Functional and morphological deficits in late-treated patients with homocystinuria: a clinical, electrophysiologic and MRI study

The Spectrum of Mutations of Homocystinuria in the MENA Region

Homocystinuria is an inborn error of metabolism due to the deficiency in cystathionine beta-synthase (CBS) enzyme activity. It leads to the elevation of both homocysteine and methionine levels in the blood and urine. Consequently, this build-up could lead to several complications such as nearsightedness, dislocated eye lenses, a variety of psychiatric and behavioral disorders, as well as vascular system complications. The prevalence of homocystinuria is around 1/200,000 births worldwide. However, its prevalence in the Gulf region, notably Qatar, is exceptionally high and reached 1:1800. To date, more than 191 pathogenic CBS mutations have been documented. The majority of these mutations were identified in Caucasians of European ancestry, whereas only a few mutations from African-Americans or Asians were reported. Approximately 87% of all CBS mutations are missense and do not target the CBS catalytic site, but rather result in unstable misfolded proteins lacking the normal biological function, designating them for degradation. The early detection of homocystinuria along with low protein and methionine-restricted diet is the best treatment approach for all types of homocystinuria patients. Yet, less than 50% of affected individuals show a significant reduction in plasma homocysteine levels after treatment. Patients who fail to lower the elevated homocysteine levels, through high protein-restricted diet or by B6 and folic acid supplements, are at higher risk for cardiovascular diseases, neurodegenerative diseases, neural tube defects, and other severe clinical complications. This review aims to examine the mutations spectrum of the CBS gene, the disease management, as well as the current and potential treatment approaches with a greater emphasis on studies reported in the Middle East and North Africa (MENA) region.

Guidelines for the diagnosis and management of cystathionine beta-synthase deficiency

Cystathionine beta-synthase (CBS) deficiency is a rare inherited disorder in the methionine catabolic pathway, in which the impaired synthesis of cystathionine leads to accumulation of homocysteine. Patients can present to many different specialists and diagnosis is often delayed. Severely affected patients usually present in childhood with ectopia lentis, learning difficulties and skeletal abnormalities. These patients generally require treatment with a low-methionine diet and/or betaine. In contrast, mildly affected patients are likely to present as adults with thromboembolism and to respond to treatment with pyridoxine. In this article, we present recommendations for the diagnosis and management of CBS deficiency, based on a systematic review of the literature. Unfortunately, the quality of the evidence is poor, as it often is for rare diseases. We strongly recommend measuring the plasma total homocysteine concentrations in any patient whose clinical features suggest the diagnosis. Our recommendations may help to standardise testing for pyridoxine responsiveness. Current evidence suggests that patients are unlikely to develop complications if the plasma total homocysteine concentration is maintained below 120 μmol/L. Nevertheless, we recommend keeping the concentration below 100 μmol/L because levels fluctuate and the complications associated with high levels are so serious.

Expanding the Spectrum of Methylmalonic Acid-Induced Pallidal Stroke: First Reported Case of Metabolic Globus Pallidus Stroke in Transcobalamin II Deficiency

A 10-year-old boy with transcobalamin II (TCII) deficiency on oral cyanocobalamin therapy presented with acute right hemiparesis and sensory axonal neuropathy in the context of an intercurrent viral illness. MRI demonstrated unilateral globus pallidus stroke with normal MRA. Echocardiogram was normal. Methylmalonic acid in serum was mildly elevated at 10.29 μmol/L (normal < 0.37 μmol/L), which was an 18-fold increase from his previous baseline. The patient was switched to IM cyanocobalamin and serum methylmalonic acid levels normalized over 6 months to 0.01 μmol/L. After 4 months of IM cyanocobalamin therapy, the neuropathy had resolved. Repeat MRI 4 months after the sentinel stroke demonstrated a chronic-appearing contralateral globus pallidus stroke of uncertain timing.

CONCLUSIONS

We are describing the first case of metabolic stroke and peripheral neuropathy in TCII deficiency. The neuropathy was responsive to parenteral hydroxycobalamin. Unilateral globus pallidus stroke in the appropriate clinical context should not exclude a metabolic etiology as it may herald contralateral involvement and may provide an opportunity for early recognition and treatment. IM hydroxycobalamin should be strongly considered in all patients with TCII, particularly when they reach later childhood. This case highlights the selective vulnerability of the globus pallidus to increased levels of methylmalonic acid of various causes, which is important for both diagnosis and ultimately understanding the mechanisms of neurological injury in this group of conditions. Metabolic stroke may occur with lower levels of methylmalonic acid than previously reported in the context of an intercurrent bioenergetic stressor.

