HCy measurement is indicated as part of the routine evaluation of a child presenting with developmental delay or with neurological features including movement disorder, seizures and myelopathy. It is also indicated in the investigation of an individual at any age with lens dislocation or an unprovoked thromboembolic event.

HCU caused by CBS deficiency is the most common condition of hyperhomocystinaemia. Its estimated worldwide prevalence from newborn screening and clinical case finding is between 1 in 200 000 and 335 000,10 11 although it varies by ethnicity with a higher prevalence seen in certain populations (1 in 65 000 in Ireland, 1 in 17 800 in Germany and 1 in 1800 in Qatar).

Thromboembolic tendency increases in direct proportion to the magnitude and duration of HCy elevation. Left untreated there’s a 30% chance of a vascular event by age 20 rising to 50% by the age of 30.12 Lens subluxation is a hallmark of HCU distinguishing it from remethylation disorders and is detectable in approximately 70% of untreated patients by the age of 10 and in over 90% diagnosed later.12 It causes a stepwise deterioration in visual acuity which may go unnoticed or be attributed to other causes unless specifically examined for. Additional ophthalmic complications of HCU include severe myopia, strabismus, glaucoma, retinal detachment and optic atrophy. Failing to consider a diagnosis of HCU in a patient presenting with lens subluxation or an unprovoked thromboembolic event leads to significant delays in initiating appropriate treatment to prevent further complications.13 14

Skeletal and connective tissue disorders affect approximately 50% of HCU patients, and may variably include elongated long bones (‘Marfanoid’ body habitus), arachnodactyly, joint laxity, osteopenia, scoliosis and vertebral collapse.

Learning disabilities are not universal or may be subtle and, when present, include global intellectual impairment, specific learning disabilities such as autism and behavioural disturbance.

HCU treatment is summarised in table 2. The primary goal is to maintain plasma tHCy concentration at less than 100 μmol/L in adults and less than 50 μmol/L in children at which thromboembolic risk is reversed, lens subluxation is halted and bone growth in children proceeds normally.15 16 Very early initiation of treatment allows for normal intellectual development, but once learning difficulties are evident they are not reversed by treatment.

Treatment and monitoring of the disorders of homocystinuria

The CBS enzyme uses pyridoxal 5’-phosphate as a cofactor. Approximately 50% of patients with HCU respond partially or wholly to high-dose pyridoxine (vitamin B6). Pyridoxine responsiveness is thought to depend on the extent to which the CBS mutations affect the cofactor binding site. Notably, the p.I278T missense mutation, present in approximately 25%, confers pyridoxine responsiveness even in the heterozygous state.17 People who are homozygous for this mutation and nutritionally replete may remain asymptomatic without treatment. Conversely, the p.G307S mutation prevalent in the Irish population is not associated with pyridoxine responsiveness.18 Testing for pyridoxine responsiveness forms an essential part of the diagnostic process for any patient newly diagnosed with HCU (figure 2). Briefly, this involves correcting dietary B12 or folate deficiency which may otherwise impair response to pyridoxine, ensuring adequate dietary protein intake, and administering high-dose pyridoxine (100–500 mg daily depending on age) with repeat measurement of tHCy after 1 week. A suggested protocol is detailed in the 2017 guideline.15

In fully pyridoxine-responsive HCU, target tHCy can be attained with oral pyridoxine alone (table 2) typically with doses in excess of the recommended daily allowance, ranging from 300 to 900 mg/day in adults and 10 mg/kg/day to a maximum of 500 mg/day in children.15 More commonly patients are partially responsive at high doses (postpyridoxine tHCy <80% baseline but >50 μmol/L). Pyridoxine excess is associated with peripheral neuropathy; therefore, it is ideally prescribed under specialist supervision with patients routinely monitored for this side effect. For the other 50% who do not respond to pyridoxine at all (postpyridoxine tHCy >80% baseline) pyridoxine does not form part of their treatment regimen.

If the tHCy target is not attainable with pyridoxine alone, oral betaine (N,N,N-trimethylglycine) is given as a methyl donor which provides an additional route for remethylation of HCy to methionine catalysed by betaine-homocysteine methyl transferase (figure 1). High methionine levels in patients on betaine can rarely lead to cerebral oedema presenting symptomatically with raised intracranial pressure or asymptomatically as white matter change on brain MRI.19 20 Therefore, the dose used is the maximum that allows for maintenance of methionine at less than 1000 μmol/L (which in adults may be higher than the UK licensed dose of 3 g twice daily). Patients who are partially pyridoxine-responsive can often be adequately and safely treated with a combination of pyridoxine and betaine only (table 2).

