohemagglutinin-stimulated ‘private’ mutations. In general a search for the most recur- lymphocytes and liver biopsies. Exceptional patients may rent mutations in a given population may be rewarding but have significant residual activity of CBS in fibroblast ex- systematic screening for mutations of the entire coding tracts but still show the typical abnormalities of the disease. region of the CBS gene is a prerequisite for reliable estab- The molecular diagnosis of CBS deficiency now provides a lishment of genotype/phenotype correlation, not least powerful additional approach to the diagnosis. because a double mutational event has been observed on a In many countries, newborn mass-screening programs single allele in some patients. DNA linkage analysis remains based on detection of hypermethioninemia have been an option for prenatal diagnosis of CBS deficiency or carrier implemented. A reduced cut-off value of methionine of detection among family members provided that DNA from 67 Pmol/l was proposed to decrease the high rate of false- an affected subject in a genetically informative family is negative results previously reported in pyridoxine-respon- available. sive patients [13]. To our knowledge, no strategy of genetic treatment has been designed so far. Prenatal Diagnosis Prenatal diagnosis of homocystinuria has been performed in at-risk pregnancies by assaying CBS in extracts of cul- 21.1.4 Diagnostic Tests tured amniocytes [14]. CBS activity is very low in uncul- tured chorionic villi from control subjects and can only be Screening of urine with the cyanide-nitroprusside test often measured after culturing. In the families where mutation(s) yields positive results but can also be negative and lacks is known, direct analysis of the CBS gene allows rapid pre- specificity. Initial diagnosis is best achieved by quantitative natal diagnosis and, in other cases, DNA linkage analysis to amino acid analysis of plasma which must be immediately the CBS locus may have diagnostic value. processed to prevent loss of disulfide amino acids by bind- ing to protein. Increased levels of methionine, homocystine Heterozygotes and cysteine-homocysteine disulfide, low cystine and no On a group basis, differences between obligate heterozy- increase of cystathionine, is typical of CBS deficiency. Ex- gotes and control subjects have been clearly demonstrated ceptional pyridoxine-responsive patients are extremely sen- using either measurements of homocyst(e)ine in plasma sitive to very small supplements of pyridoxine as contained after methionine loading or assay of CBS in liver biopsies, in multivitamin tablets so that false-negative results may cultured skin fibroblasts, or phytohemagglutinin-stimulat- be obtained. Determination of plasma total homocysteine, ed lymphocytes. However, overlap of values obtained in a after treatment of the plasma sample with reducing agents, considerable number of obligate heterozygotes with those is useful for both preliminary diagnosis and monitoring at the lower end of the control range limits the value of of treatment. The determination of total plasma homo- these two approaches for heterozygote testing in individual cysteine including the various available methods has been subjects. com prehensively reviewed [12]. Normal plasma total homo- Molecular analysis of established mutations allows cyst(e)ine values are less than 15 Pmol/l, whereas most un- heterozygote detection in individual families, and the most treated CBS patients exhibit levels above 200 Pmol/l. common CBS mutations might be considered in population Hyperhomocyst(e)inemia also occurs in remethylation screening for CBS heterozygotes. Fibroblast CBS activity defects, due to 5,10-methylene-tetrahydrofolate reductase compatible with a heterozygosity was found in a signifi- deficiency and 5-methyl-THF-homocysteine-methyltrans- cant number of vascular disease patients with hyperhomo- ferase deficiency either isolated or due to defects in cyto- cyst(e)inemia. However molecular genetic studies failed solic cobalamin metabolism (7 Chap. 28). These disorders to demonstrate a causative role of CBS heterozygosity in can mostly be distinguished from CBS deficiency by the patients affected by premature vascular disease [15]. very low to normal plasma methionine level. It must be noted that, in CBS deficiency, methionine concentrations tend to decrease with age and may even be normal in some 21.1.5 Treatment and Prognosis older patients. This decrease may be contributed to by folate depletion and a consequent reduced capacity for The aim of treatment is to reduce plasma total homo- remethylation. cyst(e)ine levels to as close to normal as possible while It needs to be borne in mind that a wide range of non- maintaining normal growth rate. Plasma cystine should be genetic causes of hyperhomocyst(e)inemia are known, kept within the normal range (67 ± 20 Pmol/l) and should including renal failure and administration of drugs such as be supplemented if necessary (up to 200 mg/kg/day). Ho- methotrexate, trimethoprin, niacin. mocysteine levels can be lowered in a number of ways, and 278 Chapter 21 · Disorders of Sulfur Amino Acid Metabolism