Postoperative psychosis in homocystinuria

Young homocystinuria patients suffering from lens dislocation frequently have to undergo eye surgery. We describe a 16-year-old girl with mild mental retardation who became psychotic-delirant immediately after the last of three lentectomia operations performed under general thiopental anaesthesia. Because methionine, homocysteine, its oxidation product homocysteate and cysteine are potent glutamate agonists, the disturbance of the sulphur containing amino acid (SCAA) metabolism in homocystinuria patients may alter the function of cerebral glutamatergic transmission. The chronic and acute neurological and psychiatric symptoms of homocystinuria patients offer a clue to studies of the neurotoxic but also antipsychotic potency of glutamate agonists like the SCAAs in humans.

Age-related deficiencies in complex I endogenous substrate availability and reserve capacity of complex IV in cortical neuron electron transport

Respiratory enzyme complex dysfunction is mechanistically involved in mitochondrial failure leading to neurodegenerative disease, but the pathway is unclear. Here, age-related differences in mitochondrial respiration were measured in both whole and permeabilized neurons from 9-month and 24-month adult rat cortex cultured in common conditions. After permeabilization, respiration increased in both ages of neurons with excess substrates. To dissect specific deficiencies in the respiratory chain, inhibitors for each respiratory chain complex were used to isolate their contributions. Relative to neurons from 9-month rats, in neurons isolated from 24-month rats, complexes I, III, and IV were more sensitive to selective inhibition. Flux control point analysis identified complex I in neurons isolated from 24-month rats as the most sensitive to endogenous substrate availability. The greatest age-related deficit in flux capacity occurred at complex IV with a 29% decrease in neurons isolated from 24-month rats relative to those from 9-month rats. The deficits in complexes I and III may contribute to a redox shift in the quinone pool within the electron transport chain, further extending these age-related deficits. Together these changes could lead to an age-related catastrophic decline in energy production and neuronal death.

Proton MR spectroscopy in hyperhomocysteinemia with elevated blood methionine levels

Proton magnetic resonance spectroscopy imaging (MRSI) and serial MRI were performed on a 10-year-old girl with B6 unresponsive cystathionine B-synthase (CBS) deficiency who developed high methionine levels while on betaine therapy. At presentation, T2-weighted sequences showed diffuse white matter (WM) hyperintensity and sulcal effacement, while MRSI metabolite concentrations were normal. Four months later, after the betaine therapy was discontinued and a methionine-restricted diet with vitamin B6, B12, and folate supplementation was initiated, blood methionine levels and MRI findings returned to normal. Normal MRSI at presentation was predictive of a positive outcome despite the markedly abnormal initial MRI results.

Effect of homocysteine-lowering treatment with folic acid plus vitamin B on cerebrovascular atherosclerosis and white matter abnormalities as determined by MRA and MRI: a placebo-controlled, randomized trial

BACKGROUND

A high plasma homocysteine concentration is an independent risk factor for large and possibly small vessel disease. We investigated the effects of homocysteine-lowering treatment with folic acid plus vitamin B(6) on markers of cerebrovascular atherosclerosis and cerebral microangiopathy.

MATERIALS AND METHODS

Using 158 healthy siblings (mean age 46.0 +/- 7.6 years) of patients with premature atherosclerotic disease, we performed a randomized, placebo-controlled trial using 5 mg of folic acid plus 250 mg of vitamin B(6) daily (n = 78) or placebo medication (n = 80). Participants were followed for 2 years with magnetic resonance angiography (MRA) (carotid stenosis; carotid and/or vertebral elongation) and magnetic resonance imaging (MRI) (white matter abnormalities; cerebral atrophy).

