Inherited Disorders of Cobalamin Metabolism: A Practical Guide for Adult Physicians

Inherited disorders of cobalamin (vitamin B12) metabolism are a clinically heterogeneous group of inborn errors of intermediary metabolism caused by defects in intestinal absorption, plasma transport, or intracellular processing of this essential cofactor. Although classically regarded as pediatric diseases, late-onset and adult presentations - notably late-onset cblC disease (MMACHC biallelic variants) and disorders of the remethylation cycle - are increasingly recognized in internal medicine. In adults, the clinical spectrum encompasses unexplained neuropsychiatric syndromes, recurrent thromboembolic events, chronic kidney disease, and atypical hematological findings, frequently in the presence of normal serum cobalamin concentrations, creating a persistent diagnostic gap. Plasma total homocysteine, often markedly elevated and accompanied by elevated methylmalonic acid, is the most sensitive initial biochemical marker, enabling localization of the defect before confirmatory multigene sequencing. Prompt parenteral hydroxocobalamin, superior to cyanocobalamin for intracellular utilization, combined with betaine and folinic acid supplementation, can halt disease progression and partially reverse neurological injury when initiated early. In refractory or cobalamin-unresponsive forms - particularly isolated methylmalonyl-CoA mutase deficiency - liver transplantation reduces systemic metabolite burden, while gene-based and mRNA therapies represent emerging disease-modifying strategies. This Seminar synthesizes current evidence on biochemistry, classification, clinical phenotypes in adults, diagnostic strategies, and therapeutic options, with practical guidance for internists and adult clinicians encountering these challenging rare disorders.

Vitamin B12 (cobalamin), isolated in 1948 as the curative factor for pernicious anemia (Biermer’s disease), is an essential water-soluble cofactor required for two enzymatic reactions: the cytosolic remethylation of homocysteine to methionine-by-methionine synthase (dependent on methylcobalamin), and the mitochondrial isomerisation of methylmalonyl-coenzyme A to succinyl-CoA by methylmalonyl-CoA mutase (dependent on adenosylcobalamin) [1,6]. Inherited defects disrupting any step of cobalamin absorption, transport, or intracellular processing impair one or both of these pathways, generating characteristic metabolic signatures - methylmalonic acidemia, hyperhomocysteinaemia, or both - that provide the biochemical key to diagnosis [19,24].

Although these conditions were historically encountered almost exclusively in pediatric practice, adult and late-onset presentations are increasingly recognized. The reasons are manifold: expanded availability of metabolomics and next-generation sequencing panels, greater awareness among internists, and - critically - the growing number of individuals with early-diagnosed disorders who survive into adulthood under treatment. In adult clinical practice, the diagnostic challenge is compounded by phenotypic heterogeneity and by the frequent finding of normal or only mildly reduced serum cobalamin concentrations in intracellular processing defects, a discordance that can mislead clinicians into alternative diagnoses for months to years [2,3,15,19].

This review provides adult physicians and internists with an evidence-based overview of the biochemistry, classification, multisystem clinical manifestations, diagnostic algorithm, and current and emerging treatments for inherited disorders of cobalamin metabolism, with particular emphasis on adult presentations and practical clinical implications.

Biochemistry and Intracellular Cobalamin Processing

Dietary cobalamin is released from food proteins by gastric acid and pepsin and transferred sequentially through haptocorrin (gastric R-binder), intrinsic factor (secreted by gastric parietal cells), and the ileal cubam receptor complex (cubilin-amnionless) into enterocytes [12,13,17]. In the portal circulation, cobalamin is bound to transcobalamin II (TCN2), which delivers it to virtually all nucleated cells via the CD320 receptor. Following receptor-mediated endocytosis and lysosomal degradation of the transcobalamin–cobalamin complex, free cobalamin is exported into the cytosol through LMBRD1 (cblF) and ABCD4 (cblJ) [10,24,29].

