Proximal Myopathy With Extrapyramidal Signs (MPXPS): Comprehensive Disease Characteristics Report

Disease: Proximal Myopathy With Extrapyramidal Signs (also "Myopathy With Extrapyramidal Signs," MPXPS) MONDO: MONDO:0014300 | OMIM: 615673 | Causal gene: MICU1 (HGNC:1530; OMIM 605084) Category: Mendelian, autosomal recessive

Summary

Proximal Myopathy With Extrapyramidal Signs (MPXPS; MONDO:0014300, OMIM 615673) is a rare, autosomal‑recessive, multisystem disorder caused by biallelic loss‑of‑function (LOF) variants in MICU1, the EF‑hand, calcium‑sensing gatekeeper of the mitochondrial calcium uniporter (MCU). Under resting cytosolic calcium, MICU1 — normally as a MICU1–MICU2 heterodimer — occludes the MCU pore and sets the threshold for mitochondrial Ca²⁺ entry. When MICU1 function is lost, this gate is removed, permitting uncontrolled mitochondrial Ca²⁺ uptake at low cytosolic Ca²⁺, mitochondrial Ca²⁺ overload, and severe fragmentation of the mitochondrial network. The two tissues most dependent on tightly tuned mitochondrial Ca²⁺ handling — central nervous system neurons (especially basal ganglia) and skeletal muscle — are preferentially injured, producing the clinical triad that defines the disease.

Clinically, MPXPS typically presents in early childhood (mean age at onset ≈5.9 years; median ≈3 years), with a core phenotype of proximal myopathy (~51%), learning difficulties (~72%), and a progressive extrapyramidal movement disorder, frequently accompanied by elevated creatine kinase (CK), fatigability, ataxia, reduced tendon reflexes, and speech impairment. The phenotype is broad: reported cases range from early congenital brain malformations and acute encephalopathy to adult‑onset and atypical distal myopathy presentations. Diagnosis rests on exome/panel sequencing demonstrating biallelic MICU1 LOF variants, supported by muscle biopsy (dystrophic plus neurogenic features with mitochondrial perturbation) and biochemistry (elevated CK, LDH, sometimes transaminases).

No disease‑specific therapy is approved; management is supportive (physiotherapy, movement‑disorder symptom control, developmental support, surveillance). The most mechanistically rational disease‑modifying strategy — pharmacologic inhibition of the MCU channel to counteract mitochondrial Ca²⁺ overload (e.g., Ru360/Ru265‑class agents) — remains preclinical/conceptual. This report synthesizes the molecular, clinical, genetic, and translational evidence across the 15 requested domains, flagging where information is robust, inferred, or unavailable.

Key Findings

Finding 1 — MICU1 loss-of-function is the cause of MPXPS, acting through primary mitochondrial Ca²⁺ dysregulation

The seminal study by Logan et al. (2014) identified recessive MICU1 mutations in individuals with proximal myopathy, learning difficulties, and a progressive extrapyramidal movement disorder — establishing both the causal gene and the defining clinical triad. In patient fibroblasts, agonist‑induced mitochondrial Ca²⁺ uptake at low cytosolic Ca²⁺ concentrations was paradoxically increased, cytosolic Ca²⁺ signals were reduced, and the mitochondrial network was severely fragmented despite an unchanged resting membrane potential. This demonstrated that the disease originates in a primary defect of mitochondrial Ca²⁺ signaling, not in a bioenergetic collapse of membrane potential.

"we report mutations of MICU1 in individuals with a disease phenotype characterized by proximal myopathy, learning difficulties and a progressive extrapyramidal movement disorder" — PMID: 24336167

"agonist-induced mitochondrial Ca(2+) uptake at low cytosolic Ca(2+) concentrations was increased, and cytosolic Ca(2+) signals were reduced … the mitochondrial network was severely fragmented" — PMID: 24336167

Finding 2 — Phenotypic spectrum and frequencies from a 62-patient aggregate cohort

Beheshti et al. (2026) combined 7 newly reported Iranian‑Turkish patients with 54 published cases (total 62; 61 deeply phenotyped), providing the most robust phenotype frequencies to date. The disease is a rare, autosomal‑recessive, multisystem disorder caused by biallelic LOF variants in MICU1, the calcium‑sensing gatekeeper of the MCU. Mean age at onset was 5.9 ± 7.3 years (median 3 years); 61.5% presented before age 5 and 9.5% after age 15. The most common features were learning difficulties (72%), myopathy (51%), and speech impairments (51%). Expressivity is largely genotype‑driven: monozygotic twins showed an indistinguishable course, while one atypical adult‑onset patient began at 29 years with faster progression.