RESULTS

Seventeen (10.8%) subjects refused MRA/MRI owing to claustrophobia and were excluded. From the remaining 141 participants, 68 received vitamin and 73 received placebo medication [42 (61.8%) and 48 (65.8%) had postmethionine hyperhomocysteinaemia, respectively]. Twenty-four participants (15.2%; 10 in the treatment and 14 in the placebo group) did not complete both years of the trial. Vitamin treatment was associated with an increase in plasma folate (13-fold vs. placebo; P < 0.001) and vitamin B(6) (8.8-fold; P < 0.001). Fasting and postmethionine total homocysteine concentrations decreased 38.7% (95% CI, 27.4-50.0) and 29.1% (95% CI, 19.2-39.0) vs. placebo (all P < 0.001). During follow up six individuals in the vitamin-treated and 11 in the placebo-treated group deteriorated in their outcome measurements. Vitamin treatment, as compared with placebo, was associated with nonsignificantly improved outcomes on both MRA and MRI outcome measurements (odds ratio 0.48; 95% CI 0.17-1.41; P = 0.18 and 0.48; CI 0.14-1.60; P = 0.23, respectively).

CONCLUSIONS

These results could indicate a possible favourable effect of homocysteine-lowering treatment on cerebrovascular atherosclerosis and cerebral microangiopathy among healthy siblings of patients with premature atherosclerotic disease, but larger trials are required to establish this with certainty.

Cerebrospinal fluid and plasma total homocysteine and related metabolites in children with cystathionine beta-synthase deficiency: the effect of treatment

The neurologic complications of cystathionine beta-synthase deficiency are thought to be secondary to accumulation of homocyst(e)ine in the CNS. Treatment of this disorder with betaine has been shown to improve the behavior of individuals, to reduce plasma total homocysteine, and to correct secondary abnormalities of serine. To test the hypothesis that homocyst(e)ine accumulates within the CNS and that this can be reduced by treatment with betaine, we measured total homocysteine and related metabolites in the plasma of 10 children with cystathionine beta-synthase deficiency and cerebrospinal fluid of five children before and during betaine therapy. In plasma, betaine significantly lowered total homocysteine (but not to the normal range) and had a variable effect on methionine. In the cerebrospinal fluid, total homocysteine was raised before treatment (mean 1.2 microM) and was significantly reduced by betaine (mean 0.32 microM) but not to the normal range (<0.10 microM). Cerebrospinal fluid methionine was raised before and during treatment, but betaine did not cause a significant further increase. Cerebrospinal fluid serine was significantly reduced before treatment and rose to the normal range with betaine. Cerebrospinal fluid S-adenosylmethionine was normal before treatment and rose significantly with treatment; there were no significant changes in cerebrospinal fluid 5-methyltetrahydrofolate. The demonstration of accumulation of homocysteine within the CNS lends support to the hypothesis that this may be one cause of the neurologic complications of cystathionine beta-synthase deficiency. Betaine is effective in reducing cerebrospinal fluid homocysteine, but concentrations are still significantly raised during treatment.

Effect of L-dopa on visual evoked potentials and neuropsychological tests in adult phenylketonuria patients

Eight adult, untreated patients with classical phenylketonuria received L-dopa and a decarboxylase inhibitor for 2 weeks. No effect of L-dopa therapy on choice reaction time tasks, sustained attention, frontal lobal function as well as latencies of visual evoked potentials was found. The results raise the question if adult patients with phenylketonuria really suffer from functional dopamine deficiency.

Hyperhomocysteinaemia; with reference to its neuroradiological aspects

Severe or even mild hyperhomocysteinaemia can cause a wide range of neurological problems. In recent years its vascular complications, including cerebral stroke, in children and young adults have gained special interest, because hyperhomocysteinaemia is treatable and recurrence of vascular incidents may be preventable. Current knowledge about biochemical mechanisms leading to hyperhomocysteinaemia, the pathogenesis of vascular pathology and neurological disfunction, and the various patterns of cerebral damage are reviewed. The significance of MRI in diagnosis, follow-up and research on hyperhomocysteinaemia is discussed.

Excitotoxicity, energy metabolism and neurodegeneration

There is increasing evidence that the neurotoxic effects of excitatory amino acids and their analogues are part of the pathogenesis of neuronal degeneration in acute and chronic neurological disease. Recent studies indicate that activation of excitatory amino acid receptors is also induced in the mechanism of neuronal damage induced by impairment of cellular energy metabolism. This article briefly summarizes the evidence for the presence of such a mechanism and discusses metabolic diseases in which excitatory amino acids alone or in combination with energy deficiency could play a pathogenetic role. In these and other metabolic diseases, antagonists to excitatory amino acid receptors may offer a therapeutic opportunity; however, there are potential limits that may prevent chronic use.