Within the cytosol, MMACHC catalyzes reductive decyanation or dealkylation of incoming cobalamin species, generating cob (II)alamin - a central intermediate from which the vitamin is directed toward one of two metabolically active cofactor forms. Methylcobalamin is synthesized via the MTR/MTRR pathway and supports methionine synthase-mediated remethylation of homocysteine using 5-methyltetrahydrofolate as the methyl donor. Adenosylcobalamin is generated in mitochondria after MMADHC-mediated trafficking and MMAB (adenosyltransferase) activity, serving as cofactor for methylmalonyl-CoA mutase [1,6,24].

Defects proximal to the metabolic branch point result in combined methylmalonic acidemia and hyperhomocysteinaemia, whereas lesions affecting only one downstream pathway produce isolated biochemical phenotypes, a distinction that underpins the diagnostic algorithm presented below [19,24].

Classification of Inherited Cobalamin Disorders

Inherited cobalamin disorders are classified according to the affected step: defects of intestinal uptake and absorption, defects of plasma transport, and defects of intracellular cobalamin processing, further subdivided by somatic-cell complementation groups (cblA-cblJ and related entities) [24,27,29].

Disorders of Absorption and Plasma Transport

Intrinsic factor deficiency (autoimmune or congenital) and Imerslund-Gräsbeck syndrome (biallelic variants in CUBN or AMN) impairs ileal uptake of the intrinsic factor-cobalamin complex and typically present in childhood with megaloblastic anemia; Imerslund-Gräsbeck syndrome is additionally characterized by proteinuria reflecting cubilin's role in proximal renal tubular protein reabsorption [12,17].

Transcobalamin deficiency (biallelic TCN2 variants) impairs cellular delivery of absorbed cobalamin despite normal or near-normal serum total cobalamin concentrations - explained by predominance of haptocorrin-bound cobalamin in circulation. Presentation in early infancy includes failure to thrive, pancytopenia, and megaloblastic anemia; the condition is highly responsive to pharmacological parenteral hydroxocobalamin [21,22]. Fewer than 50 genetically confirmed cases have been reported worldwide, underscoring the need for clinical suspicion even when serum cobalamin levels appear reassuring [22].

Disorders of Intracellular Cobalamin Processing

cblC disease (MMACHC) is the most frequent intracellular cobalamin disorder, with an estimated incidence of 1:100,000-1:200,000 live births [6,8]. Early-onset cblC disease (approximately 90% of cases) presents within the first months of life with severe multisystem disease: hypotonia, megaloblastic anemia, progressive microcephaly, seizures, maculopathy, and renal thrombotic microangiopathy (particularly with the c.80A>G variant) [2,4,5,7,34]. Late-onset cblC disease - the form most relevant to adult internists - presents from adolescence to adulthood with a heterogeneous phenotype including subacute combined degeneration of the spinal cord, cognitive decline, psychiatric manifestations, and thromboembolic events; diagnosis is frequently delayed owing to its variable and nonspecific presentation [2,3,25].

Other major complementation groups are summarized in Table 1. cblA and cblB defects (MMAA, MMAB) impair adenosylcobalamin synthesis and present with isolated methylmalonic aciduria; cblA disease typically retains cobalamin responsiveness, whereas cblB disease does not [18,20,35]. cblD (MMADHC) is phenotypically heterogeneous, producing combined or isolated biochemical phenotypes depending on the affected protein domain [17,19]. cblE and cblG (MTRR, MTR) impair methionine synthase function and present with isolated hyperhomocysteinaemia without methylmalonic aciduria [19,24]. cblF and cblJ (LMBRD1, ABCD4) impair lysosomal cobalamin export and result in combined methylmalonic aciduria and homocystinuria with intralysosomal cobalamin accumulation [24,29].

Table 1: Classification and key features of inherited cobalamin disorders.

CKD = chronic kidney disease; MMA = methylmalonic acid; tHcy = total homocysteine; TMA = thrombotic microangiopathy.

