10 Disorders of Sulfur Amino Acid Bridget Wilcken

10.1 Introduction

Disorders of sulfur amino acid metabolism include disorders of transsulfura- tion and disorders of the remethylation of homocysteine (Hcy) to methionine (Mudd et al. 2001; Rosenblatt and Fenton 2001). Disorders involving cystine – cystinuria and cystinosis – are dealt with elsewhere in the book. This introduc- tion identifies the individual disorders, the treatment aims, and the evidence, where it exists, for the different treatment modalities.

I Transsulfuration Disorders Methionine adenosyltransferase I/III deficiency is rare and can be benign, but demyelination has been reported in some patients. Methionine levels are very high, but there is a deficiency of S-adenosyl methionine (SAM), and the aim of treatment is to elevate the latter, with anecdotal success (Surtees et al. 1991), and perhaps to reduce methionine levels. One case of adenosylhomocysteine hydrolase deficiency has recently been described, in which there was elevated methionine and SAM. (Mudd et al. 2003). The phenotype is still unclear. The need for treatment in this disorder is not yet substantiated, but there is in- creasing evidence that very high levels of methionine (over at least 1500 µmol/l, which may not occur in these disorders) can possibly cause cerebral edema (Yaghmai et al. 2002). Glycine N-methyl transferase deficiency also leads to ele- vated methionine and SAM levels, but also N-methyl glycine (see below; Mudd et al. 2001). Cystathionine beta synthase (CβS) deficiency, classic homocystinuria, results in elevated levels of circulating Hcy and methionine, S-adenosylmethionine and S-adenosyl homocysteine, and reduced circulating cystathionine and cys- teine (Mudd et al.). CβS deficiency is associated with lens dislocation, skeletal and intellectual problems, and increased risk of thromboembolism. While the pathophysiology of CβS is not fully understood, the main goal of treatment is to lower Hcy levels in plasma while maintaining methionine within or above the normal range, with cysteine within the normal range (Fig. 10.1). There are few data to suggest optimal treatment targets for any of the analytes to obtain good outcomes, and in practice it is very difficult to achieve a normal level of plasma total homocysteine (tHcy) in all but a very few patients. 106 Disorders of Sulfur Amino Acid Metabolism

Untreated patient

B6 100 mg bd for 2 weeks, measure tHcy, then add 5mg/day for 2 weeks

tHcy <60 µmol/l tHcy decreased, but >60 µmol/l tHcy unchanged

B6-Responsive B6-partially responsive B6 -nonresponsive

tHcy<20 µmol/l tHcy >20 µmol/l

Consider betaine and/or Consider betaine and/or Add: betaine 3 g bd and restriction plus protein restriction plus protein restriction plus amino acid supplement amino acid supplement amino acid supplement (Methionine-free) (Methionine-free) (Methionine-free)

Monitor amino acid levels 3-6 monthly or monthly (children); adjust diet, supplement, and betaine, to keep tHcy levels as low as possible, and preferably below 60 µmol/l

Fig. 10.1. Cystathionine β-synthase deficiency: flow chart for institution of treatment and monitoring of homocysteine levels (tHcy, total homocysteine)