"the mean age at onset was 5.9 ± 7.3 years (median = 3 years); 61.5% presented before age 5, while 9.5% manifested after 15 years" — PMID: 40898676

"common symptoms included learning difficulties (72%), myopathy (51%), and speech impairments (51%)" — PMID: 40898676

"a rare, autosomal-recessive, multisystem disorder caused by biallelic loss-of-function (LOF) variants in MICU1, the calcium-sensing gatekeeper of the mitochondrial calcium uniporter" — PMID: 40898676

Finding 3 — Mechanistic chain: uncontrolled mitochondrial Ca²⁺ uptake drives neurodegeneration and myopathy

Across mouse and human evidence, Singh et al. (2022) showed that loss of MICU1 gatekeeping produces uncontrolled mitochondrial Ca²⁺ uptake underlying the pathogenesis of neurodegeneration. Kohlschmidt et al. (2021) characterized patients carrying nonsense (p.Arg18*, p.Arg185*) and exon‑2 deletion variants who presented with ataxia, developmental delay, early‑onset myopathy, clinodactyly, attention deficits, insomnia, and impaired cognitive pain perception; their muscle biopsies revealed dystrophic and neurogenic atrophy, severe mitochondrial perturbations, altered Golgi structure, vacuoles, and altered lipid homeostasis. A recent mouse model (Shams et al., 2026) confirms that MICU1 LOF is associated with proximal muscle weakness and myopathy via elevated mitochondrial Ca²⁺.

"presenting with ataxia, developmental delay and early onset myopathy, clinodactyly, attention deficits, insomnia and impaired cognitive pain perception" — PMID: 33428302

"Muscle biopsies revealed signs of dystrophy and neurogenic atrophy, severe mitochondrial perturbations, altered Golgi structure, vacuoles and altered lipid homeostasis" — PMID: 33428302

"MICU1 loss-of-function variants in human patients are associated with proximal muscle weakness and myopathy" — PMID: 42349424

Finding 4 — Brain structural involvement and expanded/variant phenotypes

Wilton et al. (2020) reported a child with MICU1 variants and multiple congenital brain malformations — anterior perisylvian polymicrogyria, dysmorphic basal ganglia, cerebellar dysplasia, and white matter abnormalities — plus acute encephalopathy, implicating MICU1 in neurodevelopment and directly tying basal‑ganglia (extrapyramidal) pathology to the gene. Finsterer & Barwari (2024) described a novel phenotype (dolichocephaly, arachnodactyly, diplopia, distal myopathy) with MICU1 c.553C>T. Bitarafan et al. (2021) reported a 44‑year‑old with learning disability, muscle weakness, easy fatigability, reduced tendon reflexes, ataxia, gait disturbance, elevated hepatic transaminases, and elevated CK and LDH (novel nonsense c.385C>T, p.R129*), illustrating the diagnostic biochemical signature and late‑recognized disease.

"she shows anterior perisylvian polymicrogyria, dysmorphic basal ganglia, and cerebellar dysplasia in addition to white matter abnormalities" — PMID: 32395406

"learning disability, muscle weakness, easy fatigability, reduced tendon reflexes, ataxia, gait disturbance, elevated hepatic transaminases, elevated serum creatine kinase (CK), and elevated lactate dehydrogenase (LDH)" — PMID: 33969448

Finding 5 — MCU inhibition is a mechanistically rational (preclinical) therapeutic strategy

Because MICU1 loss removes gatekeeping and causes mitochondrial Ca²⁺ overload, pharmacologic inhibition of the MCU channel is the logical countermeasure. The cell‑permeable MCU inhibitor Ru265 "preserves cortical neuron respiration after lethal oxygen glucose deprivation and reduces hypoxic/ischemic brain injury" (Novorolsky et al., 2020). Ru360, an MCU‑specific inhibitor, restores altered mitochondrial dynamics in independent disease models. The largest clinical cohort explicitly proposes MCU modulation as the future therapeutic direction. No MPXPS‑specific clinical trial exists; this evidence is preclinical/conceptual.

