1. CBS deficiency: clinical guidelines
Andrew Morris, Viktor Kožich, Saikat Santra, Generoso Andria, Tawfeg Ben-Omran, Anupam Chakrapani, Ellen Crushell, Mick Henderson, Michel Hochuli, Martina Huemer, Miriam Janssen, Francois Maillot, Philip Mayne, Jenny McNulty, Tara Morrison, Siobhan O’Sullivan, Helene Ogier, Markéta Pavlíková, Isabel Tavares de Almeida, Sufin Yap, Henk Blom, Kimberly Chapman

2. Methyltetrahydrofolate
Methionine
Methyl
group
B12
Methyltetrahydrofolate
Homocysteine
Serine
Cystathionine
β-synthase
Pyridoxal 5-phosphate
Cystathionine
Cysteine 2

3. CBS deficiency Adverse effects Brain Eye Skeleton Blood clotting
Complete deficiency
causes early-onset
multisystem disease
B6-responsive cases may be asymptomatic
or only suffer adult-onset thromboembolism

4. CBS deficiency Problems
Rare, maybe 3/million diagnosed globally, but many mild cases are undiagnosed
Awareness is low
Still many uncertainties about diagnosis & management
Need evidence-based guidelines
To help doctors with limited experience
To highlight uncertainties & stimulate research

5. Evidence-based Guideline
EHOD: European Network and Registry for Homocystinurias and Methylation Defects
Team of paediatricians, adult physicians, biochemists, dietitians, a geneticist, a statistician
Questions agreed, answers prepared & evidence graded (A: randomised controlled trials, B: cohort studies, C: case reports, D: expert opinion)
Guideline reviewed (Wilcken, Leonard)
Published in 2017, J Inherit Metab Dis 40:49-74, Open Access
5 years ago, the EU provided funding for EHOD, a European Network and Registry for Homocystinurias and Methylation Defects. One of the aims was to write guidelines for the management of classical homocystinuria. A team of specialists was assembled, the key questions were agreed and answers prepared with supporting evidence. Unfortunately, the evidence was generally poor: there were no randomised trials and most was based on case reports or expert opinion! The guidelines have now been published and they can be downloaded freely.

6. Clinical problems Eye Brain Lens dislocation Learning difficulties
High Myopia
Brain
Learning difficulties
Psychiatric & behaviour disorders
Seizures
Clumsiness
Strokes
Statements 1, 30 & 33: grades B-D

7. Clinical problems: Skeleton
Knock knees
Pigeon chest
Tall
Osteoporosis
Scoliosis
Statements 1 & 31: grades B-D

8. Clinical problems: Blood clots
Deep Venous Thromboses
Pulmonary Emboli
Saggital sinus thrombosis
Statements 1 & 31
grades B-D

9. Terminology Homocysteine Homocystine Cysteine Cystine Mixed disulfide
At this stage, I need to say something about terminology. Homocysteine, spelt with an e before the i, has an SH or thiol group. This reacts readily with other thiol groups. Thus, two homocysteine molecules can react to form a dimer, which is known as homocystine. The amino acid, cysteine, also has a thiol group, allowing it to form a dimer, known as cystine. Alternatively, homocysteine & cysteine can react together to form a mixed disulphide.
Mixed disulfide

10. TOTAL HOMOCYSTEINE Plasma proteins Hcy-SH FREE HOMOCYSTEINE
Hcy-S-S-Cys
HOMOCYSTEINE-CYSTEINE MIXED DISULFIDE
Hcy-S-S-Hcy
FREE HOMOCYSTINE
Hcy-S-S-R
OTHER DISULFIDES
PROTEIN BOUND HOMOCYSTEINE
Hcy │ S
Cys
Hcy │ S
Cys
If you measure the homocysteine level after breaking all the disulphide bonds, it is known as the total homocysteine concentration. If you measure the homocysteine level after breaking all the disulphide bonds, it is known as the total homocysteine concentration.
Plasma proteins

11. Diagnosis of homocystinuria
Plasma total homocysteine (tHcy)
Free homocystine (in plasma or urine) unreliable
tHcy may be normal in B6-responsive patients if taking multi-vitamins containing B6
(Plasma should be separated promptly)
Statements 4-6, grades B-C

12. Other causes of high homocysteine
Vitamin B12 or folate deficiencies
Remethylation defects
Renal failure
CBS deficiency confirmed by mutation analysis
or CBS assay (in plasma or fibroblasts)
Statements 7-8, grades B-C

13. CBS deficiency: Newborn screening
Blood methionine
Outcomes
Normal if good compliance
Problem
Some cases not detected
Statement 12, grade C

14. Dietary restriction Methionine Betaine Homocysteine Pyridoxine
Folic acid
B12
Betaine
Homocysteine
Pyridoxine
Cystathionine
Cysteine 14

15. Treatment with Pyridoxine
Presumed to stabilise the mutant CBS enzyme
Low doses normalise homocysteine in some patients
Others have partial response even on high doses
Side effects
Peripheral neuropathy with high doses (>900 mg/d)
Recommended dose
Lowest effective
Up to 10mg/kg/d, max. 500mg/d
Statements 17-18, grade D

16. Testing for B6 responsiveness
We propose
Test when patient stable, on normal protein intake
Give folate & correct B12 if deficient
Give pyridoxine 10mg/kg/day up to 500mg/day
Continue for 6 weeks with at least 2 tHcy measurements before & on pyridoxine
If detected by newborn screening, response very rare
Pyridoxine 100mg/day for 2 weeks
Statement 16, grade C-D

17. Low methionine diet
Small, measured amounts of food containing methionine
Methionine-free amino acid mixture
(includes cystine but requirement unknown)
Adequate energy from low protein foods
most fruit & vegetables
synthetic e.g. low-protein bread, biscuits, pasta
Vitamin & mineral supplements
Statements 21 & 24: grade D

18. Monitoring Statement 29, grade D Total Homocysteine Methionine
Frequency depends on severity, treatment, age
Blood spots can be used but most labs require liquid blood, separated promptly
Also B12, folate
Nutrition
Bone density
Statement 29, grade D

19. Homocysteine targets
Normal IQ & no complications in Irish pts with free Homocystine <11 umol/l (Yap 1998 & 2001)
We propose keeping tHcy <100
Safety of tHcy <120 based on few pts, up to 30yr
Lower levels hard to achieve
tHcy
120
fHcy 11
Statement 15, grade C

20. Low methionine diet Problems Diet & supplements relatively unpalatable
Restrictions unpopular
Expert dietitians required
Introduction of diet harder after infancy
Diet needed throughout life but compliance deteriorates

21. Betaine Dimethylglycine Methionine Betaine Homocysteine Roles
Add to diet if unable to reach homocysteine targets
Seldom achieves adequate control alone
Side effects
Taste, fishy odour at high doses
Methionine rises, seldom toxic but brain swelling in few patients with methionine >1000 umol/l
Statements 26 & 28, grades C-D

22. Minimise risk of thromboses
During infections, travel, surgery & pregnancy
Good hydration
Compression stockings
Early mobilisation
Anticoagulants
Contraceptives
Avoid oestrogens (combined pill)
Statements 36-39, grades C-D
Statement 35, grade D

23. Future developments Few of our recommendations are controversial
To be revised with evidence from EHOD register, mouse models etc
Dietary treatment e.g. improved supplements, data on methionine content of foods, blood spot tHcy
Novel treatments e.g. chaperones, ERT
Primary need is earlier diagnosis
Increased awareness
Newborn screening