Reversible Infantile Cytochrome c Oxidase Deficiency

Mendelian MONDO:0957524 Pathograph 13 Show in embeddings browser Mitochondrial Disease Inborn Error of Metabolism

Reversible infantile cytochrome c oxidase (COX) deficiency — historically called "benign cytochrome c oxidase deficiency myopathy" and now better termed reversible infantile respiratory chain deficiency (RIRCD) — is a maternally inherited mitochondrial myopathy that is exceptional among childhood mitochondrial encephalomyopathies in showing spontaneous recovery in infants who survive the first months of life. The principal molecular cause is a homoplasmic point mutation in the mitochondrial DNA-encoded transfer RNA for glutamate (MT-TE / mt-tRNA(Glu)), most commonly m.14674T>C and less often m.14674T>G. The mutation reduces steady-state levels of mt-tRNA(Glu) and impairs mitochondrial translation, producing a respiratory chain defect that affects predominantly Complex IV (cytochrome c oxidase) and variably Complexes I and III. Affected infants present in the first weeks to months of life with profound hypotonia, feeding difficulties, lactic acidosis, and a COX-deficient mitochondrial myopathy, and may require respiratory support. The distinctive feature is spontaneous clinical and biochemical reversal over the first one to three years of life, attributed to tissue-specific, largely nuclear-encoded compensatory mechanisms — primarily modulation of mt-tRNA 2-thiouridylation by the nuclear modifier TRMU. A proposed role for a developmental isoform switch of cytochrome c oxidase subunits (COX6A, COX7A) in skeletal muscle was investigated but refuted (Boczonadi et al. 2015). Because the MT-TE lesion is mitochondrially (not nuclear) encoded and carries no causal nuclear-gene edge, MONDO's placement of this entity under the nuclear-type Complex IV deficiency subtree (MONDO:0033885) is a classification artifact; RIRCD is mechanistically a mitochondrial-DNA translation disorder. The reversibility, the genetic test (a single homoplasmic mt-tRNA(Glu) variant), and the L-cysteine therapeutic rationale distinguish it from the relentlessly progressive infantile COX deficiencies.

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Inheritance

Maternal mitochondrial inheritance (homoplasmic) HP:0001427

The m.14674T>C and m.14674T>G MT-TE variants are carried on mitochondrial DNA and transmitted maternally. Unusually for an mtDNA disease they are homoplasmic rather than heteroplasmic, so there is no heteroplasmy threshold; penetrance is instead incomplete, and the reversible course reflects developmental compensation rather than a shifting mutant load.

Mitochondrial inheritance

Show evidence (1 reference)

PMID:21931168 SUPPORT Human Clinical

"Homoplasmic maternally inherited, m.14674T>C or m. 14674T>G mt-tRNA(Glu) mutations have recently been identified in reversible infantile cytochrome c oxidase deficiency (or 'benign COX deficiency')."

States both claims this block makes for the causal MT-TE variants: that they are maternally inherited, and that they are homoplasmic rather than heteroplasmic.

⚙

Pathophysiology

Homoplasmic mt-tRNA(Glu) Mutation and Impaired Mitochondrial Translation

A maternally inherited, homoplasmic m.14674T>C (or m.14674T>G) mutation in the mitochondrial DNA-encoded tRNA for glutamate (MT-TE) reduces steady-state levels of mt-tRNA(Glu) and impairs intramitochondrial protein synthesis. The severity of the translation defect is further modulated by mt-tRNA(Glu) 2-thiouridylation, which depends on the nuclear modifier TRMU; reduced 2-thiouridylation exacerbates the effect of the tRNA(Glu) mutation on mitochondrial translation. This is a mitochondrial-DNA tRNA / translation lesion, mechanistically distinct from the nuclear assembly-factor defects classified alongside it under Complex IV deficiency.

MT-TE hgnc:7479

mitochondrial translation GO:0032543 ↓ DECREASED mt-tRNA(Glu) 2-thiouridylation (wobble uridine thiolation) GO:0002143 ↓ DECREASED

Downstream

Respiratory Chain (Complex IV) Deficiency in Skeletal Muscle Impaired mitochondrial translation reduces synthesis of mtDNA-encoded respiratory chain subunits, producing a Complex IV-predominant respiratory chain deficiency.

Show evidence (4 references)

PMID:19720722 SUPPORT Human Clinical

"identifying a maternally inherited, homoplasmic m.14674T>C mt-tRNA(Glu) mutation in 17 patients from 12 families"

Defines the principal causal lesion — a homoplasmic mt-tRNA(Glu) (MT-TE) mutation in 17 patients from 12 families.

PMID:21194154 SUPPORT Human Clinical

"we identified a homoplasmic m.14674T>C or m.14674T>G mitochondrial transfer RNA-glutamate mutation"

Confirms both m.14674T>C and m.14674T>G mt-tRNA(Glu) variants as causal across an independent patient series.

PMID:21194154 SUPPORT In Vitro

"Northern blot analysis revealed decreased levels of mitochondrial transfer RNA-glutamate molecules"

The mutation lowers steady-state mt-tRNA(Glu), the proximate cause of impaired mitochondrial translation.

