SQSTM1-related Multisystem Proteinopathy (MSP3): A Comprehensive Disease Characterization

MONDO ID: MONDO:0800464 · OMIM: 616887 (MSP3) · Gene: SQSTM1/p62 (HGNC:11280; NCBI Gene 8878; UniProt Q13501; chr5q35) · Category: Mendelian Report type: Disease knowledge-base characterization · Evidence base: 10 confirmed findings, 60 papers reviewed

Summary

SQSTM1-related multisystem proteinopathy (MSP3) is a dominantly inherited, adult-onset, pleiotropic disorder caused by germline variants in SQSTM1, the gene encoding the autophagy receptor/signaling scaffold p62/sequestosome-1. A single genetic lesion produces a remarkably variable clinical spectrum: inclusion body/distal myopathy (IBM/MRV), amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and Paget disease of bone (PDB), unified pathologically by p62- and TDP-43–positive protein aggregates and rimmed vacuoles. Most pathogenic variants cluster in the ubiquitin-associated (UBA) domain, with p.P392L (c.1175C>T) the single most common allele across populations (PMID: 17229007).

Mechanistically, the disease branches from one p62 lesion into two arms. A gain-of-function bone arm operates through enhanced RANK→NF-κB signaling that drives osteoclast overactivity and pagetic bone remodeling — but this arm is incompletely penetrant and requires an environmental "second hit" (paramyxoviral measles nucleocapsid protein acting via IL-6), as demonstrated in knock-in mice (PMID: 24339057). A loss-of-function neuromuscular arm reflects impaired selective autophagy and mitophagy, causing TDP-43 aggregation and degeneration of muscle and motor/cortical neurons. Strikingly, biallelic loss-of-function of the same gene produces a completely distinct recessive childhood-onset neurodegeneration with ataxia, dystonia, and vertical gaze palsy (DNECAS) (PMID: 27545679).

The clinical and prognostic bottom line is organ-dependent: the bone arm is largely benign and highly treatable/preventable with zoledronic acid — including genotype-guided prophylaxis that prevented new lesions in a randomized trial of SQSTM1 carriers (PMID: 38123339) — whereas the neuromuscular arms (ALS/FTD, distal myopathy, childhood neurodegeneration) are progressive, life-limiting, and lack disease-modifying therapy. A rare but grave complication of the bone arm is malignant (osteosarcoma/giant-cell) transformation in <1% of polyostotic PDB, often associated with a ZNF687 second-hit genotype (PMID: 32106343).

Key Findings

Finding 1 — SQSTM1 causes a pleiotropic multisystem proteinopathy spanning IBM, ALS, FTD, and Paget disease of bone

Heterozygous SQSTM1 variants produce a clinical spectrum encompassing inclusion body myopathy (IBM), motor neuron disease/ALS, frontotemporal dementia (FTD), and Paget disease of bone (PDB), unified by shared TDP-43– and p62-positive aggregate pathology and rimmed vacuoles. SQSTM1 is formally designated MSP3 (OMIM 616887), one of a family of MSP genes that also includes VCP, HNRNPA2B1, and HNRNPA1.

As the landmark MSP review states: "Mutations in VCP, HNRNPA2B1, HNRNPA1, and SQSTM1, encoding RNA-binding proteins or proteins in quality-control pathways, cause multisystem proteinopathies (MSP). They share pathological findings of protein aggregation and clinical combinations of inclusion body myopathy (IBM), neurodegeneration [motor neuron disorder (MND)/frontotemporal dementia (FTD)], and Paget disease of bone (PDB)" (PMID: 36861178). In the Mayo Clinic MSP cohort, SQSTM1+TIA1 accounted for 5 of 31 MSP individuals.

Demographically, a Chinese MSP cohort of 953 ALS/IBM/dementia patients identified 29 carriers of MSP-gene variants, with onset predominantly in the 3rd–5th decade and strong male predominance: "Most patients were male (72.4%), with disease onset predominantly in the third to fifth decades of life. The majority of patients (21/29) presented with a single clinical phenotype" (PMID: 41737544). The fact that most patients present with a single phenotype is central to the disease's clinical reality — full multisystem co-occurrence in one individual is the exception, not the rule.

Ontology anchors: MONDO:0800464 (disease); HP:0003198 (Myopathy); HP:0007354 (Amyotrophic lateral sclerosis); HP:0002145 (Frontotemporal dementia); HP:0002661 (Paget disease of bone).

