A Highly Diverse Portrait: Heterogeneity of Neuropsychological Profiles in cblC Defect

The MMACHC variant c.158T>C: Mild clinical and biochemical phenotypes and marked hydroxocobalamin response in cblC patients

Mutations in MMACHC cause cobalamin C disease (cblC, OMIM 277400), the commonest inborn error of vitamin B12 metabolism. In cblC, deficient activation of cobalamin results in methylcobalamin and adenosylcobalamin deficiency, elevating methylmalonic acid (MMA) and total plasma homocysteine (tHcy). We retrospectively reviewed the medical files of seven cblC patients: three compound heterozygotes for the MMACHC (NM_015506.3) missense variant c.158T>C p.(Leu53Pro) in trans with the common pathogenic mutation c.271dupA (p.(Arg91Lysfs*14), "compounds"), and four c.271dupA homozygotes ("homozygotes"). Compounds receiving hydroxocobalamin intramuscular injection monotherapy had age-appropriate psychomotor performance and normal ophthalmological examinations. In contrast, c.271dupA homozygotes showed marked psychomotor retardation, retinopathy and feeding problems despite penta-therapy (hydroxocobalamin, betaine, folinic acid, l-carnitine and acetylsalicylic acid). Pretreatment levels of plasma and urine MMA and tHcy were higher in c.271dupA homozygotes than in compounds. Under treatment, levels of the compounds approached or entered the reference range but not those of c.271dupA homozygotes (tHcy: compounds 9.8-32.9 μM, homozygotes 41.6-106.8 (normal (N) < 14); plasma MMA: compounds 0.14-0.81 μM, homozygotes, 10.4-61 (N < 0.4); urine MMA: compounds 1.75-48 mmol/mol creatinine, homozygotes 143-493 (N < 10)). Patient skin fibroblasts all had low cobalamin uptake, but this was milder in compound cells. Also, the distribution pattern of cobalamin species was qualitatively different between cells from compounds and from homozygotes. Compared to the classic cblC phenotype presented by c.271dupA homozygous patients, c.[158T>C];[271dupA] compounds had mild clinical and biochemical phenotypes and responded strikingly to hydroxocobalamin monotherapy.

Would, early, versus late hydroxocobalamin dose intensification treatment, prevent cognitive decline, macular degeneration and ocular disease, in 5 patients with early-onset cblC deficiency?

In early-onset (EO) cblC deficiency (MMACHC), hydroxocobalamin dose-intensification (OHCBL-DI) improved biochemical and clinical outcome. In mammals, Cobalamin is reduced, in a reaction mediated by MMACHC. Pathogenic variants in MMACHC disrupt the synthesis pathway of methyl-cobalamin (MetCbl) and 5'-deoxy-adenosyl-cobalamin (AdoCbl), cofactors for both methionine synthase (MS) and methyl-malonyl-CoA mutase (MCM) enzymes. In 5 patients (pts.), with EO cblC deficiency, biochemical and clinical responses were studied following OHCbl-DI (mean ± SD 6,5 ± 3,3 mg/kg/day), given early, before age 5 months (pts. 1, 2, 3 and 4) or lately, at age 5 years (pt. 5). In all pts., total homocysteine (tHcy), methyl-malonic acid (MMA) and Cob(III)alamin levels were measured. Follow-up was performed during 74/12 years (pts. 1, 2, 3), 33/12 years (pt. 4) and 34/12 years (pt. 5). OHCbl was delivered intravenously or subcutaneously. Mean ± SD serum Cob(III)alamin levels were 42,2 × 106 ± 28, 0 × 106 pg/ml (normal: 200-900 pg/ml). In all pts., biomarkers were well controlled. All pts., except pt. 5, who had poor vision, had central vision, mild to moderate nystagmus, and with peri-foveolar irregularity in pts. 1, 2 and 4, yet none had the classic bulls' eye maculopathy and retinal degeneration characteristic of pts. with EO cblC deficiency. Only pt. 5, had severe cognitive deficiency. Both visual and cognitive functions were better preserved with early than with late OHCBL-DI. OHCBL-DI is suggested to bypass MMACHC, subsequently to be rescued by methionine synthase reductase (MSR) and adenosyl-transferase (ATR) to obtain Cob(I)alamin resulting in improved cognitive and retinal function in pts. with EO cblC deficiency.

Clinical, phenotypic and genetic landscape of case reports with genetically proven inherited disorders of vitamin B12 metabolism: A meta-analysis

Inherited disorders of B₁₂ metabolism produce a broad spectrum of manifestations, with limited knowledge of the influence of age and the function of related genes. We report a meta-analysis on 824 patients with a genetically proven diagnosis of an inherited disorder of vitamin B₁₂ metabolism. Gene clusters and age categories are associated with patients' manifestations. The "cytoplasmic transport" cluster is associated with neurological and ophthalmological manifestations, the "mitochondrion" cluster with hypotonia, acute metabolic decompensation, and death, and the "B₁₂ availability" and "remethylation" clusters with anemia and cytopenia. Hypotonia, EEG abnormalities, nystagmus, and strabismus are predominant in the younger patients, while neurological manifestations, such as walking difficulties, peripheral neuropathy, pyramidal syndrome, cerebral atrophy, psychiatric disorders, and thromboembolic manifestations, are predominant in the older patients. These results should prompt systematic checking of markers of vitamin B₁₂ status, including homocysteine and methylmalonic acid, when usual causes of these manifestations are discarded in adult patients.