+ 1 more reference

Respiratory Chain (Complex IV) Deficiency in Skeletal Muscle

The mitochondrial translation defect lowers production of mtDNA-encoded cytochrome c oxidase subunits, producing a respiratory chain deficiency that affects predominantly Complex IV and variably Complexes I and III in skeletal muscle and patient-derived cybrids. The deficiency is tissue- and differentiation-dependent: it is seen in mature muscle and cybrids but not in naive myoblasts, indicating that nuclear-encoded factors can compensate.

skeletal muscle cell CL:0000188

mitochondrial electron transport, cytochrome c to oxygen GO:0006123 ↓ DECREASED mitochondrial ATP synthesis coupled electron transport GO:0042775 ↓ DECREASED

Downstream

Infantile Mitochondrial Myopathy Bioenergetic failure from the respiratory chain deficiency produces the infantile COX-deficient mitochondrial myopathy.

Developmental Compensation and Spontaneous Recovery Nuclear-encoded compensatory mechanisms progressively restore respiratory chain activity with development, driving the characteristic reversal.

Show evidence (1 reference)

PMID:21194154 SUPPORT In Vitro

"cybrids derived from patients showed decreased activity of respiratory complexes IV, and/or I, III; however, this was normal in naive myoblasts"

Patient muscle and cybrids show a Complex IV-predominant (and variably I/III) deficiency that is absent in naive myoblasts, demonstrating the differentiation-dependent biochemical defect.

Infantile Mitochondrial Myopathy

In the symptomatic phase, bioenergetic failure in skeletal muscle produces a profound infantile myopathy with hypotonia, feeding difficulties, and lactic acidosis presenting within the first months of life, frequently requiring respiratory support. Muscle histopathology shows ragged-red and COX-negative fibres with increased lipid and/or glycogen. Clinical involvement is largely confined to skeletal muscle, although basal ganglia lesions have been reported in a subset.

skeletal muscle cell CL:0000188

Downstream

Profound Hypotonia Myopathic weakness presents as profound infantile hypotonia.

Lactic Acidosis Impaired oxidative phosphorylation shifts metabolism toward lactate, producing lactic acidosis.

Feeding Difficulties Bulbar and generalized myopathic weakness impairs feeding.

Mitochondrial Myopathy with Ragged-Red and COX-Negative Fibers The respiratory chain deficiency produces the characteristic ragged-red and COX-negative fiber histopathology in skeletal muscle.

Respiratory Insufficiency Generalized myopathic weakness of respiratory muscles produces respiratory insufficiency requiring ventilatory support.

Abnormal Basal Ganglia Morphology Basal ganglia lesions occur in a small subset, possibly reflecting energy failure extending beyond skeletal muscle.

Show evidence (3 references)

PMID:21931168 SUPPORT Human Clinical

"Patients presented with subacute onset of profound hypotonia, feeding difficulties and lactic acidosis within the first months of life"

Describes the symptomatic infantile presentation of the myopathy.

PMID:21931168 SUPPORT Human Clinical

"Histopathological findings in muscle included increased lipid and/or glycogen content, ragged-red and COX negative fibres"

Documents the COX-deficient mitochondrial myopathy on muscle histopathology.

PMID:21194154 SUPPORT Human Clinical

"Clinical symptoms were limited to skeletal muscle and improved spontaneously in all cases; however, 2 siblings had basal ganglia lesions"

Establishes the predominantly skeletal-muscle phenotype with occasional basal ganglia involvement.

Developmental Compensation and Spontaneous Recovery

The hallmark of RIRCD is spontaneous clinical and biochemical recovery over the first one to three years of life. Recovery is attributed to tissue-specific, largely nuclear-encoded compensatory mechanisms rather than change in the homoplasmic mtDNA mutation load. One proposed mechanism — a developmental isoform switch of nuclear-encoded cytochrome c oxidase subunits (COX6A, COX7A) toward the heart/muscle isoforms around three months of age — was formally tested and refuted by Boczonadi et al. (2015): patient follow-up biopsies showed that the isoform switch does not contribute to disease manifestation or spontaneous recovery. The primary active mechanism is TRMU-dependent restoration of mt-tRNA 2-thiouridylation, which modulates the severity of the translation defect and drives age-dependent recovery in skeletal muscle. A mild residual myopathy may persist into adulthood.

Show evidence (4 references)

PMID:19720722 SUPPORT Human Clinical

"tissue-specific mechanisms downstream of tRNA(Glu) may explain the spontaneous recovery"

Attributes spontaneous recovery to tissue-specific downstream (nuclear) mechanisms rather than to the mtDNA mutation itself.

PMID:21194154 SUPPORT In Vitro

"Normal respiratory chain enzyme activities in naive myoblasts suggested the compensatory influence of nuclear factors"

Normal respiratory chain activity in naive myoblasts implicates compensatory nuclear factors underlying recovery.

PMID:25666558 REFUTE Human Clinical

"the physiological isoform switch does not contribute to the clinical manifestation and to the spontaneous recovery of this disease"

Patient follow-up biopsies formally refute the COX6A/COX7A isoform switch as a mechanism of spontaneous recovery in RIRCD.