Clinical Manifestations in Adults

Inherited cobalamin disorders present in adults as multisystem diseases with highly variable expressivity. The clinical picture is shaped by the underlying biochemical defect, its functional consequences (impaired methylation, methylmalonic acid accumulation, or both), and the duration of diagnostic delay. Key organ systems and their respective manifestations are summarized in Table 2.

Table 2: Systemic clinical manifestations of inherited cobalamin disorders relevant to adult medicine.

CKD = chronic kidney disease; DVT/PE = deep vein thrombosis/pulmonary embolism; Hcy = homocysteine; MMA = methylmalonic acid; MS = methionine synthase; tHcy = total plasma homocysteine; TMA = thrombotic microangiopathy.

Neurological Manifestations

Neurological disease is the dominant determinant of long-term morbidity in adults. The classic presentation of late-onset cblC disease mimics subacute combined degeneration of the spinal cord - posterior and lateral column dysfunction with sensory ataxia, spastic paraparesis, and vibration loss - clinically indistinguishable from nutritional cobalamin deficiency if biochemical profiling is not requested [2,3]. Peripheral neuropathy, motor dysfunction, and progressive cognitive decline are also well-documented. Pathophysiology is multifactorial: impaired S-adenosylmethionine–dependent methylation of myelin basic protein, direct neurotoxicity of homocysteine and methylmalonic acid, mitochondrial dysfunction, and oxidative stress [2,3,29].

Psychiatric Manifestations

Psychiatric involvement is increasingly recognized in late-onset remethylation disorders and may antedate overt neurological signs by months to years, contributing to systematic misattribution of symptoms to primary psychiatric diagnoses. The spectrum includes mood disorders (notably depression), behavioral changes, cognitive deterioration, acute confusional states, and frank psychosis [2,3,25]. Disruption of monoamine neurotransmitter pathways (dependent on methionine-derived S-adenosylmethionine), white matter dysfunction, and impaired one-carbon metabolism collectively underlie these manifestations. Adult physicians encountering young patients with treatment-refractory psychiatric presentations should consider measurement of plasma homocysteine as a first-line screen.

Thromboembolic and Cardiovascular Manifestations

Thromboembolic events - venous thrombosis, arterial thrombosis, cerebrovascular events, and microangiopathic processes - are a recognized and potentially life-threatening complication, particularly in remethylation disorders associated with markedly elevated homocysteine [2,21].

Endothelial dysfunction, impaired nitric oxide signaling, and a prothrombotic hemostatic profile driven by hyperhomocysteinaemia collectively increase vascular risk. Cardiovascular manifestations include cardiomyopathy in severe early-onset disease, pulmonary hypertension in selected cases, and widespread atherosclerotic disease in chronic hyperhomocysteinaemia [2,3,21,33].

Renal Manifestations

Renal involvement ranges from chronic tubulointerstitial nephropathy and proteinuria (especially in Imerslund-Gräsbeck syndrome and cblC disease) to acute thrombotic microangiopathy resembling atypical hemolytic uremic syndrome, most prominent in cblC disease with the c.80A>G variant.5,34 Endothelial injury driven by hyperhomocysteinaemia, oxidative stress, and complement dysregulation has been implicated. Renal dysfunction may progress despite treatment and occasionally necessitates renal replacement therapy or transplantation [27,29].

Hematological Manifestations

Megaloblastic anemia with macrocytosis, hypersegmented neutrophils, and variable pancytopenia is common, particularly in disorders affecting methylcobalamin-dependent methionine synthase activity. These abnormalities reflect functional folate trapping within the one-carbon metabolism pathway with consequent impairment of thymidylate and purine synthesis [21,33]. A critically important caveat for adult clinicians is that normal mean corpuscular volume and normal serum cobalamin concentrations do not exclude inherited cobalamin disorders; in intracellular processing defects, hematological markers are frequently unimpressive or absent at presentation.