"preserves cortical neuron respiration after lethal oxygen glucose deprivation and reduces hypoxic/ischemic brain injury" — PMID: 32126877

"Functional studies targeting MCU modulation may provide future therapeutic options." — PMID: 40898676

Finding 6 — Structural basis of gatekeeping: the MICU1–MICU2 heterodimer occludes the MCU pore

Multiple 2020 structural studies resolved the MICU1–MICU2 heterodimer and its control of MCU. Wu et al. (2020): "The MICU1‑MICU2 heterodimer regulates the mitochondrial calcium uniporter (MCU) and mitochondrial calcium uptake." Wang et al. (2020) and Park et al. (2020) showed MICU1/MICU2 act as a Ca²⁺‑dependent gate setting the threshold for uptake. These define the molecular machinery whose loss abolishes the gating threshold in disease.

"The MICU1-MICU2 heterodimer regulates the mitochondrial calcium uniporter (MCU) and mitochondrial calcium uptake" — PMID: 32790952

Finding 7 — Paralogs MICU2 and MICU3 contextualize tissue selectivity and are candidate modifiers

MICU1 regulates the mitochondrial calcium uniporter (mtCU) by homodimerizing or heterodimerizing with MICU2 or MICU3 (Berezhnaya et al., 2025). MICU2 loss independently causes a human neurodevelopmental disorder, paralleling MICU1 biology. MICU3 is a tissue‑specific enhancer of mitochondrial Ca²⁺ uptake, enriched in neurons and skeletal muscle (Patron et al., 2019; Ashrafi et al., 2020) — precisely the two tissues preferentially affected in MICU1 disease, offering a mechanistic rationale for its tissue selectivity and nominating MICU2/MICU3 as plausible genetic modifiers.

"regulated by MICU1, which can be either homodimerized or heterodimerized with MICU2 or MICU3" — PMID: 41273721

"MICU2 loss in patients leads to a neurodevelopmental disorder" — PMID: 41273721

Finding 8 — Skeletal-muscle mechanism: impaired sarcolemmal membrane repair compounds Ca²⁺ overload

Debattisti et al. (2019) — "Dysregulation of Mitochondrial Ca²⁺ Uptake and Sarcolemma Repair Underlie Muscle Weakness and Wasting in Patients and Mice Lacking MICU1" — demonstrated in MICU1‑null patients and mice that "Muscle function is regulated by Ca," and that loss of MICU1 both dysregulates mitochondrial Ca²⁺ handling and impairs repair of the injured sarcolemma (plasma membrane), producing myofiber weakness and wasting beyond Ca²⁺ overload alone. This complements biopsy findings of combined dystrophic + neurogenic atrophy and the recent mouse model linking elevated mitochondrial Ca²⁺ to proximal weakness.

"Muscle function is regulated by Ca" — PMID: 31665639

Section-by-Section Report

1. Disease Information

MPXPS is a Mendelian, autosomal‑recessive multisystem disorder in which biallelic MICU1 LOF produces a combined neuromuscular + movement‑disorder phenotype. It is best understood as a "mitochondrial calcium‑signaling disease" rather than a classic oxidative‑phosphorylation mitochondrial disease, because the primary lesion is dysregulated Ca²⁺ gating rather than loss of membrane potential (Finding 1).

• Key identifiers: MONDO:0014300; OMIM 615673; gene MICU1 (OMIM 605084, HGNC:1530). Orphanet and ICD‑10/ICD‑11/MeSH do not assign a dedicated disease‑specific code with high confidence in the reviewed sources — it is typically captured under broad categories of hereditary myopathy/muscular dystrophy with movement disorder (not confirmed in reviewed literature; flagged as a gap).
• Synonyms / alternative names: "Myopathy with extrapyramidal signs" (MPXPS); "MICU1 deficiency"; "MICU1‑related myopathy"; "MICU1‑related disorder." The disorder has been reported within limb‑girdle muscular dystrophy cohorts (e.g., a Dutch LGMD cohort reclassified one family as MICU1‑related myopathy, 33386810).
• Information source: Predominantly aggregated disease‑level resources and published individual case reports/series (e.g., the 62‑patient aggregate, Finding 2), not EHR‑derived at population scale.

2. Etiology

Causal factor: Monogenic — biallelic LOF variants in MICU1. No environmental or infectious cause; the disorder is fully genetic (Findings 1–3).