the best approach or combination for the individual patient be proven. In the meantime, the need of such treatment will depend on the nature of the defect and social factors. should be assessed on an individual basis. About half of patients with CBS deficiency respond, Whatever the combination of regimes employed, often only partially, to large oral doses of pyridoxine. In achievement of virtually normal total homocysteine levels about 10% of these patients who respond fully, fasting plas- is very difficult in most patients. Notwithstanding this, ma total homocysteine, methionine and cystine become prevention of the severe clinical abnormalities associated normal following a period of up to a few weeks of daily with this disorder requires lifelong treatment and consider- administration of between a few milligrams and 1000 mg able impact on outcome has been achieved in patients for of pyridoxine. Response to the vitamin is also influenced by whom adequate treatment was judged as removal of free- IV folate depletion, which may be due to pyridoxine adminis- disulfide homocystine from plasma. The results of the in- tration itself. Therefore, folic acid (5–10 mg/day) should be ternational survey provide a firm baseline for the evaluation added to the treatment. An approach to assessment of py- of past and future therapeutic regimens [3]. When the low- ridoxine responsiveness is to begin with 100 mg/day and, if methionine diet was started in the newborn period, mental necessary, progressively increase this to 500–1000 mg/day retardation was prevented, the start and progression of with monitoring of plasma levels of methionine and total lens dislocation were delayed, and the incidence of seizures homocysteine every other day. decreased. When late-diagnosed, responsive subjects re- Since doses higher than 1000 mg/day have been asso- ceived pyridoxine treatment, the first thromboembolic ciated with sensory neuropathy pyridoxine should be kept episode occurred later. In fact, a normal IQ was reported in at the lowest dose able to achieve adequate metabolic con- teen-aged pyridoxine-nonresponsive CBS individuals with trol. In particular doses greater than 250 mg/day should good compliance to treatment since birth [17]. Also treat- be avoided in newborns and young infants. In patients ment regimens aimed at lowering plasma homocysteine who do not respond to pyridoxine, a low-methionine/high- significantly reduce cardiovascular risk in homocystinuric cystine diet must be introduced and must be continued patients despite imperfect biochemical control [18]. throughout life. Finally, treatment success clearly depends on early diag- A less strict low-methionine diet may also be necessary nosis and treatment, providing a case for mass newborn- to achieve adequate control in pyridoxine-responsive pa- screening. tients. Synthetic methionine-free amino acid mixtures are commercially available and are especially useful for infants. The requirement for methionine is met by small amounts 21.2 Methionine S-Adenosyltrans- of infant formula. Supplements of essential fatty acids and ferase Deficiency carbohydrates are also required if not present in the methio- nine-free amino acid mixture. After infancy, foods con- 21.2.1 Clinical Presentation taining proteins low in methionine can be introduced, in- cluding gelatin and pulses such as lentils and soybeans. More than 60 patients with methionine S-adenosyltrans- However, it should be noted that soya-modified formulas ferase (MAT) deficiency have now been described, many are usually enriched with methionine. In addition to pyrid- detected by newborn screening, and the great majority have oxine, folate and, possibly vitamin B12, the usual vitamin so far been symptom free suggesting a benign disorder [19]. and mineral supplements are recommended. However, neurological abnormalities and demyelination of Betaine, given orally at a maximum dose of 150 mg/kg/ the brain attributed to deficient formation of S-adenosyl- day (6–9 g maximum in adults) is another important homo- methionine have been observed in a few patients, possibly cysteine-lowering agent especially useful when compliance linked to the severity of the enzyme deficiency. to the diet is unsatisfactory. For older children and adults 6–9 grams of betaine are given daily divided into 3 doses. Betaine remethylates homocysteine often leading to very 21.2.2 Metabolic Derangement high methionine concentrations but with no apparent in- fluence on the pathophysiology of the disease. However, This disorder is characterized by a deficiency of the hepatic unexplained cerebral edema has been recently described in form of the enzyme MAT I/III (but not the extra hepatic form, a few cases of children receiving betaine therapy [16]. MAT II), leading to elevated methionine concentrations in Vitamin C supplementation (1 g/day) has been shown tissues and physiological fluids. The product of this enzyme to ameliorate endothelial dysfunction in CBS patients sug- reaction, S-adenosylmethionine, appears not to be deficient gesting its possible value in reducing the long-term risk of in most cases. Alternative metabolism of methionine seems to atherothrombotic complications. The value of long-term occur above a threshold plasma methionine concentration of treatment with antithrombotic agents such as dipyridamole about 300 µM, resulting in the formation of the transamina- (100 mg four times per day) either alone or combined with tion product 4-methylthio-3-oxobutyrate and dimethyl aspirin (100 mg/day with 100 mg aspirin/day) remains to sulfide, the latter resulting in a distinct odor of the breath. 279 21 21.4 · S-Adenosylhomocysteine Hydrolase Deficiency

