Cerebellar microhemorrhages have not been previously documented in methylmalonic acidemia with homocystinuria (MMA-HC), a rare inherited metabolic disorder. Herein, we reported an 18-year-old female presented with acute gait instability and dysarthria post-febrile illness. Biochemical testing revealed severe hyperhomocysteinemia. Brain MRI demonstrated bilateral cerebellar DWI/FLAIR hyperintensities. Whole-exome sequencing confirmed compound heterozygous MMACHC mutations, establishing cblC-type MMA-HC diagnosis. Symptoms resolved after one month of vitamin-based therapy. Follow-up 3.0 T MRI and 7.0 T MRI susceptibility-weighted imaging (SWI) uncovered multiple punctate cerebellar vermian microhemorrhages-a previously unreported finding. This case highlights an unusual adult-onset presentation of MMA-HC and represents the first report of SWI-detectable cerebellar vermis microhemorrhages with this condition, visualized. This finding suggests that cerebellar microhemorrhages may be an under-recognized feature in MMA-HC, particularly detectable using high-field SWI during acute exacerbations, and contributes to a more comprehensive understanding of the neurological complications in this metabolic disorder.

Combined methylmalonic acidemia and homocystinuria cblC type (cblC) is a multisystemic disease with diverse clinical presentations and known genotype-phenotype correlations. This study aims to define and explain the phenotypes and outcomes associated with the MMACHC variant c.1A>G (p.M1V), previously reported in several cases. A retrospective review of 54 Chinese patients with cblC carrying the MMACHC c.1A>G variant was conducted. Clinical features, including onset age, initial symptoms, biochemical index and prognosis were analyzed and compared with 100 cblC patients without this variant. The variant's pathogenicity was investigated by in vitro experiments. Twenty-nine (54 %) of 54 individuals with the c.1A>G variant were diagnosed via newborn screening (NBS) and 23 (79 %) remained asymptomatic. Among 19 symptomatic patients, 12 (63 %) developed symptoms after 1 year of age, with cognitive decline being the most common initial symptom (55 %). Before treatment, all analyzed biochemical indexes except homocysteine showed reduced levels in the c.1A>G group compared to the Control group. Post-treatment, the poor prognosis rate and some metabolite levels in the c.1A>G group were significantly decreased compared to those in the Control group. Western blotting indicated that c.1A>G significantly reduced MMACHC protein expression, and co-immunoprecipitation provided evidence for impaired interaction between the variant MMACHC and methionine synthase (MTR). The c.1A>G variant in MMACHC is associated with later-onset disease, milder phenotypes and improved clinical outcomes in cblC patients. Functional studies suggest that this variant reduces MMACHC translation efficiency and disrupts its interaction with MTR. Our findings underscore the utility of NBS for early diagnosis and better management in c.1A>G-associated cblC.

MMACHC protein plays a crucial role in the metabolism of vitamin B12 (cobalamin, Cbl) by catalyzing its conversion into the active forms adenosylcobalamin (AdoCbl) and methylcobalamin (MeCbl), which serve as essential cofactors in key cellular reactions. Mutations in the gene encoding MMACHC lead to the rare metabolic disorder known as methylmalonic aciduria and homocystinuria, cblC type. This condition predominantly affects children and is characterized by cardiovascular dysfunction, intellectual disability, and a severe form of maculopathy. The most common missense mutation, R161Q, impairs enzymatic activity despite not being directly involved in cobalamin binding. Here, using a comprehensive set of biophysical techniques, we demonstrate that this pathogenic variant compromises MMACHC structural stability, alters the thermal unfolding cooperativity and pathway, as well as the populations of conformational intermediates. Moreover, we show that the R161Q mutation decreases AdoCbl binding affinity and impairs the protein's ability to form homodimers, which are supposed to have a functional role. A partial recovery in protein activity upon treatment with betaine, an osmolyte known for its stabilizing effect on proteins, was observed. This suggests a direct correlation between the energetics of MMACHC thermal unfolding and its functional activity. These findings contribute to a deeper understanding of the molecular mechanisms underlying MMACHC function and open avenues for potential therapeutic interventions.