+ 1 more reference

⬡

Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.

●

Phenotypes

Digestive 1

Feeding Difficulties HP:0011968

Show evidence (1 reference)

PMID:21931168 SUPPORT Human Clinical

"profound hypotonia, feeding difficulties and lactic acidosis within the first months of life"

Feeding difficulties are part of the presenting infantile phenotype.

Metabolism 1

Lactic Acidosis HP:0003128

Show evidence (2 references)

PMID:21931168 SUPPORT Human Clinical

"profound hypotonia, feeding difficulties and lactic acidosis within the first months of life"

Lactic acidosis is a presenting biochemical feature.

PMID:21931168 SUPPORT Human Clinical

"Clinical improvement was reflected by normalisation of lactic acidosis"

The lactic acidosis resolves with the characteristic clinical recovery.

Musculoskeletal 2

Profound Hypotonia HP:0001252

Show evidence (1 reference)

PMID:21931168 SUPPORT Human Clinical

"Patients presented with subacute onset of profound hypotonia, feeding difficulties and lactic acidosis within the first months of life"

Profound hypotonia is a presenting feature within the first months of life.

Respiratory Insufficiency Respiratory insufficiency due to muscle weakness HP:0002747

Show evidence (1 reference)

PMID:21931168 SUPPORT Human Clinical

"select children with an excellent prognosis for continuing respiratory support from those with severe mitochondrial presentation in infancy"

Affected infants may need continuing respiratory support during the symptomatic phase despite an excellent prognosis.

Nervous System 1

Abnormal Basal Ganglia Morphology OCCASIONAL HP:0002134

Show evidence (1 reference)

PMID:21194154 SUPPORT Human Clinical

"2 siblings had basal ganglia lesions"

Basal ganglia lesions occur in a minority, extending the phenotype beyond skeletal muscle.

Other 1

Mitochondrial Myopathy with Ragged-Red and COX-Negative Fibers Ragged-red muscle fibers HP:0003200

Show evidence (1 reference)

PMID:21931168 SUPPORT Human Clinical

"Histopathological findings in muscle included increased lipid and/or glycogen content, ragged-red and COX negative fibres"

Ragged-red and COX-negative fibres are the characteristic muscle histopathology.

🧬

Genetic Associations

MT-TE (mt-tRNA(Glu)) homoplasmic mutations causing RIRCD

Gene: MT-TE (mitochondrially encoded tRNA-Glu) hgnc:7479

Mitochondrial (maternal)

Show evidence (1 reference)

PMID:21931168 SUPPORT Human Clinical

"patients not carrying the m.14674T>C or T>G mt-tRNA(Glu) mutations may have mutations in the TRMU gene"

Documents the genetic heterogeneity of the reversible infantile phenotype, including nuclear TRMU.

Variants (2)

m.14674T>C (MT-TE)

Gene: MT-TE hgnc:7479

Homoplasmic m.14674T>C mutation in the mitochondrial tRNA for glutamate; the most common cause of RIRCD, identified in 17 patients from 12 families in the founding report and in subsequent independent series.

Show evidence (1 reference)

PMID:19720722 SUPPORT Human Clinical

"homoplasmic m.14674T>C mt-tRNA(Glu) mutation in 17 patients from 12 families"

Defines the principal causal variant and its recurrence.

m.14674T>G (MT-TE)

Gene: MT-TE hgnc:7479

Homoplasmic m.14674T>G mutation at the same mt-tRNA(Glu) nucleotide position; a less common cause, underscoring the importance of this site.

Show evidence (1 reference)

PMID:21194154 SUPPORT Human Clinical

"Identification of a novel m.14674T>G mutation in addition to m.14674T>C indicated the importance of this site for disease causation"

Establishes m.14674T>G as a second pathogenic allele at the same critical position.

💊

Medical Actions

L-Cysteine Supplementation

Action: dietary supplementation Ontology label: Dietary Intervention NCIT:C15447

Mechanistic rationale and in vitro evidence support L-cysteine supplementation in RIRCD: cysteine is required for optimal TRMU function and 2-thiomodification of mitochondrial tRNAs, and L-cysteine (but not N-acetyl-cysteine) rescued respiratory chain enzyme activities in patient cell lines. This remains an investigational, mechanism-based therapy rather than an established standard of care.

Show evidence (2 references)

PMID:23814040 SUPPORT In Vitro

"Supplementation with l-cysteine, which is required for optimal TRMU function, rescued respiratory chain enzyme activities in human cell lines of patients with RIRCD"

Provides the in vitro rationale for L-cysteine as a mechanism-based therapy.

PMID:27854233 SUPPORT In Vitro

"supplementation with L-cysteine, but not with N-acetyl-cysteine partially rescues the mitochondrial translation defect"

Confirms L-cysteine (not NAC) partially rescues the mitochondrial translation defect in patient cells.

Supportive and Respiratory Care

Action: supportive care Ontology label: Supportive Care NCIT:C15747

Management in the symptomatic phase is supportive: respiratory support, nutritional support for feeding difficulties, and treatment of lactic acidosis. Recognizing the molecular diagnosis (a homoplasmic mt-tRNA(Glu) variant) is important because it identifies infants with an excellent prognosis who warrant continued intensive support through the reversible phase.