The outcome in 158 patients treated for up to 18 years has recently been reported (Yap et al. 2001a). Those patients responsive to pyridoxine ( B6; see below) maintain tHcy levels of < 60 µmol/l (reference < 15 µmol/l), while B6-nonresponsive patients have levels usually > 80 µmol/l. Treatment regimens vary somewhat. There is a substantial decrease in thromboembolic episodes from the number expected in untreated patients. In a subset of patients whose Introduction 107 treatment has been standardized and similar to that described below, the same clinical outcome has been seen (Wilcken et al. 1983). In patients with neona- tal diagnosis and treatment, there is also evidence of improved outcome, with avoidance of intellectual deficit and dislocation of the lens with free homocys- teine (fHcy) levels maintained at usually < 19 µmol/l (Yap et al. 2001b). Several strategies are used to lower Hcy levels (Mudd et al.): • The methionine load is reduced by a low-protein diet combined with a me- thionine-free amino acid mixture, containing supplemented cysteine. • Transsulfurationcanbeincreasedinsomepatientsbyusingpharmacological doses of the vitamin B6. • Remethylation can be increased both by the folate cycle, using folate and vitamin B12 medication, and by betaine methyl transferase, using betaine medication (Wilcken et al. 1983, 1985). About half of all CβS patients are very responsive to pharmacological doses of vitamin B6, and this treatment alone will substantially reduce plasma Hcy levels. All of these patients will eventually become folate depleted on treatment, andprobablyalsoB12 depleted, and they need these in addition. A few patients are partially responsive to B6.MostB6-responsive patients cannot achieve a normal level of Hcy on B6,folate,andB12 treatment alone, although the levels obtained evidently result in a greatly improved outcome. Addition of diet and methionine-free amino acid supplement, if tolerated, will result in near-normal tHcy levels in most patients. B6-nonresponsive patients need betaine in addition to folate, vitamins B12,andB6, and a low-protein diet with a methionine-free amino acid supplement (Wilcken et al. 1983). Usually only patients diagnosed as neonates are fully compliant with diet and the amino acid supplement. γ-Cystathionase deficiency appears to be a benign disorder, needing no treatment (Mudd et al.). Sulfite oxidase deficiency occurs both as an isolated disorder and, combined with xanthine oxidase deficiency, as a cofactor disorder. This severe disorder usually causes intractable seizures and death. No treatment has been successful except in late-onset cases, which may respond to a diet low in protein and an amino acid mixture without methionine or cystine (Touati et al. 2000).

I Remethylating Defects 5,10-Methylene tetrahydrofolate reductase (MTHFR) deficiency is associated withelevatedcirculatingHcybutloworlow-to-normallevelsofmethionine,and there is much clinical heterogeneity, with symptoms including gait disturbance, intellectual deficits, and sometimes isolated thromboembolic episodes. Treat- ment regimens aim at lowering Hcy while raising methionine and S-adenosyl methionine levels, but clinical benefit is not clear, and several aspects of treat- 108 Disorders of Sulfur Amino Acid Metabolism ment remain experimental. Key aspects of treatment include oral , be- taine and/or methionine, vitamin B12, and riboflavin (Rosenblatt and Fenton; Fowler 1998). Homozygosity for a common polymorphism in the MTHFR , 667C>T,confers a slightly increased risk of thromboembolism, especially where dietary folate is low.

G Disorders of Cobalamin Metabolism Disorders of cobalamin metabolism and transport are associated with moder- ately high levels of circulating Hcy but, as above, low or low-to-normal plasma methionine. Deficiencies may affect hydroxocobalamin, resulting in combined functional deficiencies of methylmalonyl CoA mutase (CblC, CblD, and CblF) or methyl cobalamin alone, (CblE and CblG), resulting in a functional de- ficiency of methionine synthase. All these disorders can be associated with developmental delay, and to a varying degree, psychiatric disturbance, mega- loblastosis, and other problems. Treatment aims are to increase methionine and S-adenosyl methionine levels into the normal range and to reduce plasma Hcy (and methylmalonic acid in CblC, -D, and -F). Initial treatment with in- tramuscular vitamin B12 is certainly life-saving in cases presenting in infancy, and early treatment clearly improves the outcome. Other treatment modalities, including folates and betaine, are probably important, but their clinical efficacy has not been studied systematically (Rosenblatt and Fenton).

I Adverse Effects of Specific Treatments

• Vitamin B6: doses > 400 mg daily have been associated with peripheral neu- ropathy (Bendeich and Cohen 1990). • Betaine: accidental inhalation of the powder has been reported to cause very serious pulmonary problems. • Methionine levels: very high plasma levels, > 1500 µmol/l may possibly be associated with cerebral edema, although this is uncertain (Mudd et al. 2001). Nomenclature 109

10.2 Nomenclature

No. Disorder/deficiency Definition/comment Gene symbol OMIM No.