The congenital, autosomal recessive disorder combined methylmalonic acidemia and homocystinuria - cblC type, is the most common inborn error of cobalamin (vitamin B12) metabolism. In its early onset form, cblC profoundly impacts fetal development of the central nervous system, hematopoietic system, and other tissues. Previously, mutations in the MMACHC gene, which encodes a protein required for the intracellular trafficking and enzymatic processing of free cobalamin into active coenzyme forms, were found to cause cblC. These coenzymes are required in two metabolic pathways which produce either succinyl-CoA in the mitochondria or methionine in the cytosol. However, due to a lack of sufficient animal models, the exact pathophysiology of cblC remains unknown. Moreover, there is evidence to suggest that MMACHC may have roles outside of cobalamin metabolism and that cobalamin itself may be required for additional, unknown metabolic pathways. Here, we report the generation and characterization of three new mouse lines aimed at further defining the role of MMACHC and cobalamin in mammalian development. CRISPR/Cas9 genome editing was used to develop an HA-tagged version of Mmachc, which will aid in affinity purification and spatiotemporal localization of the MMACHC protein. To clarify which metabolic perturbations downstream of Mmachc loss give rise to tissue-specific developmental defects, we also created floxed alleles for both methionine synthase (Mtr) and methylmalonyl-CoA mutase (Mmut), which are the only known cobalamin dependent enzymes in mammals. In total, these new mouse models significantly expand upon the repertoire of genetic reagents to clarify the pathophysiology of cblC as well as define both the canonical and hypothesized noncanonical roles of MMACHC in mammalian development.

The aim of this study was to characterize the variable phenotypes and outcomes associated with the methylmalonic aciduria and homocystinuria type C protein gene (MMACHC) c.482G > A mutation in 195 Chinese cases with CblC disease. We carried out a national, retrospective multicenter study of 195 Chinese patients with CblC disease attributable to the MMACHC c.482G > A variant either in a homozygous or compound heterozygous state. The control group consisted of 200 patients diagnosed with CblC disease who did not possess the c.482G > A mutation. Clinical features, including disease onset, symptoms, biochemical metabolites, gene mutation, and follow-up outcomes were reviewed and analyzed in detail. The median follow-up period spanned 3 years and 8 months, with a range of 1 year and 2 months to 12 years and 10 months. Among 195 patients carrying the c.482G > A variant, 125 (64.1%) cases were diagnosed by newborn screening (NBS), 60 (30.8%) cases were detected due to disease onset, and 10 (5.1%) cases were identified from sibling diagnoses. One hundred and seventeen (93.6%) individuals who were diagnosed by NBS, and nine patients who came from sibling diagnoses remained asymptomatic in this study. From 69 symptomatic patients of the c.482G > A group, more patients presented with later onset, and the top six common clinical symptoms at disease onset were developmental delay (59.4%), lower limb weakness and poor exercise tolerance (50.7%), cognitive decline (37.7%), gait instability and abnormal posture (36.2%), seizures (26.1%), and psychiatric and behavioral disturbances (24.6%). In the 159 symptomatic patients lacking c.482G > A variants, the most frequently observed clinical manifestations at disease onset included developmental delay (81.8%), lethargy and feeding difficulty (62.9%), lower limb weakness and poor exercise tolerance (54.7%), prolonged neonatal jaundice (51.6%), vomiting (47.2%), and seizures (32.7%). Before treatment, the levels of blood propionylcarnitine, propionylcarnitine/acetylcarnitine ratio, and homocysteine in the c.482G > A group were significantly lower (P < 0.05) than those in the non-c.482G > A group, while the concentration of urinary methylmalonic acid was slightly lower (P > 0.05). The degree of decline in the above metabolites after treatment in different groups significantly differed in both plasma total homocysteine values and urinary methylmalonic acid levels (P < 0.05). In patients carrying the c.482G > A variant compared with the non-c.428G > A group, there were markedly lower rates of mortality (0.5% vs. 2.0%) and developmental delay (20.5% vs. 65.5%). When compared with individuals diagnosed due to disease onset, those identified through NBS in either group exhibited a reduced proportion of disease onset (6.7% vs. 100% in the c.482G > A group, 54.4% vs. 100% in the non-c.482G > A group), lower mortality (0.0% vs. 1.7% in the c.482G > A group, 0.0% vs. 3.6% in the non-c.482G > A group), and had a higher percentage of patients exhibiting normal psychomotor and language development (99.3% vs. 33.3% in the c.482G > A group, 58.9% vs. 10.9% in the non-c.482G > A group). The c.482G > A variant in MMACHC is associated with late-onset and milder phenotypes of CblC disease. Patients with this mutation tend to have a relatively better response to hydroxocobalamin, better metabolic control, and more favorable neurological outcomes. NBS and other appropriate pre-symptomatic treatments seem to be helpful in early diagnosis, resulting in favorable clinical outcomes. Video Abstract (MP4 136794 kb).