Show evidence (1 reference)

PMID:21931168 SUPPORT Human Clinical

"enabling them to select children with an excellent prognosis for continuing respiratory support from those with severe mitochondrial presentation in infancy"

Molecular diagnosis guides continued supportive (including respiratory) care for infants with the favorable, reversible form.

{ }

Source YAML

click to show

name: Reversible Infantile Cytochrome c Oxidase Deficiency
category: Mendelian
creation_date: "2026-06-24T00:00:00Z"
synonyms:
- Reversible infantile respiratory chain deficiency
- RIRCD
- Benign infantile mitochondrial myopathy
- Benign cytochrome c oxidase deficiency myopathy
- Reversible infantile cytochrome c oxidase deficiency myopathy
- COX deficiency, benign infantile mitochondrial myopathy
description: >
  Reversible infantile cytochrome c oxidase (COX) deficiency — historically
  called "benign cytochrome c oxidase deficiency myopathy" and now better termed
  reversible infantile respiratory chain deficiency (RIRCD) — is a maternally
  inherited mitochondrial myopathy that is exceptional among childhood
  mitochondrial encephalomyopathies in showing spontaneous recovery in infants
  who survive the first months of life. The principal molecular cause is a
  homoplasmic point mutation in the mitochondrial DNA-encoded transfer RNA for
  glutamate (MT-TE / mt-tRNA(Glu)), most commonly m.14674T>C and less often
  m.14674T>G. The mutation reduces steady-state levels of mt-tRNA(Glu) and
  impairs mitochondrial translation, producing a respiratory chain defect that
  affects predominantly Complex IV (cytochrome c oxidase) and variably Complexes
  I and III. Affected infants present in the first weeks to months of life with
  profound hypotonia, feeding difficulties, lactic acidosis, and a
  COX-deficient mitochondrial myopathy, and may require respiratory support. The
  distinctive feature is spontaneous clinical and biochemical reversal over the
  first one to three years of life, attributed to tissue-specific, largely
  nuclear-encoded compensatory mechanisms — primarily modulation of mt-tRNA
  2-thiouridylation by the nuclear modifier TRMU. A proposed role for a
  developmental isoform switch of cytochrome c oxidase subunits (COX6A, COX7A)
  in skeletal muscle was investigated but refuted (Boczonadi et al. 2015). Because
  the MT-TE lesion is mitochondrially (not nuclear) encoded and carries no causal
  nuclear-gene edge, MONDO's placement of this entity under the nuclear-type
  Complex IV deficiency subtree (MONDO:0033885) is a classification artifact;
  RIRCD is mechanistically a mitochondrial-DNA translation disorder. The
  reversibility, the genetic test (a single homoplasmic mt-tRNA(Glu) variant),
  and the L-cysteine therapeutic rationale distinguish it from the relentlessly
  progressive infantile COX deficiencies.
disease_term:
  preferred_term: Reversible infantile cytochrome c oxidase deficiency (RIRCD)
  term:
    id: MONDO:0957524
    label: COX deficiency, benign infantile mitochondrial myopathy
parents:
- Mitochondrial Disease
- Inborn Error of Metabolism
inheritance:
- name: Maternal mitochondrial inheritance (homoplasmic)
  description: >-
    The m.14674T>C and m.14674T>G MT-TE variants are carried on mitochondrial
    DNA and transmitted maternally. Unusually for an mtDNA disease they are
    homoplasmic rather than heteroplasmic, so there is no heteroplasmy
    threshold; penetrance is instead incomplete, and the reversible course
    reflects developmental compensation rather than a shifting mutant load.
  inheritance_term:
    preferred_term: Mitochondrial inheritance
    term:
      id: HP:0001427
      label: Mitochondrial inheritance
  evidence:
  - reference: PMID:21931168
    reference_title: Reversible infantile respiratory chain deficiency is a unique, genetically heterogenous mitochondrial disease.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Homoplasmic maternally inherited, m.14674T>C or m. 14674T>G mt-tRNA(Glu)
      mutations have recently been identified in reversible infantile cytochrome
      c oxidase deficiency (or 'benign COX deficiency').
    explanation: >-
      States both claims this block makes for the causal MT-TE variants: that
      they are maternally inherited, and that they are homoplasmic rather than
      heteroplasmic.
pathophysiology:
- name: Homoplasmic mt-tRNA(Glu) Mutation and Impaired Mitochondrial Translation
  description: >
    A maternally inherited, homoplasmic m.14674T>C (or m.14674T>G) mutation in
    the mitochondrial DNA-encoded tRNA for glutamate (MT-TE) reduces steady-state
    levels of mt-tRNA(Glu) and impairs intramitochondrial protein synthesis. The
    severity of the translation defect is further modulated by mt-tRNA(Glu)
    2-thiouridylation, which depends on the nuclear modifier TRMU; reduced
    2-thiouridylation exacerbates the effect of the tRNA(Glu) mutation on
    mitochondrial translation. This is a mitochondrial-DNA tRNA / translation
    lesion, mechanistically distinct from the nuclear assembly-factor defects
    classified alongside it under Complex IV deficiency.
  genes:
  - preferred_term: MT-TE
    term:
      id: hgnc:7479
      label: MT-TE
  biological_processes:
  - preferred_term: mitochondrial translation
    term:
      id: GO:0032543
      label: mitochondrial translation
    modifier: DECREASED
  - preferred_term: mt-tRNA(Glu) 2-thiouridylation (wobble uridine thiolation)
    term:
      id: GO:0002143