Genetic risk factors: The causal variants are the MICU1 LOF alleles themselves. Reported variant classes include nonsense (p.Arg18*, p.Arg185*, p.R129* [c.385C>T], c.553C>T) and whole‑exon deletions (exon 2 / first coding exon homozygous deletions). Consanguinity is a major contributor — many cases arise in consanguineous families (Iranian, Turkish, Indian, Lebanese cohorts), consistent with recessive inheritance of rare alleles.

Modifier genes (candidate / inferred): MICU2 and MICU3, paralogous mtCU gatekeeping/enhancing subunits, are mechanistically plausible modifiers of severity and tissue involvement (Finding 7), though direct human modifier evidence in MPXPS is not yet established.

Environmental / protective / gene–environment interactions: No established environmental risk factors, protective factors, or GxE interactions. As a highly penetrant recessive disorder, disease occurrence is determined by genotype. (No data — flagged as not applicable/gap.)

3. Phenotypes

Characteristics: Onset is predominantly childhood (median ~3 years; 61.5% before age 5) but ranges from congenital to adult‑onset (up to 29 years). Severity is variable; the movement disorder is progressive. Expressivity is largely consistent within genotype (monozygotic twin concordance; Finding 2).

Quality‑of‑life impact: Combined motor (proximal weakness, movement disorder, ataxia) and cognitive/speech impairment substantially affect mobility, communication, education, and independence. Formal EQ‑5D/SF‑36/PROMIS data for MPXPS are not available (gap).

4. Genetic / Molecular Information

• Causal gene: MICU1 (HGNC:1530; OMIM 605084), encoding the EF‑hand Ca²⁺‑sensing gatekeeper of the MCU.
• Pathogenic variants: Nonsense (p.Arg18*, p.Arg185*, p.R129*/c.385C>T, c.553C>T) and structural (homozygous deletion of exon 2 / first coding exon). All reported disease alleles are loss‑of‑function; the mechanism is loss of gatekeeping function of the mtCU (Findings 1–3, 8).
• Classification: Reported variants align with ACMG pathogenic/likely pathogenic (nonsense and exon deletions are canonical LOF).
• Allele frequency: Individual alleles are rare/absent in gnomAD (consistent with severe recessive disease). Exact per‑variant frequencies not tabulated here (gap).
• Origin: Germline, biallelic (homozygous or compound heterozygous), autosomal recessive.
• Functional consequence: Loss of function → loss of MICU1‑mediated threshold gating of MCU → uncontrolled mitochondrial Ca²⁺ uptake.
• Modifier genes: MICU2, MICU3 (candidate; Finding 7).
• Epigenetic / chromosomal: No disease‑specific epigenetic signature or recurrent chromosomal abnormality reported (gap), aside from the exon‑level deletions noted.

5. Environmental Information

No environmental, lifestyle, or infectious agents are implicated. MPXPS is purely monogenic. (Not applicable.)

6. Mechanism / Pathophysiology

Ordered causal chain (initiating lesion → clinical manifestation):

1. Biallelic LOF mutation in MICU1 (nonsense, exon deletion) → leads to loss/absence of functional MICU1 protein.
2. Loss of MICU1 → removes the MICU1–MICU2 heterodimeric gate that normally occludes the MCU pore at resting cytosolic Ca²⁺ (demonstrated structurally, Finding 6).
3. Loss of gating → results in a lowered activation threshold and uncontrolled mitochondrial Ca²⁺ uptake at low cytosolic Ca²⁺, with reduced cytosolic Ca²⁺ signals (demonstrated in patient fibroblasts, Finding 1).
4. Uncontrolled uptake → causes mitochondrial Ca²⁺ overload and severe fragmentation of the mitochondrial network (demonstrated, Finding 1).
5. Mitochondrial Ca²⁺ overload → drives (branch A, CNS) neuronal dysfunction and neurodegeneration, with structural basal‑ganglia/cerebellar/cortical abnormalities → progressive extrapyramidal movement disorder, ataxia, learning/speech difficulties (mouse + patient, Findings 3, 4).
6. Mitochondrial Ca²⁺ overload → drives (branch B, muscle) myofiber injury; additionally MICU1 loss impairs mitochondria‑dependent sarcolemmal (plasma membrane) repair → combined dystrophic + neurogenic atrophy → proximal myopathy, weakness, wasting, elevated CK (patient + mouse, Findings 3, 8).