In non-pyridoxine-responsive HCU and some partially responsive patients, target tHCy can only be attained by additionally following a protein restricted diet to limit intake of the essential amino acid, methionine, the precursor to HCy production in the transsulphuration pathway (figure 1).12 21 Protein intake typically ranges from 5 to 40 g daily, with the shortfall met by taking a methionine-free amino acid supplement enriched with vitamins and minerals and additional cystine. The low protein diet is complex to manage and difficult to adhere to and patients need expert dietetic support throughout life without which they may not only fail to maintain tHCy at target but are also at risk of nutritional deficiencies.

In all forms of HCU, routine supplementation with folate and vitamin B12 is advisable as a low risk non-evidence-based intervention to prevent nutritional deficiencies of these vitamins contributing to treatment resistance. Their routine use may also enhance the remethylation pathway for HCy metabolism. Additional supportive measures include optician and ophthalmology input for eyesight and lens correction, bone density surveillance, and where necessary, lifelong educational and social support.

Remethylation defects are caused by MTHFR deficiency (known as 'MTHFR') and disorders of cobalamin metabolism. Neurological features are prominent often from an early age and include developmental delay, seizures, encephalopathy, dystonia, ataxia and hypotonia or hypertonia. There is considerable overlap between conditions which precludes making a reliable diagnosis on the basis of clinical presentation alone.22

However, attenuated forms are increasingly recognised. In MTHFR, there may be a period of normal development followed by regression of previously acquired intellectual and motor skills. Cerebral atrophy and white matter change is often evident on brain MRI. Sudden respiratory failure is described and contributes to a poor outcome in both early and later onset forms of untreated MTHFR.23 24

Cobalamin disorders may present later with progressive neurological features such as sensory deficit, weakness and gait disturbance caused by subacute combined degeneration of the cord as the clinical manifestation of functional vitamin B12 deficiency, or with thromboembolic complications associated with hyperhomocystinaemia. The cobalamin disorders associated with high MMA have a well-recognised late presentation with microangiopathic complications of atypical haemolytic uraemic syndrome (aHUS) and pulmonary artery hypertension.25

MTHFR enzyme converts 5,10-MTHF to 5-MTHF which is the methyl group donor for synthesis of methionine from HCy (figure 1). Hence in MTHFR, high tHCy is accompanied by a low/low-normal methionine (figure 2). The metabolic block does not affect the availability of folate for nucleic acid synthesis which accounts for why there is no associated megaloblastic anaemia.

Direct enzyme assays are available for confirmation of the diagnosis. Mutational analysis should be interpreted with caution as there are a number of MTHFR polymorphisms described, including the extensively studied thermolabile variant, c.677C>T which, with coexisting dietary folate deficiency, contribute to a modestly elevated tHCy (40–100 μmol/L, table 1). These variants may contribute to risk of neural tube defects in the developing fetus of affected mothers26 but do not cause the clinical features associated with true MTHFR.

In this condition, 5-MTHF, distal to the metabolic defect, is low. 5-MTHF is the only folate derivative available to the central nervous system and hence both 5-MTHF and methionine levels in cerebrospinal fluid (CSF) are low (figure 1). The methionine deficit, rather than hyperhomocystinaemia, is implicated in the prominent neurological features seen in MTHFR even at modest degrees of tHCy elevation compared with other disorders. Hence, in addition to lowering tHCy, a major goal of treatment is to increase availability of methionine to the brain (table 2). This is primarily achieved with betaine which does not itself cross the blood–brain barrier but rather, by acting as an alternative methyl donor, increases the peripheral supply of methionine which does enter CSF. There is a substantial evidence base demonstrating its benefit in both improved survival and neurological development.27 Betaine also reduces plasma tHCy and the associated risk of vascular complications.

High-dose folate or its derivatives also form part of the treatment regimen but there remains no clear consensus as to the optimal formulation. The metabolic defect means that 5-MTHF cannot be generated from folate or folinic acid and therefore biochemically it is logical to administer 5-MTHF directly as calcium mefolinate which then enters the CSF.28 However, in vivo instability of mefolinate means that, even at high doses, it does not completely correct CSF 5-MTHF concentration. No neurological outcome studies demonstrating clinical efficacy of mefolinate vs other forms of folate in MTHFR have been conducted. However, in other disorders of folate metabolism associated with low CSF 5-MTHF, small increases, not amounting to complete correction, have been shown to improve neurological outcomes.29

Nitrous oxide administered as an anaesthetic agent has been reported to have catastrophic consequences in undiagnosed MTHFR30 31 and its use is contraindicated in this condition.