21.2.3 Genetics 21.3.2 Metabolic Derangement

Three forms of MAT are known: MAT-I, -II, and -III. The constellation of high methionine, elevated S-adenosyl- MAT-I and -III are encoded by the same gene, MAT1A, and methionine in plasma without elevated S-adenosylhomo- correspond to tetrameric and dimeric forms of a single D1 cysteine and sarcosine provides strong circumstantial evi- subunit, respectively. MAT-II is encoded by a separate gene, dence of deficiency of glycine N-methyltransferase (GMT). mainly expressed in fetal liver and in kidney, brain, testis, No direct demonstration of deficiency of this liver enzyme and lymphocytes. Mutations of the MAT1A gene account has been possible but mutations in the GMT gene, although for both autosomal recessive [20] and autosomal dominant yet to be expressed, strongly support GMT deficiency as a hypermethioninemia [21]. The rarer, latter form is caused cause of these metabolic changes. by mutation on a single allele with a dominant-negative effect. 21.3.3 Genetics

21.2.4 Diagnostic Tests The finding of compound heterozygosity for mutations in the GMT gene also occurring in either parent confirm High methionine in plasma and urine, detected by usual autosomal recessive inheritance of this defect [27]. chromatographic methods, without increased homo- cyst(e)ine of the degree seen in CBS deficiency and associ- ated with no increase of S-adenosylmethionine is suggestive 21.3.4 Diagnostic Tests of this defect, but several other causes of hypermethio- ninemia are possible and must be excluded. Careful inter- Differentiation between this defect and other forms of pretation of results is needed since elevated plasma total isolated hypermethioninemia is possible by measurement homocysteine of up to 28.6 µmol/L has been reported [20]. of S-adenosylmethionine and S-adenosylhomocysteine in plasma. S-adenosylmethionine levels are approximately 10- to 30-fold higher than the upper limit of controls in 21.2.5 Treatment and Prognosis GMT deficiency with normal levels in methionine ade- nosyltransferase deficiency. Sarcosine in plasma as well as Treatment is generally not indicated but, in patients with S-adenosylhomocysteine and total homocysteine are not evidence of demyelination, administration of S-adenosyl- elevated. methionine corrects deficiency of this compound. If the postulated association between specific mutations leading to a severe enzyme deficiency holds true [22, 23], treatment 21.3.5 Treatment and Prognosis with S-adenosylmethionine may be advisable in such cases. It has been speculated that treatment with a low methionine Four pregnancies in a woman with severe MAT I/III diet supplemented with cystine might be beneficial but no deficiency resulted in the birth of three normal children data is available. with fetal arrest at 10–11 weeks in the other. Four women with mild hypermethioninemia due to heterozygosity for the dominant R264H mutant allele gave birth in total to 16 21.4 S-Adenosylhomocysteine normal children with just one recorded miscarriage [24]. Hydrolase Deficiency