Supporting detail by category: - Molecular pathway: Mitochondrial Ca²⁺ uniporter (mtCU) signaling — MICU1/MICU2 gating of the MCU pore (Ca²⁺ transport across inner mitochondrial membrane). GO suggestions: GO:0036444 (mitochondrial calcium uptake), GO:0051560 (mitochondrial calcium ion homeostasis), GO:0006851 (mitochondrial calcium ion transmembrane transport). - Cellular processes: Mitochondrial fragmentation/altered dynamics, neuronal apoptosis, impaired membrane repair, altered Golgi/lipid homeostasis. GO: GO:0008637 (apoptotic process; mitochondrial fragmentation), GO:0001778 (plasma membrane repair). - Protein dysfunction: Loss of function of the MICU1 EF‑hand Ca²⁺ sensor; abolished gatekeeping of MCU (UniProt Q9BPX6). - Metabolic changes: Secondary bioenergetic stress from Ca²⁺ overload; altered lipid homeostasis in muscle. - Tissue damage mechanisms: Mitochondrial Ca²⁺ overload → oxidative/mitochondrial stress and cell death in neurons; myofiber injury with defective repair. - Cell types (CL): Skeletal muscle myofiber (CL:0000188), neuron (CL:0000540), basal‑ganglia neurons. - Immune involvement: Not a primary feature.

7. Anatomical Structures Affected

• Primary organs/systems: Skeletal muscle (UBERON:0001134) and central nervous system / brain (UBERON:0000955), especially basal ganglia (UBERON:0002420), cerebellum (UBERON:0002037), cerebral cortex (polymicrogyria), and white matter.
• Secondary: Liver biochemistry abnormalities (elevated transaminases) reported; likely reflects systemic mitochondrial stress rather than primary hepatopathy.
• Tissue/cell level: Muscle tissue (myofibers; CL:0000188); neurons (CL:0000540).
• Subcellular (GO Cellular Component): Mitochondrion (GO:0005739), mitochondrial inner membrane (GO:0005743); also implicated Golgi apparatus and plasma membrane/sarcolemma (repair defect).
• Lateralization: Bilateral/symmetric proximal muscle involvement and bilateral CNS involvement.

8. Temporal Development

• Onset: Predominantly early childhood (median ~3 years; 61.5% before age 5); ranges congenital → adult (to 29 years). Onset pattern insidious/chronic, occasionally punctuated by acute encephalopathy.
• Progression: The extrapyramidal movement disorder is progressive; myopathy is chronic. Rate is variable and partly genotype‑dependent (adult‑onset case progressed faster; Finding 2).
• Course: Chronic, lifelong. No spontaneous remission reported. A critical window for any future MCU‑targeted intervention is plausibly early (before irreversible neurodegeneration), but this is inferred, not demonstrated.

9. Inheritance and Population

• Inheritance: Autosomal recessive; biallelic LOF.
• Penetrance / expressivity: Appears highly penetrant; expressivity variable but genotype‑consistent (MZ‑twin concordance).
• Consanguinity / founder effects: Strongly enriched in consanguineous populations (Iran, Turkey, India, Lebanon). No defined global founder allele, but recurrent population‑specific variants exist.
• Epidemiology: Ultra‑rare; exact prevalence/incidence undefined. The literature comprises ~62 aggregated patients worldwide (Finding 2) — prevalence is below defined rare‑disease thresholds and not formally quantified (gap).
• Sex ratio: Autosomal recessive — expected ~1:1; no strong sex bias reported.
• Carrier frequency: Not established (gap).

10. Diagnostics

• Laboratory: Elevated creatine kinase (CK), LDH, and sometimes hepatic transaminases are characteristic supportive findings (Finding 4).
• Muscle biopsy: Dystrophic + neurogenic atrophy, severe mitochondrial perturbations, altered Golgi structure, vacuoles, altered lipid homeostasis (Finding 3).
• Imaging: Brain MRI may reveal basal‑ganglia dysmorphism, polymicrogyria, cerebellar dysplasia, and white‑matter abnormalities in severe/early cases (Finding 4).
• Genetic testing (definitive): WES/WGS or a muscular‑dystrophy/neuromuscular gene panel including MICU1; confirm biallelic LOF variants. Exon‑deletion detection may require MLPA/CNV analysis (homozygous exon 2 deletions have been reported). The disorder is frequently diagnosed within broad LGMD/muscular‑dystrophy WES programs (PMIDs 33386810, 42553504).
• Differential diagnosis: Limb‑girdle muscular dystrophies, congenital/metabolic/mitochondrial myopathies, and other inherited movement disorders; the combination of proximal myopathy + progressive extrapyramidal signs + learning difficulty + raised CK points to MICU1.
• Screening: Cascade/carrier testing in consanguineous families; no newborn screening exists.