      label: tRNA wobble position uridine thiolation
    modifier: DECREASED
  evidence:
  - reference: PMID:19720722
    reference_title: "Molecular basis of infantile reversible cytochrome c oxidase deficiency myopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "identifying a maternally inherited, homoplasmic m.14674T>C mt-tRNA(Glu) mutation in 17 patients from 12 families"
    explanation: Defines the principal causal lesion — a homoplasmic mt-tRNA(Glu) (MT-TE) mutation in 17 patients from 12 families.
  - reference: PMID:21194154
    reference_title: "Reversible infantile respiratory chain deficiency: a clinical and molecular study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "we identified a homoplasmic m.14674T>C or m.14674T>G mitochondrial transfer RNA-glutamate mutation"
    explanation: Confirms both m.14674T>C and m.14674T>G mt-tRNA(Glu) variants as causal across an independent patient series.
  - reference: PMID:21194154
    reference_title: "Reversible infantile respiratory chain deficiency: a clinical and molecular study."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Northern blot analysis revealed decreased levels of mitochondrial transfer RNA-glutamate molecules"
    explanation: The mutation lowers steady-state mt-tRNA(Glu), the proximate cause of impaired mitochondrial translation.
  - reference: PMID:23814040
    reference_title: "Altered 2-thiouridylation impairs mitochondrial translation in reversible infantile respiratory chain deficiency."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "down-regulation of TRMU in RIRCD impairs 2-thiouridylation and exacerbates the effect of the mt-tRNA(Glu) mutation by triggering a mitochondrial translation defect in vitro"
    explanation: Links reduced mt-tRNA(Glu) 2-thiouridylation (a TRMU-dependent modification) to worsening of the translation defect.
  downstream:
  - target: Respiratory Chain (Complex IV) Deficiency in Skeletal Muscle
    causal_link_type: DIRECT
    description: Impaired mitochondrial translation reduces synthesis of mtDNA-encoded respiratory chain subunits, producing a Complex IV-predominant respiratory chain deficiency.
- name: Respiratory Chain (Complex IV) Deficiency in Skeletal Muscle
  description: >
    The mitochondrial translation defect lowers production of mtDNA-encoded
    cytochrome c oxidase subunits, producing a respiratory chain deficiency that
    affects predominantly Complex IV and variably Complexes I and III in skeletal
    muscle and patient-derived cybrids. The deficiency is tissue- and
    differentiation-dependent: it is seen in mature muscle and cybrids but not in
    naive myoblasts, indicating that nuclear-encoded factors can compensate.
  cell_types:
  - preferred_term: skeletal muscle cell
    term:
      id: CL:0000188
      label: cell of skeletal muscle
  biological_processes:
  - preferred_term: mitochondrial electron transport, cytochrome c to oxygen
    term:
      id: GO:0006123
      label: mitochondrial electron transport, cytochrome c to oxygen
    modifier: DECREASED
  - preferred_term: mitochondrial ATP synthesis coupled electron transport
    term:
      id: GO:0042775
      label: mitochondrial ATP synthesis coupled electron transport
    modifier: DECREASED
  evidence:
  - reference: PMID:21194154
    reference_title: "Reversible infantile respiratory chain deficiency: a clinical and molecular study."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "cybrids derived from patients showed decreased activity of respiratory complexes IV, and/or I, III; however, this was normal in naive myoblasts"
    explanation: Patient muscle and cybrids show a Complex IV-predominant (and variably I/III) deficiency that is absent in naive myoblasts, demonstrating the differentiation-dependent biochemical defect.
  downstream:
  - target: Infantile Mitochondrial Myopathy
    causal_link_type: DIRECT
    description: Bioenergetic failure from the respiratory chain deficiency produces the infantile COX-deficient mitochondrial myopathy.
  - target: Developmental Compensation and Spontaneous Recovery
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: Nuclear-encoded compensatory mechanisms progressively restore respiratory chain activity with development, driving the characteristic reversal.
- name: Infantile Mitochondrial Myopathy
  description: >
    In the symptomatic phase, bioenergetic failure in skeletal muscle produces a
    profound infantile myopathy with hypotonia, feeding difficulties, and lactic
    acidosis presenting within the first months of life, frequently requiring
    respiratory support. Muscle histopathology shows ragged-red and COX-negative
    fibres with increased lipid and/or glycogen. Clinical involvement is largely
    confined to skeletal muscle, although basal ganglia lesions have been
    reported in a subset.
  cell_types:
  - preferred_term: skeletal muscle cell
    term:
      id: CL:0000188
      label: cell of skeletal muscle
  evidence:
  - reference: PMID:21931168
    reference_title: "Reversible infantile respiratory chain deficiency is a unique, genetically heterogenous mitochondrial disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Patients presented with subacute onset of profound hypotonia, feeding difficulties and lactic acidosis within the first months of life"
    explanation: Describes the symptomatic infantile presentation of the myopathy.
  - reference: PMID:21931168