21.4.1 Clinical Presentation 21.3 Glycine N-Methyltransferase Deficiency A single patient with this disorder was recently reported [28]. Clinical signs included severely delayed psychomotor 21.3.1 Clinical Presentation development and severe myopathy. When diagnosed at 12.7 months development had ceased and MRI of the brain Persistent isolated hypermethioninaemia associated with a showed delayed myelination and atrophy of white matter. history of persistent elevated plasma transaminases, and mild hepatomegaly was found in two siblings, documented 21.4.2 Metabolic Derangement from the age of 1 year and 5 years [25] and in an unrelated boy from 2 years of age [26]. Deficiency of this enzyme has been proven and leads to a block in the degradation and accumulation of S-adenosyl- homocysteine as well as increased levels of S-adenosyl- 280 Chapter 21 · Disorders of Sulfur Amino Acid Metabolism

methionine and methionine. Elevated levels of guanidino- 21.5.3 Genetics acetate and low levels of phosphatidylcholine and choline are compatible with inhibition of the respective transme- Inheritance is autosomal recessive. The cystathionase gene thylases by S-adenosylhomocysteine. A number of other was cloned [29] and mutant alleles in individuals with this abnormalities such as slightly elevated total homocysteine, condition have been reported [30]. betaine, dimethylglycine and cystathionine remain unclear.

21.5.4 Diagnostic Tests 21.4.3 Genetics IV High urinary excretion of cystathionine without homo- An autosomal recessive inheritance is indicated and cystine and with normal plasma methionine points to this sequencing of the S-adenosyl homocysteine hydrolase gene defect. Transient cystathioninuria in newborns is due to revealed the presence of a maternally derived nonsense known secondary causes, such as vitamin-B6 deficiency, mutation and a missense mutation of paternal origin. generalized liver disease, thyrotoxicosis, and neural tumors. Milder increases of plasma and urine levels of cystathionine can also occur in the remethylation defects due to over- 21.4.4 Diagnostic Tests production of this metabolite. While y-cystathionase activ- ity is certainly expressed in cultured skin fibroblasts, the Differentiation between this and the two other forms of level of activity is probably too small to allow reliable mea- hypermethioninemia described here can be achieved by surement by specific enzyme assay [31]. measurement of S-adenosylhomocysteine, S-adenosyl- methionine and sarcosine in plasma. Each of these is elevate d with approximately 100- and 30-fold elevations of S-adeno- 21.5.5 Treatment and Prognosis sylhomocysteine and S-adenosylmethionine, respectively . Most subjects respond to administration of about 100 mg of pyridoxine daily though, as a benign disorder, it remains 21.4.5 Treatment and Prognosis debatable whether treatment is needed.

Treatment was attempted by severely restricting methio- nine intake and administering phosphatidylcholine in the 21.6 Isolated Sulfite Oxidase form of egg yolk. This resulted in lowering plasma metabo- Deficiency lites and clinical improvement but the long term outcome remains unknown. 21.6.1 Clinical Presentation

Characteristic findings in the severe form of this enzyme 21.5 γ-Cystathionase Deficiency deficiency, whether isolated (approximately 20 patients reported) or due to molybdenum cofactor deficiency (more 21.5.1 Clinical Presentation than 50 described, 7 Chap. 35), are early refractory con- vulsions, severe psychomotor retardation, failure to thrive, This is considered to be a benign disorder. Subjects detected microcephaly, hypotonia passing into hypertonia, lens dis- without ascertainment bias are mainly asymptomatic; sub- location, and early death [32]. Milder presentation has been jects with mental retardation have had healthy siblings with reported. the same defect or without the defect but showing the same symptoms. 21.6.2 Metabolic Derangement

21.5.2 Metabolic Derangement Sulfite oxidase catalyses the last step in the oxidation of the sulfur atom of cysteine into inorganic sulfate (. Fig. 20.1). Deficiency of the pyridoxal-phosphate-requiring J-cysta- Its deficiency results in accumulation of the suspected toxic thionase leads to tissue accumulation of cystathionine. compound sulfite together with its detoxification products, Increased plasma concentrations and markedly increased S-sulfocysteine and thiosulfate, with reduced formation excretion of cystathionine occur and N-acetylcystathionine of sulfate. is also excreted. 281 21 References