11. Outcome / Prognosis

• Survival/mortality: No formal survival statistics; the disorder is chronic and multisystem. Severe early cases with brain malformation/acute encephalopathy carry a worse outlook; many patients survive into adulthood (adult cases reported to 44 years).
• Morbidity/function: Substantial — combined motor disability (weakness, movement disorder, ataxia) and cognitive/speech impairment.
• Disease course: Progressive movement disorder; chronic myopathy. Complications may include falls, contractures, dysarthria, and developmental/educational impact.
• Prognostic factors: Age/mode of onset and genotype appear to influence course (adult‑onset faster progression in one case; MZ‑twin concordance indicates genotype‑driven prognosis). No validated prognostic biomarkers (gap).

12. Treatment

• Current standard: Supportive/symptomatic only — physiotherapy/occupational therapy for proximal weakness, symptomatic management of the extrapyramidal movement disorder, speech therapy, developmental/educational support, and surveillance.
• Disease‑modifying (preclinical/conceptual): MCU channel inhibition to counteract mitochondrial Ca²⁺ overload — e.g., Ru360 and the cell‑permeable Ru265 (neuroprotective in ischemia models; Finding 5). The largest cohort explicitly nominates MCU modulation as the future therapeutic avenue. No MPXPS‑specific clinical trial (no NCT) currently exists.
• Pharmacogenomics / gene / cell / RNA therapies: None established; gene‑replacement or ASO strategies are theoretically plausible for a recessive LOF gene but untested (gap).
• NCIT suggestions: Physical therapy (NCIT:C15351), supportive care (NCIT:C15285). MCU inhibitors are investigational and lack NCIT clinical‑intervention codes.

13. Prevention

• Primary prevention: Not applicable (monogenic). Genetic counseling for consanguineous families and at‑risk couples; carrier/cascade testing; prenatal or preimplantation genetic testing when a familial variant is known.
• Secondary/tertiary: Early diagnosis to enable supportive interventions and complication surveillance (contractures, falls, educational support). No population screening program exists.

14. Other Species / Natural Disease

• Model-organism relevance (see Section 15). No naturally occurring MPXPS‑equivalent disease is documented in companion animals or wildlife in the reviewed literature (gap; OMIA not confirmed).
• Orthology: MICU1 is evolutionarily conserved (mouse Micu1; NCBI Gene). Conservation of the mtCU gatekeeping mechanism across vertebrates underpins the validity of mouse models.
• Zoonotic potential: Not applicable (genetic disease).

15. Model Organisms

• Mouse (Mus musculus): MICU1‑deficient/LOF mice recapitulate core disease biology — uncontrolled mitochondrial Ca²⁺ uptake and neurodegeneration (Singh et al., 2022, 35302860), muscle weakness and wasting with defective sarcolemma repair (Debattisti et al., 2019, 31665639), and proximal muscle weakness from elevated mitochondrial Ca²⁺ (Shams et al., 2026, 42349424). These models validate both the CNS and muscle branches of the causal chain.
• Cellular / in vitro: Patient‑derived fibroblasts demonstrating increased mitochondrial Ca²⁺ uptake at low cytosolic Ca²⁺ and network fragmentation (Logan et al., 2014); structural studies of MICU1–MICU2 heterodimers.
• Recapitulation: Mouse models reproduce the neurodegeneration and myopathy well; the progressive extrapyramidal movement phenotype and human cognitive/speech features are less precisely modeled (limitation).
• Resources: MGI (Micu1); model details in the cited primary literature.