    reference_title: "Reversible infantile respiratory chain deficiency is a unique, genetically heterogenous mitochondrial disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Histopathological findings in muscle included increased lipid and/or glycogen content, ragged-red and COX negative fibres"
    explanation: Documents the COX-deficient mitochondrial myopathy on muscle histopathology.
  - reference: PMID:21194154
    reference_title: "Reversible infantile respiratory chain deficiency: a clinical and molecular study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Clinical symptoms were limited to skeletal muscle and improved spontaneously in all cases; however, 2 siblings had basal ganglia lesions"
    explanation: Establishes the predominantly skeletal-muscle phenotype with occasional basal ganglia involvement.
  downstream:
  - target: Profound Hypotonia
    causal_link_type: DIRECT
    description: Myopathic weakness presents as profound infantile hypotonia.
  - target: Lactic Acidosis
    causal_link_type: DIRECT
    description: Impaired oxidative phosphorylation shifts metabolism toward lactate, producing lactic acidosis.
  - target: Feeding Difficulties
    causal_link_type: DIRECT
    description: Bulbar and generalized myopathic weakness impairs feeding.
  - target: Mitochondrial Myopathy with Ragged-Red and COX-Negative Fibers
    causal_link_type: DIRECT
    description: The respiratory chain deficiency produces the characteristic ragged-red and COX-negative fiber histopathology in skeletal muscle.
  - target: Respiratory Insufficiency
    causal_link_type: DIRECT
    description: Generalized myopathic weakness of respiratory muscles produces respiratory insufficiency requiring ventilatory support.
  - target: Abnormal Basal Ganglia Morphology
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: Basal ganglia lesions occur in a small subset, possibly reflecting energy failure extending beyond skeletal muscle.
- name: Developmental Compensation and Spontaneous Recovery
  description: >
    The hallmark of RIRCD is spontaneous clinical and biochemical recovery over
    the first one to three years of life. Recovery is attributed to
    tissue-specific, largely nuclear-encoded compensatory mechanisms rather than
    change in the homoplasmic mtDNA mutation load. One proposed mechanism — a
    developmental isoform switch of nuclear-encoded cytochrome c oxidase subunits
    (COX6A, COX7A) toward the heart/muscle isoforms around three months of age —
    was formally tested and refuted by Boczonadi et al. (2015): patient follow-up
    biopsies showed that the isoform switch does not contribute to disease
    manifestation or spontaneous recovery. The primary active mechanism is
    TRMU-dependent restoration of mt-tRNA 2-thiouridylation, which modulates the
    severity of the translation defect and drives age-dependent recovery in
    skeletal muscle. A mild residual myopathy may persist into adulthood.
  evidence:
  - reference: PMID:19720722
    reference_title: "Molecular basis of infantile reversible cytochrome c oxidase deficiency myopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "tissue-specific mechanisms downstream of tRNA(Glu) may explain the spontaneous recovery"
    explanation: Attributes spontaneous recovery to tissue-specific downstream (nuclear) mechanisms rather than to the mtDNA mutation itself.
  - reference: PMID:21194154
    reference_title: "Reversible infantile respiratory chain deficiency: a clinical and molecular study."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Normal respiratory chain enzyme activities in naive myoblasts suggested the compensatory influence of nuclear factors"
    explanation: Normal respiratory chain activity in naive myoblasts implicates compensatory nuclear factors underlying recovery.
  - reference: PMID:25666558
    reference_title: "Investigating the role of the physiological isoform switch of cytochrome c oxidase subunits in reversible mitochondrial disease."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "the physiological isoform switch does not contribute to the clinical manifestation and to the spontaneous recovery of this disease"
    explanation: Patient follow-up biopsies formally refute the COX6A/COX7A isoform switch as a mechanism of spontaneous recovery in RIRCD.
  - reference: PMID:21931168
    reference_title: "Reversible infantile respiratory chain deficiency is a unique, genetically heterogenous mitochondrial disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "a mild myopathy persisted into adulthood"
    explanation: Recovery is remarkable but a mild residual myopathy may persist into adulthood.
phenotypes:
- name: Profound Hypotonia
  description: Profound infantile hypotonia in the symptomatic phase.
  phenotype_term:
    preferred_term: Hypotonia
    term:
      id: HP:0001252
      label: Hypotonia
  evidence:
  - reference: PMID:21931168
    reference_title: "Reversible infantile respiratory chain deficiency is a unique, genetically heterogenous mitochondrial disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Patients presented with subacute onset of profound hypotonia, feeding difficulties and lactic acidosis within the first months of life"
    explanation: Profound hypotonia is a presenting feature within the first months of life.
- name: Feeding Difficulties
  description: Feeding difficulties accompanying the infantile myopathy.
  phenotype_term:
    preferred_term: Feeding difficulties
    term:
      id: HP:0011968
      label: Feeding difficulties
  evidence:
  - reference: PMID:21931168