Finding 2 — Pathogenic SQSTM1 variants cluster in the UBA domain and impair selective autophagy with NF-κB activation

In familial PDB, all 14 initially reported SQSTM1 mutations affect the ubiquitin-associated (UBA) domain, with P392L the most prevalent, present in both familial and sporadic cases across populations: "The 14 mutations reported in SQSTM1/p62 all affect the ubiquitin-associated domain of the protein. The P392L mutation is the most commonly observed mutation in PDB patients" (PMID: 17229007).

Functionally, the p62 protein is a multidomain scaffold: it binds ubiquitinated cargo via the UBA domain, LC3 via its LIR motif (acting as a selective autophagy receptor), and sequesters KEAP1 to activate the NFE2L2/Nrf2 antioxidant response. Pathogenic variants disrupt this machinery. Studies of the p.R321C variant demonstrate the dual mechanism: "In vitro studies associated the p62-321C variant with a blockade of autophagy and with the activation of the NF-kB pathway" (PMID: 40374720). A humanized p62-321C mouse developed stimulus-induced seizures, CNS p62/cFos changes, and degenerating spinal motor neurons.

Ontology anchors: GO:0005515 (protein binding); GO:0061136 (regulation of proteasomal protein catabolic process); GO:0000421 (autophagosome membrane); GO:0038061 (NIK/NF-κB signaling). Protein: UniProt Q13501, UBA domain residues ~389–434.

Finding 3 — Biallelic SQSTM1 loss-of-function causes a distinct recessive childhood-onset neurodegeneration (DNECAS)

In sharp mechanistic contrast to the dominant adult disease, biallelic (recessive) loss of p62 produces a childhood/adolescence-onset neurodegenerative disorder. Haack et al. "identified three different biallelic loss-of-function variants in SQSTM1 in nine affected individuals from four families with a childhood- or adolescence-onset neurodegenerative disorder characterized by gait abnormalities, ataxia, dysarthria, dystonia, vertical gaze palsy, and cognitive decline" (PMID: 27545679).

Affected fibroblasts showed complete absence of p62 protein and investigators "found evidence of a defect in the early response to mitochondrial depolarization and autophagosome formation" (PMID: 27545679) — directly implicating a mitophagy/autophagy-initiation defect. A novel homozygous frameshift (c.790del) was subsequently reported in childhood-onset progressive neurodegeneration with ataxia and gaze palsy (PMID: 35957775).

Ontology anchors: HP:0001251 (Ataxia); HP:0001332 (Dystonia); HP:0031605 (Vertical gaze palsy); HP:0100543 (Cognitive impairment); GO:0000423 (mitophagy).

Finding 4 — Epidemiology: SQSTM1 is the most common PDB gene; PDB prevalence ~1–2% in older adults but declining, male-predominant, with strong geographic variation

Paget disease of bone, the most frequent SQSTM1-associated phenotype, affects ~1–2% of adults over 55 in high-prevalence regions (UK, Western Europe), with marked geographic clustering (highest in NW England) and a continuing secular decline. In the UK, "the overall standardized incidence decreased from 0.75/10,000 person-years in 1999 to 0.20/10,000 person-years in 2015" (PMID: 33742666).

Prevalence is far lower in Asian populations. In South Korea, researchers reported "a mean prevalence of 1.20 per 100,000 and an age-adjusted incidence ranging from 0.38 to 1.26 per 100,000 person-years" (PMID: 39906256). PDB is male-predominant and strongly age-dependent, with crude incidence rising from ~0.04–0.07/10,000/yr at age 45–49 to 3.7–6.3/10,000/yr at ≥85.

The full SQSTM1-MSP syndrome (neuromuscular + bone) is much rarer than isolated PDB: MSP overall was found in only 3.0% (29/953) of a Chinese ALS/IBM/dementia cohort, and a Japanese nationwide survey ascertained only 47 MSP patients (PMID: 38287512). The secular decline and a rural/livestock (cattle density) association point to an environmental contribution: "A moderately positive correlation (R2 = 0.236) was identified between PDB incidence and the density of female breeding cattle" (PMID: 40408225).

Finding 5 — Treatment: Zoledronic acid durably controls SQSTM1-associated PDB, and genetic-test-guided prophylaxis can prevent bone lesions

Intravenous zoledronic acid (an aminobisphosphonate; NCIT:C1699) is the mainstay for the bone component. In a long-term retrospective cohort, biochemical response favored treatment strongly: "Primary outcome was more prevalent in the ZOL compared to the NZOL group [42 patients (88%) VS 11 patients (52%) respectively, P = 0.004]" (PMID: 38555314), with sustained normalization of alkaline phosphatase.