Ophthalmological Manifestations

Pigmentary maculopathy with progressive visual impairment is a hallmark of cblC disease and may persist or progress despite biochemical control. Fundoscopic and optical coherence tomography examination should be incorporated into the initial work-up of all adults diagnosed with cblC or related disorders [2,4].

Diagnostic Strategy for Adult Clinicians

The diagnostic approach in adults begins with a high index of clinical suspicion, followed by a stepwise biochemical and molecular evaluation.19,24 A structured algorithm is presented in Figure 1.

Figure 1: Diagnostic algorithm for inborn errors of cobalamin metabolism.

Figure Legend: Clinical suspicion arises from neurologic, hematologic (e.g., megaloblastic anemia), metabolic presentations, and/or abnormal newborn screening (↑ C3 and C3/C2 ratio). Initial biochemical testing includes plasma total homocysteine (tHcy) and methylmalonic acid (MMA). Isolated MMA elevation indicates defects in adenosylcobalamin synthesis or methylmalonyl-CoA mutase (MMUT, MMAA/cblA, MMAB/cblB). Isolated tHcy elevation suggests remethylation defects (MTR/cblG, MTRR/cblE, MTHFR deficiency). Combined MMA and tHcy elevation indicate intracellular cobalamin processing or transport disorders, most commonly MMACHC (cblC), as well as MMADHC (cblD), LMBRD1 (cblF), ABCD4 (cblJ), or transport defects (TCN2, CUBN, AMN). Serum vitamin B12 may be normal; holotranscobalamin may provide additional functional information. Definitive diagnosis relies on multigene panel or exome sequencing targeting the above genes. Newborn screening by tandem mass spectrometry (↑ C3, ↑ C3/C2) enables presymptomatic diagnosis and improved outcomes.

First-Line Biochemical Screening

Plasma total homocysteine (tHcy) and urine or plasma methylmalonic acid (MMA) are the pivotal initial investigations.19,24 Isolated MMA elevation without hyperhomocysteinaemia indicates impairment of adenosylcobalamin synthesis (cblA, cblB) or methylmalonyl-CoA mutase (mut⁰/mut⁻). Isolated tHcy elevation without MMA elevation suggests a defect in methylcobalamin synthesis or methionine synthase function (cblE, cblG) or methylenetetrahydrofolate reductase deficiency. Combined elevation of both MMA and tHcy is the biochemical fingerprint of proximal intracellular processing defects - most commonly cblC (MMACHC), cblD, cblF, cblJ - or of absorption and transport disorders [19,24,30].

A critical practical point: serum total cobalamin concentrations are frequently normal or only minimally reduced in inherited intracellular processing defects, because circulating cobalamin concentrations do not reflect intracellular cobalamin utilization. Holotranscobalamin (the biologically active fraction bound to transcobalamin) may provide a more sensitive functional marker in selected cases and should be measured when clinical suspicion is high despite apparently normal serum cobalamin [10,38].

Confirmatory Molecular Diagnosis

Definitive diagnosis requires molecular genetic testing. Multigene panels or exome-based sequencing covering MMACHC, MMAA, MMAB, MMADHC, MTRR, MTR, LMBRD1, ABCD4, MMUT, TCN2, CUBN, AMN, and MTHFR are preferred, given the substantial phenotypic overlap across complementation groups [19,24,36]. Functional cell-based complementation assays in fibroblasts, although less widely available, remain the reference standard for categorization into complementation groups when genotype-phenotype correlation is unclear.

Newborn Screening and Its Implications for Adult Internists

Tandem mass spectrometry-based newborn screening - detecting elevated propionylcarnitine (C3) and an increased C3/C2 acylcarnitine ratio - identifies most infants with methylmalonic acidemia, including cblC disease, before symptom onset [36,37,41]. Adult internists are increasingly encountering patients who were diagnosed through such programs and are transitioning from pediatric metabolic services. These patients require long-term multidisciplinary monitoring and may present new complications (renal, ophthalmological, neurological) in adulthood despite early treatment.