    reference_title: "Reversible infantile respiratory chain deficiency is a unique, genetically heterogenous mitochondrial disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "profound hypotonia, feeding difficulties and lactic acidosis within the first months of life"
    explanation: Feeding difficulties are part of the presenting infantile phenotype.
- name: Lactic Acidosis
  description: >
    Lactic acidosis in the symptomatic phase, which normalizes with spontaneous
    recovery.
  phenotype_term:
    preferred_term: Lactic acidosis
    term:
      id: HP:0003128
      label: Lactic acidosis
  evidence:
  - reference: PMID:21931168
    reference_title: "Reversible infantile respiratory chain deficiency is a unique, genetically heterogenous mitochondrial disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "profound hypotonia, feeding difficulties and lactic acidosis within the first months of life"
    explanation: Lactic acidosis is a presenting biochemical feature.
  - reference: PMID:21931168
    reference_title: "Reversible infantile respiratory chain deficiency is a unique, genetically heterogenous mitochondrial disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Clinical improvement was reflected by normalisation of lactic acidosis"
    explanation: The lactic acidosis resolves with the characteristic clinical recovery.
- name: Mitochondrial Myopathy with Ragged-Red and COX-Negative Fibers
  description: >
    Muscle biopsy in the symptomatic phase shows ragged-red and COX-negative
    fibres with increased lipid and/or glycogen content.
  phenotype_term:
    preferred_term: Ragged-red muscle fibers
    term:
      id: HP:0003200
      label: Ragged-red muscle fibers
  evidence:
  - reference: PMID:21931168
    reference_title: "Reversible infantile respiratory chain deficiency is a unique, genetically heterogenous mitochondrial disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Histopathological findings in muscle included increased lipid and/or glycogen content, ragged-red and COX negative fibres"
    explanation: Ragged-red and COX-negative fibres are the characteristic muscle histopathology.
- name: Respiratory Insufficiency
  description: >
    Respiratory insufficiency from myopathic weakness in the symptomatic phase,
    frequently necessitating ventilatory support; prognosis for those carrying
    the mt-tRNA(Glu) mutation is nonetheless excellent.
  phenotype_term:
    preferred_term: Respiratory insufficiency due to muscle weakness
    term:
      id: HP:0002747
      label: Respiratory insufficiency due to muscle weakness
  evidence:
  - reference: PMID:21931168
    reference_title: "Reversible infantile respiratory chain deficiency is a unique, genetically heterogenous mitochondrial disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "select children with an excellent prognosis for continuing respiratory support from those with severe mitochondrial presentation in infancy"
    explanation: Affected infants may need continuing respiratory support during the symptomatic phase despite an excellent prognosis.
- name: Abnormal Basal Ganglia Morphology
  description: Basal ganglia lesions reported in a subset of patients.
  phenotype_term:
    preferred_term: Abnormal basal ganglia morphology
    term:
      id: HP:0002134
      label: Abnormal basal ganglia morphology
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:21194154
    reference_title: "Reversible infantile respiratory chain deficiency: a clinical and molecular study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "2 siblings had basal ganglia lesions"
    explanation: Basal ganglia lesions occur in a minority, extending the phenotype beyond skeletal muscle.
genetic:
- name: MT-TE (mt-tRNA(Glu)) homoplasmic mutations causing RIRCD
  gene_term:
    preferred_term: MT-TE (mitochondrially encoded tRNA-Glu)
    term:
      id: hgnc:7479
      label: MT-TE
  inheritance:
  - name: Mitochondrial (maternal)
    evidence:
    - reference: PMID:19720722
      reference_title: "Molecular basis of infantile reversible cytochrome c oxidase deficiency myopathy."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "a maternally inherited, homoplasmic m.14674T>C mt-tRNA(Glu) mutation"
      explanation: The causal mtDNA variant is maternally inherited and homoplasmic, consistent with mitochondrial inheritance.
  variants:
  - name: m.14674T>C (MT-TE)
    description: >
      Homoplasmic m.14674T>C mutation in the mitochondrial tRNA for glutamate;
      the most common cause of RIRCD, identified in 17 patients from 12 families
      in the founding report and in subsequent independent series.
    gene:
      preferred_term: MT-TE
      term:
        id: hgnc:7479
        label: MT-TE
    evidence:
    - reference: PMID:19720722
      reference_title: "Molecular basis of infantile reversible cytochrome c oxidase deficiency myopathy."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "homoplasmic m.14674T>C mt-tRNA(Glu) mutation in 17 patients from 12 families"
      explanation: Defines the principal causal variant and its recurrence.
  - name: m.14674T>G (MT-TE)
    description: >
      Homoplasmic m.14674T>G mutation at the same mt-tRNA(Glu) nucleotide
      position; a less common cause, underscoring the importance of this site.
    gene:
      preferred_term: MT-TE
      term:
        id: hgnc:7479
        label: MT-TE
    evidence:
    - reference: PMID:21194154