Most importantly, the ZiPP/PREVENT-PDB randomized trial of 222 SQSTM1-pathogenic-variant carriers showed that prophylactic zoledronic acid prevented new bone lesions: "Eight of the placebo group had a poor outcome (lesions which were new, unchanged or progressing) compared with none of the ZA group (OR 0.08, 95% CI 0.00 to 0.42, p=0.003)" over a median 84 months (PMID: 38123339). This is a rare example of genotype-guided precision prevention in a Mendelian disease.

The acute phase response (fever, myalgia) after zoledronate can be attenuated by a short course of dexamethasone (PMID: 36970850). By contrast, no disease-modifying therapy exists for the myopathy/ALS/FTD components, which are managed supportively (physiotherapy, occupational therapy, respiratory care).

Ontology anchors: NCIT:C1699 (Zoledronic Acid); NCIT:C1455 (Bisphosphonate); NCIT:C575 (Dexamethasone); CHEBI:46557 (zoledronic acid).

Finding 6 — TIA1/SQSTM1 digenic late-onset distal myopathy defines a homogeneous SQSTM1 muscle phenotype, with SQSTM1 acting as a modifier

The muscle arm of SQSTM1 disease is frequently digenic. Co-inheritance of TIA1 p.Asn357Ser and SQSTM1 p.Pro392Leu produces a strikingly homogeneous phenotype: "These twenty-four patients (75% males) had late-onset (52,6 ± 10,1 years), mainly asymmetric, distal ankle and hand finger extension weakness (75%), mild CK elevation (82.4%) and myopathic EMG" (PMID: 39142003).

The TIA1 allele is enriched in distal myopathy panels and is proposed as a genetic modifier: "We confirm an increased burden of the TIA1 p.Asn357Ser variant in distal myopathy patients which could act as a genetic modifier" (PMID: 39142003). An independent Spanish report confirmed that "a similar phenotype has been attributed to the digenic inheritance of TIA1 c.1070A>G (p.Asn357Ser) and SQSTM1 c.1175C>T (p.Pro392Leu) variants" (PMID: 38016875). Muscle biopsy shows rimmed vacuoles (combined 44–83%) with p62/TDP-43 aggregates. Notably, SQSTM1 can also cause proximal myopathy with rimmed vacuoles via a frameshift variant, broadening the muscle phenotype (PMID: 36998782).

Ontology anchors: HP:0003693 (Distal amyotrophy); HP:0003805 (Rimmed vacuoles); HP:0003236 (Elevated circulating creatine kinase); HP:0003458 (EMG: myopathic abnormalities); HP:0030319 (Finger extensor weakness).

Finding 7 — Mutant p62 potentiates RANK-induced NF-κB activation and osteoclastogenesis (bone arm) and accelerates ALS progression (neuro arm)

On the bone side, PDB-associated p62 mutants act by gain-of-function on NF-κB: "p62 mutants increased NF-kappaB activation and significantly potentiated osteoclast formation and bone resorption in human primary cell cultures" (PMID: 16813535). This study described a novel K378X truncation causing a severe polyostotic phenotype alongside P392L and E396X.

On the neurodegeneration side, SQSTM1 missense variants span the PB1, TRAF6-binding, PEST, and UBA domains and worsen prognosis. In a Chinese ALS cohort, "the ΔFS analysis revealed that the risk of death or tracheostomy was significantly increased in SQSTM1 mutation carriers (p < 0.05)" (PMID: 25708934). Neuropathologically, p62 immunohistochemistry reliably labels TDP-43–positive and C9RANT-positive neuronal cytoplasmic inclusions in FTLD/ALS tissue.

Ontology anchors: GO:0038061 (NIK/NF-κB signaling); GO:0045670 (regulation of osteoclast differentiation); CL:0000092 (osteoclast); CL:0000100 (motor neuron).

