Encephalitis/Encephalopathy, Mild, with Reversible Myelin Vacuolization (MMERV): Research Report

MONDO:0020853 · OMIM #618113 · Gene: MYRF (OMIM *608329; hgnc:1181)

Scope: Fewer than 20 genotyped patients have been published. The quotes below marked "verified" were read in this session from the repository's local reference cache (references_cache/) or from OMIM. I have not checked PMIDs marked "unverified". Run just fetch-reference on them before citing. The PubMed and OLS MCP servers need authorization in claude.ai connector settings and were not available, so no new ontology lookups were made. Every CURIE below either already appears in kb/disorders/Encephalitis_Encephalopathy_Mild_With_Reversible_Myelin_Vacuolization.yaml or is marked as a suggestion to be looked up.

1. Disease Information

Overview. MMERV is an autosomal dominant disorder. It causes repeated or single episodes of acute, mild encephalopathy, usually triggered by fever or infection, in people whose development is otherwise normal. During an episode, MRI shows extensive symmetric restricted diffusion in the corpus callosum (especially the splenium) and the deep cerebral white matter. The lesions and the symptoms resolve within days to about 2 weeks. The disorder lies on the same clinical spectrum as sporadic MERS (mild encephalitis/encephalopathy with a reversible splenial lesion), but it is familial and the white matter involvement is more extensive.

OMIM summary (verified via OMIM mirror): MMERV is an "episodic acute reversible encephalopathy that occurs in children and is frequently associated with a trigger, such as a febrile illness."

Identifiers | System | ID | |---|---| | MONDO | MONDO:0020853 | | OMIM (phenotype) | #618113 | | OMIM (gene) | *608329 (MYRF); allelic variant .0001 = Q403R | | MedGen | C4722446 | | KEGG Disease | H02489 | | Orphanet / ICD-10 / ICD-11 | No specific code found. ICD-10 G93.4 (other and unspecified encephalopathy) or G93.40 is the nearest generic code. |

Synonyms: MMERV; MYRF-related mild encephalopathy with reversible myelin vacuolization (the name proposed by Kurahashi et al. 2018); familial MERS; familial mild encephalopathy with a reversible splenial lesion and extensive white matter involvement.

Source of the data: Everything comes from individual pedigree and case reports. No registry or EHR cohort exists.

Name origin (verified, PMID:29265453): "We propose the term 'MYRF-related mild encephalopathy with reversible myelin vacuolization.'"

2. Etiology

Primary cause. A heterozygous germline missense variant in MYRF. The recurrent variant is c.1208A>G, p.Gln403Arg (Q403R), which sits in the conserved Ndt80-like DNA-binding domain.

"we identified a heterozygous c.1208A>G predicting p.Gln403Arg in the highly conserved DNA-binding domain in the myelin regulatory factor (MYRF) gene." (PMID:29265453, verified)

Triggers (environmental and infectious). Febrile infections trigger episodes. Reported triggers include unspecified febrile viral illnesses, SARS-CoV-2 (PMID:35990002), and influenza or COVID-like fever in the Chinese proband (PMID:38155716). One allelic case with a different variant was triggered by influenza A H1N1 (PMID:41190523). OMIM also lists stress as a trigger.

"Infectious diseases in children are a major triggering factor for MMERV" (PMID:38155716, verified)

Gene–environment interaction. The published working model is that mutant MYRF supports myelin adequately at baseline but becomes insufficient under the metabolic and inflammatory stress of infection. OMIM attributes this hypothesis to Kurahashi et al. It is plausible but has not been tested experimentally.

Genetic risk and modifiers. No modifier genes are known. Penetrance is incomplete (Section 9).

Protective factors. None are known. The 2023 review suggests reducing infections:

"Minimizing infections can help reduce disease recurrence." (PMID:38155716, verified) This is an expert suggestion without supporting data.

Sporadic MERS is genetically different. Kurahashi et al. sequenced MYRF in 33 sporadic MERS cases and found the variant only in the second family. MYRF therefore does not account for typical sporadic MERS.

3. Phenotypes

All phenotypes are episodic, start in childhood (as young as 2–3 years) through young adulthood, are mild, and reverse completely. Frequencies cannot be estimated reliably from roughly 9–15 patients; the qualitative estimates below are my own.