      reference_title: "Reversible infantile respiratory chain deficiency: a clinical and molecular study."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Identification of a novel m.14674T>G mutation in addition to m.14674T>C indicated the importance of this site for disease causation"
      explanation: Establishes m.14674T>G as a second pathogenic allele at the same critical position.
  features: >
    RIRCD is most often caused by the homoplasmic m.14674T>C (or less commonly
    m.14674T>G) mt-tRNA(Glu) mutation. The disorder is genetically heterogeneous:
    a phenotypically similar reversible infantile respiratory chain deficiency
    can arise from biallelic mutations in the nuclear modifier gene TRMU, which
    2-thiouridylates mt-tRNA(Glu).
  evidence:
  - reference: PMID:21931168
    reference_title: "Reversible infantile respiratory chain deficiency is a unique, genetically heterogenous mitochondrial disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "patients not carrying the m.14674T>C or T>G mt-tRNA(Glu) mutations may have mutations in the TRMU gene"
    explanation: Documents the genetic heterogeneity of the reversible infantile phenotype, including nuclear TRMU.
treatments:
- name: L-Cysteine Supplementation
  description: >
    Mechanistic rationale and in vitro evidence support L-cysteine
    supplementation in RIRCD: cysteine is required for optimal TRMU function and
    2-thiomodification of mitochondrial tRNAs, and L-cysteine (but not
    N-acetyl-cysteine) rescued respiratory chain enzyme activities in patient
    cell lines. This remains an investigational, mechanism-based therapy rather
    than an established standard of care.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: dietary supplementation
    term:
      id: NCIT:C15447
      label: Dietary Intervention
  evidence:
  - reference: PMID:23814040
    reference_title: "Altered 2-thiouridylation impairs mitochondrial translation in reversible infantile respiratory chain deficiency."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Supplementation with l-cysteine, which is required for optimal TRMU function, rescued respiratory chain enzyme activities in human cell lines of patients with RIRCD"
    explanation: Provides the in vitro rationale for L-cysteine as a mechanism-based therapy.
  - reference: PMID:27854233
    reference_title: "Cysteine Supplementation May be Beneficial in a Subgroup of Mitochondrial Translation Deficiencies."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "supplementation with L-cysteine, but not with N-acetyl-cysteine partially rescues the mitochondrial translation defect"
    explanation: Confirms L-cysteine (not NAC) partially rescues the mitochondrial translation defect in patient cells.
- name: Supportive and Respiratory Care
  description: >
    Management in the symptomatic phase is supportive: respiratory support,
    nutritional support for feeding difficulties, and treatment of lactic
    acidosis. Recognizing the molecular diagnosis (a homoplasmic mt-tRNA(Glu)
    variant) is important because it identifies infants with an excellent
    prognosis who warrant continued intensive support through the reversible
    phase.
  treatment_term:
    preferred_term: supportive care
    term:
      id: NCIT:C15747
      label: Supportive Care
  evidence:
  - reference: PMID:21931168
    reference_title: "Reversible infantile respiratory chain deficiency is a unique, genetically heterogenous mitochondrial disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "enabling them to select children with an excellent prognosis for continuing respiratory support from those with severe mitochondrial presentation in infancy"
    explanation: Molecular diagnosis guides continued supportive (including respiratory) care for infants with the favorable, reversible form.
notes: >
  Reversible infantile cytochrome c oxidase deficiency / reversible infantile
  respiratory chain deficiency (RIRCD) is caused predominantly by the homoplasmic
  m.14674T>C (or m.14674T>G) mutation in MT-TE (mt-tRNA(Glu); hgnc:7479,
  OMIM:590025), reported by Horvath et al. (Brain 2009; PMID:19720722), Mimaki
  et al. (Ann Neurol 2010; PMID:21194154), and Uusimaa et al. (J Med Genet 2011;
  PMID:21931168). Recovery mechanisms are reviewed in Boczonadi et al. (Int J
  Biochem Cell Biol 2015; PMID:25666558, COX6A/COX7A isoform switch) and the
  2-thiouridylation / L-cysteine work of Boczonadi et al. (Hum Mol Genet 2013;
  PMID:23814040) and Bartsakoulia et al. (J Neuromuscul Dis 2016; PMID:27854233).

  Scope / classification note (issue #4239): MONDO:0957524 is filed by MONDO under
  the nuclear-type Complex IV deficiency subtree (MONDO:0033885), but the primary
  cause of RIRCD is a mitochondrial-DNA-encoded mt-tRNA(Glu) lesion impairing
  mitochondrial translation — not a nuclear assembly-factor defect — and the MONDO
  term carries no causal nuclear-gene edge. For this reason the entry is curated as
  a standalone mtDNA mitochondrial translation/myopathy disorder and is
  deliberately NOT conformed to the nuclear complex_iv_assembly_deficiency module
  nor added to the Mitochondrial_Complex_IV_Deficiency grouping. Whether RIRCD
  should be admitted to that (nuclear-type) grouping is left as a maintainer scope
  decision per the assessment in issue #4239. Note also the genetic heterogeneity:
  a phenotypically similar reversible infantile respiratory chain deficiency can be
  caused by biallelic nuclear TRMU mutations.

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