10.1.1 Methionine adenosyl Hepatic form MAT1A 250850 transferase I/III 10.1.2 S-Adenosylhomocysteine Onecase,withmyopathy AHCY 180960 hydrolase 10.1.3 Glycine Possibly benign GNMT 606664 N-methyltransferase 10.2 Cystathionine β-synthase CBS 236200 10.2.1 Cystathionine β-synthase Pyridoxine-responsive form CBS 236200 10.2.2 Cystathionine β-synthase Pyridoxine intermediate form CBS 236200 10.2.3 Cystathionine β-synthase Pyridoxine-nonresponsive form CBS 236200 10.3 γ-Cystathionase Appears benign CTH 219500 10.4.1 Molybdenum cofactor Sulfite oxidase plus xanthine and aldehyde MOCS1 252150 deficiency oxidase deficiencies MOCS2 10.4.2 Sulfite oxidase Isolated SUOX 272300 10.5 5,10-Methylene MTHFR 236250 tetrahydrofolate reductase 10.5.1 5,10-Methylene MTHFR 236250 tetrahydrofolate reductase severe 10.5.2 5,10-Methylene Common in most populations, benign in MTHFR, 236250 tetrahydrofolate reductase presence of adequate folate intake 667C>T thermolabile variant 10.6 Methionine synthase Functional defect 10.6.1 Cobalamin E defect Methionine synthase reductase CblE 236270 10.6.2 Cobalamin G defect Defects within methionine synthase CblG 250940 10.7 Methylmalonyl mutase and Functional defect methionine synthase 10.7.1 Cobalamin C defect Cytosolic reduction of hydroxocobalamin CblC 277400 10.7.2 Cobalamin D defect Cytosolic reduction of hydroxocobalamin CblD 277410 10.7.3 Cobalamin F defect Lysosomal transport CblF 277380

10.3 Treatment

G 10.1.1 Methionine adenosyltransferase I/III deficiency

No. Symbol Age Medication/diet Dosage

10.1.1 MAT I/III All ages? S-Adenosyl methioninea a Treatment reported in one patient with MAT I/III, with restoration of normal CSF S-adenosylmethionine levels and remyelination seen on magnetic resonance image (MRI) (Surtees et al. 1991) 110 Disorders of Sulfur Amino Acid Metabolism

G 10.1.2 S-Adenosyl hydrolase deficiency Only one patient with this disorder has been reported. Treatment with methio- nine restriction, phosphatidyl choline and creatine appear to have improved myopathy (Mudd et al. 2003).

G 10.1.3 Glycine N-methyl transferase deficiency Recently described. May be a benign disorder.

I 10.2 Cystathionine β-synthase deficiency 10.2.1 CβS deficiency, pyridoxine responsive 10.2.2 CβS deficiency, pyridoxine intermediate