Key clinical quotes (verified): - PMID:29265453: "recurrent encephalopathy with extensive but reversible cerebral myelin vacuolization and neurological symptoms similar to those of MERS spanning 3 generations." - PMID:35990002: "a 14-year-old girl with a heterozygous p. Gln403Arg variant in the MYRF gene, who had five episodes of encephalopathy. She experienced reduced consciousness, numbness in the arm, and impaired verbal communication from day 4 of SARS-CoV-2 infection." - PMID:38155716: "The clinical manifestations of MMERV include seizures, impaired speech (such as lack of fluency, dysarthria, and language difficulties), recurrent episodes of impaired consciousness (confusion or delirium), spontaneous motor difficulties, and limb numbness"

Quality of life. No quality-of-life instrument data exist. Between episodes patients function normally. The burden comes from recurrent acute hospitalizations and the anxiety that goes with them.

4. Genetic and Molecular Information

• Gene: MYRF (myelin regulatory factor; formerly C11orf9). Location 11q12.2. hgnc:1181 (in the local HGNC cache).
• Variant: NM_001127392 c.1208A>G, p.Gln403Arg. It is in the N-terminal Ndt80/PhoG-like DNA-binding domain and was found in both Japanese families. OMIM reports that haplotype analysis suggested a founder effect. The Chinese family (PMID:38155716) carries a heterozygous MYRF variant described as related. Check the exact variant in that paper's full text before recording it.
• Classification: OMIM lists it as allelic variant .0001. I did not check its ClinVar classification in this session.
• Origin: Germline, segregating with disease.
• Population frequency: Not established here. Kurahashi et al. filtered for rare variants. Check gnomAD.
• Functional effect: Partial loss of function, a hypomorphic reduction in transactivation:

  "Luciferase assay revealed that transcriptional activity of the N-terminal region of MYRF was significantly diminished by introducing the c.1208A>G variant." (PMID:29265453, verified)

Whether Q403R acts dominant-negatively on the MYRF trimer has not been tested. The trimerization requirement (PMID:23966832) makes this a reasonable open question. - Allelic MYRF disorders (spectrum context). These are the reason to keep MMERV as its own entry: - Cardiac-urogenital syndrome: de novo, mostly loss-of-function variants; congenital heart disease, diaphragmatic hernia, and genital anomalies. Key paper: Pinz et al. 2018 (unverified PMID). - Nanophthalmos / high hyperopia: Garnai et al. 2019 and others (unverified PMIDs). - Persistent (non-reversible) leukoencephalopathy after H1N1 infection, caused by the C-terminal variant p.Gly735Asp: > "establishes persistent leukoencephalopathy as a novel clinical phenotype within the MYRF disease spectrum." (PMID:41190523, verified) - Epigenetic or chromosomal changes: None reported.

5. Environmental Information

• Infectious triggers: non-specific febrile viral infections; SARS-CoV-2 (NCBITaxon:2697049, look up); influenza A virus (NCBITaxon:11320, look up) in the allelic persistent-leukoencephalopathy case.
• Lifestyle factors: none known.
• Toxins: none implicated.
• Suggested exposure term: an ECTO "exposure to virus" class. Search ECTO before binding one; the repository forbids writing CURIEs from memory.

6. Mechanism and Pathophysiology

Causal chain

1. Heterozygous MYRF c.1208A>G (p.Gln403Arg) changes a conserved residue in the N-terminal DNA-binding domain (PMID:29265453).
2. This reduces the transcriptional activation activity of the cleaved N-terminal MYRF fragment. This was shown in luciferase assays (in vitro).
3. MYRF is normally a membrane-anchored ER protein. It trimerizes and cleaves itself through an intramolecular chaperone domain related to bacteriophage tailspike proteins, which releases the N-terminal transcription factor into the nucleus (PMID:23966832, PMID:23966833). The variant therefore weakens a normally constitutive myelin-gene transcription program after release; the cleavage step itself is unaffected.
4. Less direct activation of myelin and oligodendrocyte gene enhancers. The link to specific genes is inferred. ChIP-seq in the normal state shows that the N-terminal fragment "directly binds the enhancer regions of oligodendrocyte-specific and myelin genes" (PMID:23966833, verified). Target genes include MBP, PLP1, MOG, and MAG.
5. Mature oligodendrocytes have a reduced reserve for maintaining myelin. This is inferred from mouse models: adult deletion of Myrf causes loss of oligodendrocyte identity and myelin integrity (Koenning et al. 2012, J Neurosci, PMID:22956843, unverified).
6. Febrile or systemic infection acts as a second hit, through fever, cytokines, and possibly metabolic stress. The mechanism is hypothesized, not demonstrated.
7. Intramyelinic edema and myelin vacuolization follow, through splitting of myelin lamellae at the intraperiod line. The authors infer this from imaging; there is no human histopathology.
8. Branch, imaging: restricted diffusion (low ADC) and T2/FLAIR hyperintensity in the corpus callosum (splenium first) and the centrum semiovale.
9. Transiently impaired axonal conduction in the affected white matter tracts (inferred) causes encephalopathy, confusion or delirium, seizures, dysarthria, and ataxia.
10. The vacuolization resolves once the trigger clears, so imaging and symptoms normalize within about 1–2 weeks. Steroids may speed recovery, but there are no controlled data.