Treatment

Parenteral Hydroxocobalamin: Drug of Choice

For all cobalamin-responsive inherited intracellular processing disorders, parenteral hydroxocobalamin is the treatment of choice over cyanocobalamin, owing to superior plasma retention, cellular uptake, and intracellular conversion efficiency to the active cofactor forms [19,22]. In cblC, cblF, and cblJ disease, treatment typically consists of intramuscular or subcutaneous hydroxocobalamin 1 mg daily during acute decompensation, followed by individualized maintenance (several times weekly to daily), guided by plasma tHcy and MMA response [6,19].

Adjunctive Therapies

Betaine (trimethylglycine) promotes alternative remethylation of homocysteine via the betaine-homocysteine methyltransferase pathway, complementing the methionine synthase-dependent route, and is a standard adjunct in remethylation disorders [6,19]. Folinic acid (vitamin B9) provides additional one-carbon units for the remethylation cycle. Carnitine supplementation is indicated in cases of secondary carnitine depletion associated with organic acidemia. Dietary protein restriction is not routinely indicated in cblC, cblF, or cblJ disorders but may be considered in isolated methylmalonic acidemia with persistent metabolic instability [6,19].

Management of Cobalamin-Unresponsive Forms

Isolated methylmalonic acidemia due to methylmalonyl-CoA mutase deficiency (mut⁰/mut⁻) is generally cobalamin-unresponsive and is managed conservatively with protein restriction, aggressive prevention of catabolic stress, and carnitine supplementation [20,35]. By contrast, cblA disease retains cobalamin responsiveness and is associated with a more favorable metabolic and clinical outcome under hydroxocobalamin therapy [18,20].

Organ Transplantation

In severe methylmalonic acidemia refractory to optimal medical management, liver transplantation substantially reduces the systemic methylmalonic acid burden and the frequency of metabolic crises, improving short-term survival [25,27,28,32,33]. However, transplantation does not fully normalize metabolite production, and renal and neurological complications may persist post-transplantation, reflecting ongoing extra-hepatic MMA generation [25,27]. Kidney transplantation provides partial metabolic correction through restoration of enzymatic activity in renal tissue and has been used in selected patients with advanced renal disease [27,29]. Combined liver-kidney transplantation offers the greatest metabolite reduction in patients with both hepatic and renal dysfunction [25,27].

Emerging Therapies

Adeno-associated viral vector-mediated gene addition, lipid nanoparticle-delivered mRNA encoding methylmalonyl-CoA mutase, and CRISPR-based genome editing strategies are under active preclinical and early clinical investigation [30,31]. Preclinical studies in animal models demonstrate a selective growth advantage for corrected hepatocytes, supporting the feasibility of durable metabolic correction without solid-organ transplantation. These approaches represent the most promising future disease-modifying strategies for cobalamin-unresponsive methylmalonic acidaemia [30,31].

Practical Approach for the Adult Internist

When an adult patient presents with an unexplained combination of neurological or psychiatric symptoms, recurrent thromboembolic events, renal impairment, or atypical hematological findings, inherited disorders of cobalamin metabolism should be included in the differential diagnosis, irrespective of serum cobalamin concentration (Panel).

Panel: Diagnostic and Therapeutic Approach in Adult Medicine.

Transition from Pediatric to Adult Care

The transition from pediatric to adult care in inherited metabolic disorders, here on cobalamin metabolism, represents a critical and increasingly prominent challenge in contemporary metabolic medicine. Advances in diagnostic and therapeutic strategies have transformed many inherited metabolic disorders from rapidly fatal childhood diseases into chronic, life-long conditions, resulting in a growing population of adolescents who survive into adulthood and require ongoing specialist care within adult-oriented health systems. However, transition remains heterogeneous and often poorly structured, with risks of loss to follow-up, treatment non-adherence, and metabolic instability during late adolescence.

