Multisystem proteinopathy (MSP), also known as inclusion body myopathy associated with Paget disease of bone and frontotemporal dementia (IBMPFD), is a progressive inherited disorder characterized by a combination of muscle weakness (myopathy), bone disease (Paget disease), neurodegeneration (frontotemporal dementia), and sometimes additional features including parkinsonism and motor neuron disease. The disease is caused by pathogenic variants in the VCP (valosin-containing protein) gene, which encodes a critical AAA+ ATPase involved in ubiquitin-dependent protein quality control. MSP exemplifies how defects in a single molecular hub can produce diverse tissue-specific phenotypes across multiple organ systems.
¶ Genetics and Molecular Biology
The VCP gene (also known as CDC48 in yeast) is located on chromosome 9p13.3 and encodes a 806-amino acid protein that functions as a AAA+ (ATPases Associated with diverse cellular Activities) ATPase. Over 40 pathogenic variants in VCP have been linked to MSP, with the most common being:
- p.R155H — The most frequent mutation, accounting for ~50% of cases
- p.R155P, p.R155C — Other common R155 variants
- p.A232G — Associated with predominantly myopathic phenotype
- p.D592N — Less severe phenotype
VCP variants exhibit autosomal dominant inheritance with variable penetrance, estimated at approximately 50% by age 50.
VCP/p97 is a highly conserved hexameric AAA+ ATPase that participates in numerous cellular processes:
- Ubiquitin-dependent protein quality control — Extraction of ubiquitinated proteins from membranes and protein complexes
- Endoplasmic reticulum-associated degradation (ERAD) — Retrotranslocation of misfolded proteins for proteasomal degradation
- Autophagy and lysosomal degradation — Processing of autophagosomes and recruitment of autophagy receptors
- Mitophagy — Selective degradation of damaged mitochondria
- DNA damage response — Chromatin remodeling and repair factor recruitment
- Cell cycle regulation — Mitotic spindle assembly and sister chromatid separation
VCP mutations cause disease through several interconnected mechanisms:
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Impaired substrate extraction — Mutant VCP has reduced ability to extract ubiquitinated substrates from cellular structures
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Autophagic flux blockade — Defective VCP disrupts autophagosome maturation and lysosomal fusion
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ER stress — Accumulation of misfolded proteins in the ER triggers unfolded protein responses
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Mitochondrial dysfunction — Impaired mitophagy leads to accumulation of damaged mitochondria
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Proteostasis network collapse — Chronic stress eventually overwhelms cellular quality control systems
¶ Inclusion Body Myopathy
Muscle weakness is the most common presenting feature, typically beginning in the third or fourth decade:
- Progressive muscle weakness — Affects proximal muscles first, particularly hip girdle muscles
- Scapular winging — Shoulder girdle involvement common
- Respiratory muscle involvement — Can lead to respiratory insufficiency
- Facial weakness — Mild facial muscle involvement in some patients
- Distal weakness — May develop later in disease course
Paget disease involves abnormal bone remodeling, typically affecting the pelvis, spine, skull, and long bones:
- Bone pain — Often in the pelvis, lumbar spine, or skull
- Deformities — Bone enlargement and shape abnormalities
- Fractures — Increased fracture risk
- Hearing loss — Temporal bone involvement can affect hearing
- High alkaline phosphatase — Elevated serum marker of bone turnover
Cognitive decline typically presents 10-20 years after myopathy onset:
- Behavioral variant FTD — Most common pattern
- Language variant FTD — Less common
- Executive dysfunction — Impaired planning, judgment, problem-solving
- Personality changes — Apathy, disinhibition, social inappropriateness
- Memory relatively preserved — Unlike Alzheimer's disease
- Motor neuron disease — ALS-like features in 10-15% of cases
- Parkinsonism — Tremor, bradykinesia, rigidity
- Cardiac involvement — Cardiomyopathy in some families
- Cataracts — Premature cataract formation
- Diabetes mellitus — Metabolic abnormalities reported
- Rimmed vacuoles — Autophagic vacuoles with basophilic debris
- TDP-43 inclusions — Phosphorylated TDP-43 aggregates
- Muscle fiber atrophy — Type 2 fiber predominance
- Inflammatory infiltrates — Mild perivascular inflammation
- Mitochondrial abnormalities — Ragged-red fibers in some cases
- Increased osteoclast activity — Excessive bone resorption
- Disorganized bone matrix — Mosaic pattern of lamellar bone
- Elevated bone turnover markers — Alkaline phosphatase, N-telopeptide
- TDP-43 proteinopathy — Phosphorylated TDP-43 inclusions in neurons and glia
- Frontotemporal atrophy — Frontotemporal lobe volume loss
- Neuronal loss — Particularly in frontal and temporal cortices
- Spinal cord involvement — Anterior horn cell loss in cases with ALS features
The diagnosis is based on meeting clinical criteria for the combination of:
- Definite MSP — Pathogenic VCP variant plus clinical syndrome
- Probable MSP — Clinical features in at least two domains (myopathy, Paget, FTD)
- Possible MSP — Single domain features with family history
- Sequencing — VCP gene sequencing identifies pathogenic variants
- Variant interpretation — ACMG guidelines for variant classification
- Family testing — Cascade screening for at-risk relatives
- Creatine kinase — Usually mildly elevated
- Electromyography — Myopathic changes with possible neurogenic features
- Muscle MRI — Fat infiltration pattern characteristic
- Bone scan — Increased uptake in Paget lesions
- Brain MRI — Frontotemporal atrophy
- Neuropsychological testing — Cognitive profile characterization
¶ Treatment and Management
- Physical therapy — Maintain strength and function
- Assistive devices — As needed for mobility
- Respiratory monitoring — Pulmonary function testing
- Cardiac surveillance — Regular echocardiography
- Bisphosphonates — First-line therapy
- Zoledronic acid — 5 mg IV single dose; long-lasting effect
- Alendronate — 40 mg daily orally
- Risedronate — 30 mg daily orally
- Calcitonin — Second-line option
- Vitamin D and calcium — Bone health maintenance
- Behavioral interventions — Structured environment, caregiver support
- SSRIs — For depression, anxiety, disinhibition
- Occupational therapy — Safety assessments, functional adaptation
- Speech therapy — For language variant
Currently no approved disease-modifying treatments, but several approaches are in development:
- AAV-VCP gene therapy — Experimental gene replacement
- Autophagy modulators — Enhancing autophagic flux
- Small molecule VCP modulators — Targeting mutant protein function
- Antisense oligonucleotides — Reducing mutant VCP expression
- Annual neurological examination
- Pulmonary function testing
- Cardiac evaluation
- Bone density scanning
- Cognitive assessment
- Neurofilament light chain (NfL) — Blood marker for neurodegeneration
- VCP protein levels — In muscle and blood
- Autophagy markers — LC3, p62 in muscle biopsies
- Autophagy enhancement — mTOR-independent activators
- ER stress modulation — Chemical chaperones
- Proteostasis network support — Heat shock protein inducers
- TDP-43 aggregation inhibitors — Small molecule approaches
AAV-mediated VCP gene delivery is being explored in preclinical models, with challenges including:
- Targeting multiple tissue types
- Achieving sufficient CNS delivery
- Balancing wild-type and mutant expression
This section highlights recent publications relevant to this disease.