21.6.3 Genetics 8. Miles EW, Kraus JP (2004) Cystathionine beta-synthase: structure, function, regulation, and location of homocystinuria-causing mu- This autosomal recessive disease has been explained at the tations. J Biol Chem 279:29871-29874 9. Kraus JP, Janosik M, Kozich V et al (1999) M Cystathionine E-syn- molecular level by cloning of the gene and characterization thase mutations in homocystinuria. Hum Mutat 13:368-375 of mutations in several patients affected by the isolated 10. Sperandeo MP, de Franchis R, Andria G, Sebastio G (1996) A 68 bp sulfite oxidase deficiency [33, 34]. The gene for molybdenum insertion found in a homocystinuric patient is a common variant cofactor deficiency has been localized to chromosome 6. and is skipped by alternative splicing of the cystathionine E-syn- thase mRNA. Am J Hum Genet 59:1391-1393 11. 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Kozich V, Kraus E, de Franchis R et al (1995) Hyperhomocysteinemia in premature arterial disease: examination of cystathionine beta- Cystine levels are always very low. Thiosulfate can also synthase alleles at the molecular level. Hum Mol Genet 4:623-629 be searched for by thin-layer-chromatography. The absence 16. Yaghmai R, Kashani AH, Geraghty MT et al (2002) Progressive of xanthinuria distinguishes the isolated deficiency from cerebral edema associated with high methionine levels and betaine the molybdenum-cofactor defect, in which xanthinuria is therapy in a patient with cystathionine beta-synthase (CBS) defi- ciency. Am J Med Genet 108:57-63 observed (7 Chap. 35). Sulfite oxidase activity can be deter- 17. Yap S, Rushe H, Howard PM, Naughten ER (2001) The intellectual mined in cultured skin fibroblasts. abilities of early-treated individuals with pyridoxine-nonrespon- sive homocystinuria due to cystathionine beta-synthase deficiency. 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Mudd SH, Tangerman A, Stabler SP et al (2003) Maternal methio- Eur J Pediatr 157:867-870 nine adenosyltransferase I/III deficiency: reproductive outcomes 3. Mudd SH, Skovby F, Levy HL et al (1985) The natural history of in a woman with four pregnancies. J Inherit Metab Dis 26:443-458 homocystinuria due to cystathionine R-synthase deficiency. Am J 25. Mudd SH, Cerone R, Schiaffino MC et al (2001) Glycine N-methyl- Hum Genet 37:1-31 transferase deficiency: a novel inborn error causing persistent iso- 4. Levy HL, Vargas JE, Waisbren SE et al (2002) Reproductive fitness lated hypermethioninaemia. J Inherit Metab Dis 24:448-464 in maternal homocystinuria due to cystathionine beta-synthase 26. Augoustides-Savvopoulou P, Luka Z, Karyda S et al (2003) Glycine deficiency. J Inherit Metab Dis 25:299-314 N -methyltransferase deficiency: a new patient with a novel muta- 5. Fowler B (1985) Recent advances in the mechanism of pyridoxine- tion. J Inherit Metab Dis 26:745-759 responsive disorders. J Inherit Metab Dis 8[Suppl 1]:76-83 27. Luka Z, Cerone R, Phillips JA 3rd et al (2002) Mutations in human 6. Davi G, Di Minno G, Coppola A et al (2001) Oxidative stress and glycine N-methyltransferase give insights into its role in methio- platelet activation in homozygous homocystinuria.Circulation nine metabolism. Hum Genet 110:68-74 104:1124-1128 28. Baric I, Fumic K, Glenn B et al (2004) S-adenosylhomocysteine 7. Gaustadnes M, Ingerslev J, Rutiger N (1999) Prevalence of con- hydrolase deficiency in a human: a genetic disorder of methionine genital homocystinuria in Denmark. N Engl J Med 340:1513 metabolism. Proc Natl Acad Sci USA 101:4234-4239 282 Chapter 21 · Disorders of Sulfur Amino Acid Metabolism

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