No. Symbol Age Medication/diet Dosage Frequency Target plasma (years) Hcy

10.2.1 CβS-R > 2 Pyridoxine 50 mg Daily tHcy < 20 µmol/l 10.2.2 CβS-I 2–15 Folic acid 1–2 mg Daily Diet and aminoacid supplement if requireda Pyridoxine 50–100 mg Twice daily tHcy < 60 µmol/l Over 15 Folic acid 5 mg Daily Hydroxocobalamin, oralb, 1 mg Daily from c. 5 years Betaine, if indicatedc 1.5–3 g Twice daily Diet and aminoacid supplement if requireda Pyridoxine 50–100 mg Twice daily tHcy < 60 µmol/l Folic acid 5 mg Daily Hydroxocobalamin, oral 1 mg Oral, daily Betaine, if indicatedc 3 g Twice daily Aspirin, if indicatedd 100 mg Daily Vitamin Ce Daily a Protein-restricted diet and methionine-free supplement can be used in patients who cannot maintain target Hcy levels. See schedule for CβS-NR patients, below. Modest protein restriction is recommended for all patients b Hydroxocobalamin could alternatively be given as an intramuscular injection, 1 mg,monthly.Theoptimalfrequencyof IMI hydroxocobalamin in CβS deficiency has not been determined c Betaine is indicated in all CβS-I patients, and in CβS-R patients who cannot maintain target levels of total homocysteine (tHcy) and cannot tolerate a formal low-protein diet with aminoacid supplementation d Aspirin is indicated if there are other thrombophilic factors present, such as factor V Leyden, or if there has been a thromboembolic event e has been shown to improve the impairment of nitric oxide-dependent vasodilatation that occurs in CβS-deficient patients (Pullin et al. 2002) Treatment 111

G 10.2.3 CβS deficiency, pyridoxine-nonresponsive

No. Symbol Age Medication/diet Dosage Frequency Target plasma (years) Hcy

10.2.3 CβS-NR > 2 Pyridoxinea 50 mg Daily tHcy < 20 µmol/l Folic acid 2 mg c. Daily Low-protein diet c.2 g/kg per day Methionine-free amino With meals acid supplement 2–15 Betaine 1.5–3 g Twice daily tHcy < 60 µmol/l Pyridoxinea 50–100 mg Daily Folic acid 5 mg Daily Hydroxocobalamin, oralb 1 mg Daily from c. 5 years Low-protein diet Methionine-free amino With meals acid supplement Over 15 Betainec 3–4.5 g Twice daily tHcy < 60 µmol/l Pyridoxinea 50–100 mg Daily Folic acid 5 mg Daily Low-protein diet Methionine-free amino 1 g/kg per day With meals acid supplement Hydroxocobalamin, oral 1 mg Daily Aspirin, if indicatedd 100 mg Daily Vitamin Ce Daily a Pyridoxine appears to improve the response to betaine in some pyridoxine-nonresponsive patients, but its use in this situation has not been rigorously investigated b Hydroxocobalamin could alternatively be given as an intramuscular injection, 1 mg, monthly. The optimal frequency of IMI hydroxocobalamin in CβS deficiency has not been determined c Anecdotally, betaine has been given in much higher doses, with no evidence of adverse effect. There is no evidence of advantage in a daily dosage of greater than 150 mg/kg (Matthews et al. 2002) d Aspirin is indicated if there are other thrombophilic factors present, such as factor V Leyden, or if there has been a thromboembolic event e Vitamin C has been shown to improve the impairment of nitric-oxide-dependent vasodilatation that occurs in CβS-deficient patients (Pullin et al. 2002) 112 Disorders of Sulfur Amino Acid Metabolism

I 10.3 γ-Cystathionase deficency This defect appears benign, and no treatment is indicated.

G 10.4.1, 10.4.2 Molybdenum cofactor deficiency, and isolated sulfite oxidase deficiency

No. Symbol Age Medication Dose/kg Frequency Comment

10.4.1 MOCS1 Child Low-protein diet Reportedly useful in late- presenting cases. No treat- ment effective in early pre- senting cases 10.4.2 SUOX Methionine + cysteine-free With meals amino acid mixture Dextromethorphan 12.5 mg (NMDA receptor inhibitor)

I 10.5 5,10-Methylenetetrahydrofolate (MTHFR) deficiency

G No.10.5.1 SymbolMTHFR deficiency, Age severe Medication Dosage Frequency Target 10.8.1 MTHFR 1–2 years Folic acida 2 mg Daily Maximize MTHFR activity Methyl THFb Replacement Betaine – oral 150 mg/kg Tw ice To increase methionine daily and SAM Hydroxocobalamin – oralc 0.5 mg Daily Cofactor for methionine synthase Riboflavind 5 mg Daily MTHFR cofactor 2years Folic acid 5 mg Daily As above to adult MethylTHFifavailable Betaine 3–4.5 G Tw ice daily Hydroxocobalamin – oral 1 mg Daily Riboflavin 5–10 mg Daily a Folinic acid, 7.5–15 mg daily may be tried instead, but is more expensive b Methyl THF may not be available, and there is little experience with this as a medication c Intramuscular hydroxocobalamin could be used instead, perhaps 1 mg monthly d A trial of riboflavin should be given. Dosages up to 50 mg/day are safe even for babies