Mechanistic Model / Interpretation

 Biallelic MICU1 LOF (nonsense / exon deletion)
            │  (removes gatekeeper subunit)
            ▼
 Loss of MICU1–MICU2 heterodimeric gate on MCU pore   [structurally demonstrated]
            │  (lowers Ca2+ activation threshold)
            ▼
 Uncontrolled mitochondrial Ca2+ uptake at LOW cytosolic Ca2+;  reduced cytosolic Ca2+
            │
            ▼
 Mitochondrial Ca2+ OVERLOAD  +  severe mitochondrial network FRAGMENTATION
            │
       ┌────┴─────────────────────────────┐
       ▼ (CNS branch)                      ▼ (Muscle branch)
 Neuronal dysfunction /            Myofiber Ca2+ stress  +  IMPAIRED
 neurodegeneration; basal-         sarcolemma (plasma membrane) REPAIR
 ganglia/cerebellar/cortical       │
 structural abnormality            ▼
       │                           Dystrophic + neurogenic atrophy; ↑CK/LDH
       ▼
 Progressive extrapyramidal        Proximal MYOPATHY, weakness, wasting
 movement disorder, ataxia,
 learning & speech difficulty

The unifying interpretation is that MPXPS is a "too much Ca²⁺ in the mitochondrion" disease caused by loss of a molecular brake. This stands in instructive contrast to several other conditions in the reviewed literature where the MICU1:MCU ratio falls (sepsis‑induced ICU‑acquired weakness, 37695737; diabetic vascular remodeling, 40673552) — in those settings reduced MICU1 similarly dysregulates mitochondrial Ca²⁺ and weakens muscle, reinforcing that MICU1 dosage/gatekeeping is a pivotal node for both muscle and neuronal integrity. The convergent human disease caused by MICU2 loss (Finding 7) and the neuron/muscle enrichment of the MICU3 enhancer together explain why the brain and skeletal muscle — not other organs — bear the brunt of MICU1 deficiency.

Evidence Base

Limitations and Knowledge Gaps

1. Rarity limits epidemiology. With ~62 aggregated patients worldwide, prevalence, incidence, carrier frequency, and survival statistics are unquantified.
2. ICD/Orphanet/MeSH coding for MPXPS was not confirmed in the reviewed sources; identifiers beyond MONDO/OMIM/gene are uncertain.
3. Phenotype frequencies derive largely from literature‑aggregated case reports (ascertainment and reporting bias likely over‑represent severe cases).
4. Therapy is unproven. MCU inhibition is mechanistically rational but entirely preclinical; no NCT, no human efficacy/safety data in MPXPS; chronic MCU inhibition risks impairing physiological mitochondrial Ca²⁺ signaling.
5. Modifier evidence for MICU2/MICU3 is inferred from paralog biology and convergent diseases, not demonstrated as MPXPS modifiers in patients.
6. Movement‑disorder phenotype is imperfectly modeled in mice; the mechanistic link from basal‑ganglia mitochondrial Ca²⁺ overload to the specific extrapyramidal signs remains partly inferred.
7. No quality‑of‑life (EQ‑5D/SF‑36/PROMIS) or natural‑history registry data specific to MPXPS.

Proposed Follow-up Experiments / Actions

1. Establish a natural‑history registry for MICU1/MICU2 patients to quantify onset, progression rates, survival, and genotype–phenotype correlations prospectively.
2. Preclinical MCU‑inhibitor efficacy studies (Ru265/next‑generation cell‑permeable MCU inhibitors) in MICU1‑LOF mice, with CNS and muscle endpoints, defining the therapeutic window and chronic‑dosing safety.
3. Test MICU2/MICU3 as genetic modifiers — correlate paralog expression/variants with severity in patient cohorts; cross models to assess rescue or aggravation.
4. Deep muscle‑repair mechanistic work to determine whether enhancing sarcolemmal repair (independent of Ca²⁺) ameliorates myopathy.
5. Standardized brain imaging + movement‑disorder phenotyping protocol to improve detection of basal‑ganglia involvement and track extrapyramidal progression.
6. Variant curation — compile all reported MICU1 alleles in ClinVar with ACMG classifications, gnomAD frequencies, and CNV (exon‑deletion) detection recommendations for diagnostic labs.
7. Formal ontology/identifier reconciliation (Orphanet, ICD‑11, MeSH) to complete the knowledge‑base entry.

Evidence source legend: human clinical (Logan 2014; Beheshti 2026; Kohlschmidt 2021; Wilton 2020; Bitarafan 2021; Debattisti 2019 patients), model organism (Singh 2022; Shams 2026; Debattisti 2019 mice), in vitro/structural (Wu/Wang/Park 2020; Patron 2019; Ashrafi 2020), computational/mechanistic inference (MCU‑inhibition rationale; modifier hypotheses).