Methylenetetrahydrofolate reductase deficiency

MTHFR deficiency is a multisystem disorder affecting principally the central nervous system, the cerebrovascular system, and the eyes. Onset ranges from the neonatal period to adolescence or adulthood, with higher residual enzyme activity associated with later presentation. Neonatal-onset disease is the more severe form, presenting with apnoea, feeding difficulties, seizures, microcephaly, and developmental delay, and progressing to intellectual disability, motor and gait abnormalities, and neurological deterioration. Stroke may occur. Adolescent- or adult-onset disease is generally milder, presenting with thromboembolic events and lens dislocation, and less commonly with cognitive decline, psychiatric manifestations, seizures, or gait abnormalities. The clinical course is highly variable.

The estimated prevalence is 1/10,000 to 1/50,000 worldwide. MTHFR deficiency is more prevalent in populations with founder variants, notably the Bukharan Jewish from Usbekistan and Amish population from Somerset County, Pennsylvania, US.

MTHFR deficiency results from biallelic germline pathogenic variants in MTHFR (1p36.22). The gene encodes methylenetetrahydrofolate reductase, which converts 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate, the cofactor required for remethylation of homocysteine to methionine. Loss-of-function variants reduce enzyme activity, producing elevated homocysteine and reduced methionine.

MTHFR deficiency is inherited in an autosomal recessive manner. The parents of an affected individual are typically heterozygous carriers, and each sibling has a 25% risk of being affected. Prenatal molecular testing is technically possible once the familial pathogenic variants have been identified.

The diagnosis is established by identification of biallelic pathogenic MTHFR variants on molecular genetic testing. Newborn screening quantifies methionine and homocysteine in dried blood spots using tandem mass spectrometry. MTHFR deficiency is included in newborn screening programmes in some countries (for example selected US states) but is not part of the German newborn screening panel. Standard tandem mass spectrometry screening targets elevated methionine, which detects cystathionine beta-synthase deficiency; because severe MTHFR deficiency instead produces low methionine, only dedicated low-methionine or homocysteine-based protocols identify affected individuals. Affected individuals show low plasma methionine with raised plasma and urinary total homocysteine, together with reduced 5-methyltetrahydrofolate in plasma and cerebrospinal fluid.

Main differential diagnoses include:

• : combined methylmalonic acidaemia and homocystinuria (cblC, cblD-combined, cblF, cblJ) or isolated homocystinuria with low/normal methionine (cblD-homocystinuria, cblE, cblG); cblC is the most common form and typically presents in infancy with poor feeding, slow growth, and neurological abnormality; MMACHC, MMADHC, MTR, MTRR, LMBRD1, ABCD4, HCFC1, THAP11, ZNF143
• Homocystinuria caused by cystathionine beta-synthase (CBS) deficiency: markedly increased plasma total homocysteine and methionine (methionine high, not low), ectopia lentis, marfanoid skeletal involvement and thromboembolism; CBS
• Imerslund-Gräsbeck syndrome: methylmalonic acidaemia and homocystinuria secondary to vitamin B12 deficiency, macrocytic anaemia with or without proteinuria; AMN, CUBN
• MTHFD1 deficiency: megaloblastic anaemia, haemolytic uraemic syndrome, microangiopathy with retinopathy and SCID-like immunodeficiency; homocysteine may or may not be elevated; MTHFD1
• Transcobalamin II deficiency: infantile megaloblastic anaemia, poor growth, neurological and immunological involvement with methylmalonic acidaemia and homocystinuria; TCN2

Further differentials include transcobalamin receptor deficiency (CD320), giving six entities in total.

• Establishing the diagnosis: molecular genetic testing confirms the clinical diagnosis and distinguishes it from other disorders with overlapping biochemical features.
• Supporting management: identification of the germline pathogenic variants supports condition-specific monitoring and management decisions made by the treating physicians in consultation with the affected individual.
• Family member testing: once the familial pathogenic variants are known, at-risk relatives can be tested to clarify their carrier or affected status.

• Umair M, Alfadhel M. Homocystinuria due to deficiency of N(5,10)-methylenetetrahydrofolate reductase activity. 2025 May 29 [updated 2025 Dec 4]. In: Adam MP, Bick S, Mirzaa GM, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2026. Available from: . PMID: .
• Huemer M, Diodato D, Schwahn B, Schiff M, Bandeira A, Benoist JF, et al. Guidelines for diagnosis and management of the cobalamin-related remethylation disorders cblC, cblD, cblE, cblF, cblG, cblJ and MTHFR deficiency. J Inherit Metab Dis. 2017;40(1):21-48. doi:10.1007/s10545-016-9991-4. PMID: .
• Al-Tai MR, Kareem AA, Saadi NW, Omran TB, Abdul Majeed BA, Ibrahim IF, et al. A study of Iraqi patients with homocysteine remethylation disorders in a tertiary pediatric centre. Mol Genet Metab Rep. 2025;43:101217. doi:10.1016/j.ymgmr.2025.101217. PMID: . PMCID: PMC12011169.