Higher vitamin B12 levels in neurodevelopmental disorders than in healthy controls and schizophrenia: A comparison among participants between 2 and 53 years

Recent studies suggest that both high and low levels of vitamin B12 (vitB12) may have negative health impacts. We measured VitB12 in patients with the Neurodevelopmental disorders (ND) (n = 222), comprised of Autism Spectrum Disorders, specific Developmental disorders, and Intellectual Disability (aged 2-53 years), schizophrenia (n = 401), and healthy controls (HC) (n = 483). Age-and gender-adjusted vitB12 z-scores were calculated by comparisons with a reference population (n = 76 148). We found higher vitB12 in ND (median 420 pmol/L, mean z-score: 0.30) than in HC (316 pmol/L, z-score: 0.06, P < .01) and schizophrenia (306 pmol/L, z-score: -0.02, P < .001), which was significant after adjusting for age, gender, vitB12 supplement, folate, hemoglobin, leukocytes, liver, and kidney function (P < .02). In ND, 20% (n = 44) had vitB12 above 650 pmol/L, and 1% (n = 3) had below 150 pmol/L (common reference limits). In 6.3% (n = 14) of ND, vitB12 was above 2SD of mean in the age-and gender-adjusted reference population, which was more frequent than in HC (n = 8, 1.6%), OR: 4.0, P = .001. Low vitB12 was equally frequent as in HC, and vitB12 z-scores were equal across the age groups. To conclude, vitB12 was higher in ND than in HC and schizophrenia, suggesting a specific feature of ND, which warrants further studies to investigate the underlying mechanisms.

Parenteral hydroxocobalamin dose intensification in five patients with different types of early onset intracellular cobalamin defects: Clinical and biochemical responses

Intracellular cobalamin metabolism (ICM) defects can be present as autosomal recessive or X-linked disorders. Parenteral hydroxocobalamin (P-OHCbl) is the mainstay of therapy, but the optimal dose has not been determined. Despite early treatment, long-term complications may develop. We have analyzed the biochemical and clinical responses in five patients with early onset of different types of ICM defects (cblC: patients 1-3; cblA: patient 4; cblX: patient 5) following daily P-OHCbl dose intensification (DI). In patient 4, P-OHCbl was started at age 10 years and in patient 5 at age 5 years. OHCbl was formulated at either, 5, 25, or 50 mg/mL. P-OHCbl was intravenously or subcutaneously (SQ) delivered, subsequently by placement of a SQ injection port except in patient 4. In all patients, homocysteine and methylmalonic acid levels, demonstrated an excellent response to various P-OHCbl doses. After age 36 months, patients 1-3 had a close to normal neurological examination with lower range developmental quotient. In patient 3, moderate visual impairment was present. Patient 4, at age 10 years, had normal renal, visual and cognitive function. In cblX patient 5, epilepsy was better controlled. In conclusion, P-OHCbl-DI caused an excellent control of metabolites in all patients. In the three cblC patients, comparison with patients, usually harboring identical genotype and similar metabolic profile, was suggestive of a positive effect, in favor of clinical efficacy. With P-OHCbl-DI, CblA patient has been placed into a lower risk to develop renal and optic impairment. In cblX patient, lower P-OHCbl doses were administrated to improve tolerability.

Phenotype, treatment practice and outcome in the cobalamin-dependent remethylation disorders and MTHFR deficiency: Data from the E-HOD registry

AIM

To explore the clinical presentation, course, treatment and impact of early treatment in patients with remethylation disorders from the European Network and Registry for Homocystinurias and Methylation Defects (E-HOD) international web-based registry.

RESULTS

This review comprises 238 patients (cobalamin C defect n = 161; methylenetetrahydrofolate reductase deficiency n = 50; cobalamin G defect n = 11; cobalamin E defect n = 10; cobalamin D defect n = 5; and cobalamin J defect n = 1) from 47 centres for whom the E-HOD registry includes, as a minimum, data on medical history and enrolment visit. The duration of observation was 127 patient years. In 181 clinically diagnosed patients, the median age at presentation was 30 days (range 1 day to 42 years) and the median age at diagnosis was 3.7 months (range 3 days to 56 years). Seventy-five percent of pre-clinically diagnosed patients with cobalamin C disease became symptomatic within the first 15 days of life. Total homocysteine (tHcy), amino acids and urinary methylmalonic acid (MMA) were the most frequently assessed disease markers; confirmatory diagnostics were mainly molecular genetic studies. Remethylation disorders are multisystem diseases dominated by neurological and eye disease and failure to thrive. In this cohort, mortality, thromboembolic, psychiatric and renal disease were rarer than reported elsewhere. Early treatment correlates with lower overall morbidity but is less effective in preventing eye disease and cognitive impairment. The wide variation in treatment hampers the evaluation of particular therapeutic modalities.

CONCLUSION

Treatment improves the clinical course of remethylation disorders and reduces morbidity, especially if started early, but neurocognitive and eye symptoms are less responsive. Current treatment is highly variable. This study has the inevitable limitations of a retrospective, registry-based design.