Consensus recommendations emphasize that transition should be understood not as a single administrative transfer but as a purposeful, planned, and multidisciplinary process that progressively fosters patient autonomy while ensuring continuity of highly specialized metabolic care across pediatric and adult services. Key elements include early initiation of transition planning, joint pediatric-adult clinics, structured education regarding disease self-management, and the involvement of internal medicine or adult metabolic specialists experienced in rare diseases. Importantly, successful transition requires integration of medical, psychological, and social dimensions, acknowledging the evolving cognitive and emotional maturity of adolescents with IMDs and the complexity of lifelong dietary, pharmacological, and biochemical monitoring.

Despite the availability of expert consensus frameworks, implementation remains inconsistent across health systems, underscoring the need for standardized transition models embedded within national reference networks for rare metabolic diseases.

Inherited disorders of cobalamin metabolism, long regarded as exclusively pediatric entities, represent a genuinely relevant diagnostic consideration for adult physicians and internists. The protean clinical manifestations - neuropsychiatric, thromboembolic, renal, and hematological - combined with the frequently normal serum cobalamin concentrations in intracellular processing defects, create a diagnostic gap that results in years of delay and preventable irreversible organ damage.

The diagnostic key is biochemical: plasma total homocysteine and methylmalonic acid, two readily available tests, provide the metabolic fingerprint that localizes the defect before confirmatory molecular analysis. Treatment with parenteral hydroxocobalamin, initiated promptly in confirmed or highly suspected cases, can halt and partially reverse multisystem complications. Even in delayed or advanced disease, treatment reduces metabolic burden and stabilizes progression.

As expanded newborn screening programs generate a growing cohort of adult survivors with inherited cobalamin disorders, and as gene-based and mRNA therapies advance toward clinical use, internists and adult metabolic medicine specialists must develop familiarity with these conditions. Collaborative management involving internal medicine, neurology, nephrology, ophthalmology, and inherited metabolic disease services remains essential to optimize long-term outcomes in this challenging but increasingly manageable group of disorders [36,37,40].

Key Messages

Inherited disorders of intracellular cobalamin metabolism - particularly late-onset cblC disease and related remethylation defects - should be suspected in adults with unexplained neurological or psychiatric syndromes, recurrent thrombosis, renal impairment, or atypical hematological findings, even when serum vitamin B12 concentrations are within normal limits.

Plasma total homocysteine and methylmalonic acid are the pivotal first-line biochemical tests; combined elevation indicates proximal intracellular processing defects (cblC, cblD, cblF, cblJ), whereas isolated elevation of either metabolite narrows the differential to specific pathway lesions.

Parenteral hydroxocobalamin is the treatment of choice, offering superior intracellular retention and cofactor activation compared with cyanocobalamin; adjunctive betaine, folinic acid, and carnitine supplementation optimize metabolic control.

Delayed diagnosis leads to irreversible neurological injury, progressive chronic kidney disease, and recurrent vascular events; early treatment can stabilize and, in some cases, partially reverse multisystem complications.

In cobalamin-unresponsive forms, liver or combined liver–kidney transplantation reduces systemic metabolite burden; gene-addition and mRNA-based therapies currently under investigation offer the prospect of durable enzymatic correction without solid-organ transplantation.

Contributors

EA conceived and supervised the work, provided critical intellectual revision, and approved the final manuscript. JET and NLV contributed to literature review, data synthesis, and drafting. All authors critically revised the manuscript and approved the final version.

Data Sources and Tools

Literature review and manuscript preparation were informed by structured searches of PubMed and Google Scholar. Reference management was performed using EndNote. Artificial intelligence-assisted drafting and language refinement were performed using ChatGPT and Claude (Anthropic) as supportive tools; all scientific content was verified and curated by the authors.

Conflict of Interest

The authors declare that they have no competing interests.

Acknowledgments

The authors thank the patients, their families, and the clinicians and researchers of the CARE B12 Group (Groupe d'Étude des CAREnces en vitamine B12, Strasbourg) for their contributions to the advancement of knowledge in inherited disorders of cobalamin metabolism.