Contrast with sporadic MERS / cytotoxic lesions of the corpus callosum (CLOCCs). In sporadic MERS, cytokine-mediated cytotoxic edema and hyponatremia have been proposed. MMERV adds a constitutional myelin-maintenance defect, which probably explains why its lesions are more extensive and why episodes recur.

Annotations (from the current entry unless marked)

• Molecular function: GO:0000981, DNA-binding transcription factor activity, RNA polymerase II-specific.
• Biological processes: GO:0045944 positive regulation of transcription by RNA polymerase II; GO:0043217 myelin maintenance; GO:0022010 central nervous system myelination; GO:0019227 neuronal action potential propagation. Suggested additions, to look up: protein autoprocessing and protein homotrimerization.
• Cell types: CL:0000128 oligodendrocyte; CL:0002453 mature oligodendrocyte.
• Cellular component, suggested: endoplasmic reticulum membrane (full-length MYRF) and nucleus (cleaved N-terminal fragment).
• Biological scale tags: the variant node is MOLECULAR; oligodendrocyte myelin maintenance is CELLULAR; vacuolization and edema are TISSUE; encephalopathy is ORGANISM.

Molecular profiling: No transcriptomic, proteomic, metabolomic, or single-cell data exist for MMERV patients. No GEO datasets were found.

7. Anatomical Structures Affected

• Organ and system: brain, central nervous system (UBERON:0001017). No other organs are involved, unlike the syndromic MYRF disorders.
• Specific sites: corpus callosum (UBERON:0002336), splenium of the corpus callosum (UBERON:0013750), cerebral hemisphere white matter (UBERON:0002437; centrum semiovale), white matter (UBERON:0002316).
• Cells: oligodendrocytes, mature oligodendrocytes, and the myelin sheath (GO cellular component "myelin sheath"; look up).
• Lateralization: bilateral and symmetric.

8. Temporal Development

• Onset: early childhood (seizures from about 32 months in Family A; ages 2–9 in Family B, with the mother affected at age 6). The Chinese proband had adult episodes from age 19, after febrile seizures in childhood.
• Pattern: acute, starting within days of a febrile illness (day 4 of SARS-CoV-2 infection in PMID:35990002).
• Course: episodic, relapsing and remitting. The disease does not progress between episodes. Up to five episodes have been reported in one patient.
• Duration of an episode: a few days to about 1 week clinically. MRI normalized within days to 13 days (PMID:38155716).
• Critical window: the febrile phase of infection.
• Remission: spontaneous, or after steroid treatment.

9. Inheritance and Population

• Inheritance: autosomal dominant (HP:0000006). Three-generation transmission is documented.
• Penetrance: incomplete. Verified quote from PMID:38155716: "His father, aunt, and grandmother all carry the same variant in the MYRF gene, despite having no history of recursive neurological symptoms or cognitive deterioration." (Note: the cached sentence says "His", although the proband is described elsewhere in the paper as female. Check this against the full text before quoting.)
• Expressivity: variable. It ranges from a single childhood febrile seizure, to adult speech dysfluency, to five or more full encephalopathy episodes.
• Founder effect: suggested in the two Japanese families (OMIM, from Kurahashi et al.).
• Epidemiology: ultra-rare. About 9 individuals in the two Japanese families (4 definite, 5 suspected affected according to OMIM), plus the Saito 2022 patient (one of the original family members followed up) and the Chinese family (2023). There is no prevalence figure.
• Populations: Japanese and Chinese so far. This probably reflects ascertainment bias, since MERS is recognized most often in East Asia.
• Sex ratio: both sexes affected.
• Anticipation, consanguinity: not reported.