Vitamin B12 is required to generate the methylcobalamin (MeCbl) co-factor for methionine synthase which catalyses the conversion of HCy to methionine. Hence, cobalamin disorders directly or indirectly impair methionine synthase activity resulting in high tHCy accompanied by a low or low/normal methionine (figure 1). They are accompanied by a macrocytic anaemia, with or without neutropenia or pancytopaenia, because accumulated 5-MTHF traps folate and disrupts bone marrow nucleotide synthesis. They are grouped into disorders of absorption and transport, in which serum B12 is low and urine/plasma MMA is elevated and disorders of intracellular cobalamin metabolism in which there is a functional B12 deficit with normal serum vitamin B12. In the latter, presence of MMA in plasma or urine depends on the defect. Diagnostic confirmation of the genetic disorders is by mutation analysis, though enzyme analysis is available for two of the intracellular cobalamin disorders (cbl E and G).

Inherited disorders of cobalamin absorption and transport include mutations in gastric intrinsic factor, the cubam receptor subunits cubilin and amnionless (Imerslund-Gräsbeck syndrome) and TC-2 (figure 1).32–35 They typically present in childhood with neurological manifestations of B12 deficiency. Pernicious anaemia caused by autoantibodies to intrinsic factor required for vitamin B12 absorption is the most common acquired disorder and presents later in life.

Recreational nitrous oxide misuse is increasingly common and its presentation can mimic that of an intracellular cobalamin disorder with subacute neurological features of functional B12 deficiency (although on a background of normal neurological development), high tHCy and MMA and normal serum vitamin B12 concentration.36

The inherited intracellular disorders of cobalamin metabolism are denoted cblC-G, cblJ and cblX in brackets after the affected enzyme. LMBD1 (cblF) and ABCD4 (cblJ) are transporter proteins, which release stored cobalamin from hepatocyte lysosomes. Thereafter cobalamin is processed by methylmalonic aciduria homocystinuria type C protein (MMACHC) (cblC) which is dependent on a transcription regulatory complex (cblX) and directed by the enzyme, MMADHC (cblD), either towards synthesis of MeCbl by methionine synthase reductase (cblE) or towards synthesis of adenosylcobalamin (AdoCbl)37–43 (figure 1).

Plasma and urine MMA is concurrently high in cblC, some forms of cblD depending on the affected MMADHC subunit, and the cobalamin disorders upstream of MMACHC (cblX, F, J). This arises from dual impairment of AdoCbl synthesis, the cofactor for the enzyme, methylmalonyl CoA mutase, required for methylmalonyl coA metabolism. In these conditions, dual elevation of MMA and tHCy appears to exert a ‘multitoxic’ effect that causes microangiopathic complications of aHUS and pulmonary hypertension which is not observed in the other cobalamin disorders (cbl E, G and some forms of cblD) in which MMA is not elevated.22 Cobalamin disorders which only impact on AdoCbl synthesis cause high MMA and normal HCy. CblC is the most common inherited cobalamin disorder, with an estimated prevalence of 1 in 100 000. The others are much rarer with only a small number of cases described.

The outcome in all cobalamin disorders is dramatically improved by prompt initiation of high-dose parenteral hydroxocobalamin, initially 1 mg/day intramuscularly (table 2) and later titrated down to the minimum dose required to achieve a target tHCy level and to normalise Hb and MCV.44 Betaine may be added to improve biochemical tHCy and methionine response. Methionine supplementation is considered if a normal range methionine level is not achieved with hydroxocobalamin and betaine. There are several reports of microangiopathic complications associated with cblC being halted or even reversed by treatment25 45 and an isolated case report of a patient with cblC in whom prospective treatment of the mother with hydroxocobalamin during pregnancy and the baby from birth prevented onset of neurological complications.46 The authors are aware of a similarly favourable outcome in a prospectively treated patient with cblG following the diagnosis in an older sibling.

General practitioners, if asked to prescribe hydroxocobalamin, must be made aware of the very high doses used to treat cobalamin disorders and cautioned against measuring vitamin B12 levels to monitor treatment adherence or to guide the prescribing dose.