G 10.5.2 MTHFR 667C>T Homozygosity for this thermolabile variant is common (10–20% or more in many populations). Treatment is not indicated unless there has been a related adverse event, when 2–5 mg folic acid is given daily. Treatment 113

I 10.6 Functional defects of methionine synthase 10.6.1 Cobalamin E defect 10.6.2 Cobalamin G defect

I 10.7 Functional defects of methylmalonyl mutase plus methionine synthase 10.7.1 Cobalamin C defect 10.7.2 Cobalamin D defect 10.7.3 Cobalamin F defect

No: Symbol Age Medicationa Dosage Comment

10.6.1 CblE 0–6 months Hydroxocobalamin, IMI 1 mg/day For CblC, D and F, 10.6.2 CblG Folic acid, oral 1 mg daily the mutase defect does 10.7.1 CblC Betaine, oral 250–500 mg not produce sufficient 10.7.2 CblD Twice daily methylmalonic acid to 10.7.3 CblF require specific treatment other than B12

10.6.1 CblE 6 months– Hydroxocobalamin, IMI 1 mg twice weekly See footnote for CblF 10.6.2 CblG 5 years Hydroxocobalamin, oralb 1 mg/day 10.7.1 CblC Folic acid, oral 2 mg/day 10.7.2 CblD Betaine, oral 75 mg/kg per day 10.7.3 CblF Twice daily

10.6.1 CblE 5 years + Hydroxocobalamin, IMI 1 mg twice weekly 10.6.2 CblG Hydroxocobalamin, oralb 1 mg/day 10.7.1 CblC Folic acid, oral 5 mg/day 10.7.2 CblD Betaine, oral 75 mg/kg per day 10.7.3 CblF twice daily a There is evidence to support these medications, but the suggested dosage schedule for hydroxocobalamin does not have published data to support it. b Oral hydroxocobalamin is not indicated for use in CblF, as there is probably a transport defect also affecting ileal transcytosis. 114 Disorders of Sulfur Amino Acid Metabolism

10.4 Follow-up/Monitoring

I 10.2 Cystathionine β-synthase deficiency

Age Biochemical Frequency Clinical Frequency

0–5 years Plasma amino acids 1–3 monthly Outpatient visit 1–3 monthly Total homocysteine 5–16 years Total homocysteine 3-monthly Outpatient visit 3–6 monthly Plasma amino acids 3–6 monthly Bone mineral density Baseline, then every 3–4 years Serum B12 (unless on B12) Yearly Opthalmology Yearly 16 years + Total homocysteine 6 monthly Outpatient and 6monthly other monitoring as indicated Plasma amino acids 6 monthly Lipids 2–3 yearly Thrombophilic factors Once

I 10.5–10.7 Disorders of folate and B12 metabolism and transport The monitoring of these patients depends heavily on the clinical circumstances, and the schedule given below is only a rough guide.

Age Biochemical Frequency Clinical Frequency

0–6 months Plasma total homocysteine c. monthly Outpatient visit Monthly Plasma amino acids Plasma methylmalonic acid 6months– As above 3 monthly Outpatient visit 3-monthly 5years Developmental At c. age 4–5 years assessment 5years As above 6–12 months Outpatient visit 6–12 monthly to adult Thrombophilic screen Once Lipid screen As adult

Dangers/Pitfalls Nitrous oxide should not be used as an anesthetic agent (it irreversibly deactivates methionine synthase). References 115

References

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