10. Diagnostics

• Neuroimaging (the key test): brain MRI during an episode shows symmetric diffusion-weighted hyperintensity with low ADC in the corpus callosum and centrum semiovale, and follow-up imaging shows it resolving. Suggested NCIT term: NCIT:C16809 Magnetic Resonance Imaging (already in the entry). Repeat MRI to document reversal is required for the diagnosis.
• EEG: may show slowing during an episode (Kurahashi et al. used EEG; the details are in the full text).
• Laboratory: cerebrospinal fluid is generally unremarkable. Serum sodium is normal, unlike in many sporadic MERS cases. Infectious workup identifies the trigger. Mitochondrial DNA testing was negative in the Chinese proband.
• Genetic testing: exome sequencing found the gene. Targeted MYRF sequencing, or a leukoencephalopathy or epilepsy panel that includes MYRF, is appropriate for familial or recurrent MERS. Testing of relatives (cascade testing) is reasonable. Suggested NCIT term: NCIT:C15709 Genetic Testing (already in the entry).

  "The definitive diagnosis of MMERV can be confirmed through genetic analysis." (PMID:38155716, verified)

• Diagnostic criteria: none formal. In practice: familial or recurrent MERS-like episodes, plus extensive reversible white matter diffusion restriction, plus a heterozygous MYRF variant.
• Differential diagnosis:
• Sporadic MERS / CLOCCs (single episode, mostly splenial lesion, often hyponatremia)
• X-linked Charcot-Marie-Tooth disease type 1 (CMTX1, GJB1), which causes transient stroke-like white matter lesions
• Acute disseminated encephalomyelitis (ADEM)
• Acute necrotizing encephalopathy (RANBP2)
• Mitochondrial disease
• Toxic or metabolic leukoencephalopathy (for example, metronidazole)
• Persistent MYRF leukoencephalopathy (PMID:41190523)
• Screening: none at the population level.

11. Outcome and Prognosis

• Mortality: none reported.
• Recovery: complete after each episode. OMIM states there are no permanent neurologic sequelae. The Saito patient recovered "completely within a week" (PMID:35990002, verified).
• Morbidity: recurrent acute admissions. No cognitive decline was seen in follow-up up to adulthood.
• Prognostic factors: unknown. Allelic variants outside the DNA-binding domain (G735D) can leave persistent lesions, which suggests that genotype affects reversibility.

12. Treatment

No disease-specific therapy exists, and there are no trials. - Corticosteroids (empirical): intravenous methylprednisolone pulse, 1 g/day for 3 days (PMID:35990002), or a methylprednisolone 500 mg taper followed by oral prednisone (PMID:38155716). Suggested annotation: NCIT:C15986 Pharmacotherapy as treatment_term, with methylprednisolone (CHEBI term, look up) as therapeutic_agent and modality SMALL_MOLECULE. Recovery also happens spontaneously, so the benefit of steroids is unproven.

"She was treated with a single course of intravenous methylprednisolone pulse therapy (1 g/day for 3 days), which resulted in an immediate improvement in her symptoms." (PMID:35990002, verified) - Supportive care: antiseizure medication for acute seizures, fever control, and treatment of the infection. Suggested NCIT term: NCIT:C15747 Supportive Care. - Gene, cell, or RNA therapies; clinical trials: none. No NCT or ICTRP records were found.

13. Prevention

• Primary prevention: not applicable beyond genetic counseling. Each child of a carrier has a 50% chance of inheriting the variant, and penetrance is incomplete. Suggested NCIT term: NCIT:C15240 Genetic Counseling.
• Secondary and tertiary prevention: avoid infections and keep vaccinations up to date, including influenza and COVID-19 (expert suggestion, PMID:38155716). Prompt MRI and supportive care during febrile illness in known carriers.
• Prenatal or preimplantation testing: technically possible once the familial variant is known. Its use for a mild, reversible disorder would need careful counseling.

14. Other Species and Natural Disease

• No naturally occurring MYRF-related reversible myelin vacuolization has been reported in animals. No OMIA entry was found.
• Orthologs: mouse Myrf (MGI), zebrafish myrf, and C. elegans myrf-1 (which acts in synaptic rewiring). The Ndt80-like DNA-binding domain is conserved down to yeast Ndt80 and Dictyostelium.
• Comparative note: some veterinary toxic and inherited spongiform leukoencephalopathies show intramyelinic vacuolization, but none has been linked to MYRF.

15. Model Organisms

Key References

Web sources consulted: - OMIM #618113 (mirror) - OMIM *608329 MYRF (mirror) - MedGen C4722446 - KEGG H02489 - Frontiers in Genetics 2023 (PMID:38155716) - Saito et al. 2022, PMC9386134 - Imamura et al. 2010 bibliographic record - Recurrent MERS case series, PMC8722321