Finding 8 — Diagnostic workup is organ-specific

Diagnosis depends on which arm is manifest:

• Bone arm (PDB): Usually detected incidentally via isolated elevated serum total/bone alkaline phosphatase (elevated in ~90% of PDB; LOINC 6768-6) or on radiographs. "Most cases of PDB are diagnosed incidentally on radiographs or as an isolated elevation of serum alkaline phosphatase... Diagnosis is made on the basis of typical radiological features on plain films, while a radionuclide bone scan may be used to assess the extent of disease" (PMID: 33543158). Pathognomonic radiographic features include cortical thickening, medullary expansion, osteolytic "blade of grass" fronts, and mixed lytic-sclerotic skull; histology shows mosaic cement lines. Bone-turnover markers (CTX, P1NP) are elevated.
• Neuromuscular arm: Requires needle EMG (myopathic ± neurogenic), muscle MRI (distal fatty replacement, e.g., medial gastrocnemius), and muscle biopsy: "Muscle biopsy showed rimmed vacuoles (44.4%), myofibrillar disorganization (16.7%) or both (38.9%), with P62/TDP43 aggregates" (PMID: 39142003).
• Definitive diagnosis is molecular: Targeted SQSTM1 sequencing or multigene MSP/ALS/distal-myopathy NGS panels (including TIA1), with WES/WGS for atypical cases.

Ontology anchors: LOINC 6768-6 (Alkaline phosphatase); NCIT:C17007 (genetic testing); HP:0003236 (elevated CK).

Finding 9 — Mouse models show p62 P394L (= human P392L) causes osteoclast RANKL-hyperresponsiveness but requires a second hit for full Paget lesions

The gene-environment nature of the bone arm is directly demonstrated in mice. "We generated p62(P394L) knock-in mice (p62KI) and found that although OCL numbers were increased, the mice did not develop pagetic lesions. However, mice expressing both MVNP and p62KI developed more exuberant pagetic lesions than mice expressing MVNP alone" (PMID: 24339057). The downstream mediator is IL-6: "Interleukin-6 (IL-6) production is essential for the pagetic phenotype, because transgenic mice with MVNP targeted to OCLs develop pagetic OCLs and lesions, but this phenotype is absent when MVNP mice are bred to IL-6(-/-) mice" (PMID: 24339057).

This explains incomplete, age-dependent penetrance of PDB: the p62 UBA mutation primes osteoclasts to RANKL but, alone, is insufficient; a paramyxoviral/inflammatory "second hit" acting through IL-6 is needed for full lesions. Available model systems therefore include: p62(P394L) knock-in mice (bone), the humanized p62-R321C mouse (CNS: seizures, motor-neuron degeneration; PMID: 40374720), Sqstm1/p62-null mice (mitophagy defect; models the biallelic human phenotype), and patient fibroblasts/iPSC and RAW264.7 osteoclast cultures.

Ontology anchors: NCBI Taxon 10090 (Mus musculus); MGI Sqstm1; CL:0000092 (osteoclast); GO:0032635 (interleukin-6 production).

Finding 10 — Prognosis is organ-specific: PDB arm usually benign/treatable but <1% undergo osteosarcoma transformation; neuro arm is progressive and life-limiting

Most SQSTM1-associated PDB is mild and monostotic (77% monostotic; only 3% fracture in one cohort) and controllable with bisphosphonates. Complications include bone pain, deformity, fractures, deafness (skull involvement), compression neuropathy/spinal cord compromise, secondary osteoarthritis, and high-output cardiac states.

The gravest bone complication is malignant transformation: "Neoplastic transformation is a rare but serious complication of Paget's disease of bone (PDB), occurring in fewer than 1% of individuals with polyostotic disease. Their prognosis is poor, with less than 50% surviving 5 years" (PMID: 32106343). Spinal pagetic osteosarcoma has a 5-year survival as low as 10% (PMID: 42359209). Transformation is associated with a germline ZNF687 P937R founder mutation and dual SQSTM1+ZNF687 genotypes — a case report documented "genetic testing confirming dual SQSTM1 and ZNF687 mutations" (PMID: 41630218).

In contrast, the neuromuscular components are relentlessly progressive: distal myopathy causes slowly progressive disability over 10+ years; SQSTM1-ALS carriers have accelerated decline and shortened survival (PMID: 25708934); FTD and the biallelic childhood neurodegeneration are progressive and fatal.

Mechanistic Model / Interpretation

Ordered causal chain (initiating lesion → clinical manifestation)

Shared initiating lesion: 1. A germline SQSTM1 variant (most commonly a UBA-domain missense, e.g., p.P392L) alters p62 function → impairs ubiquitin-cargo binding and p62's dual role as autophagy receptor and signaling scaffold (PMID: 17229007).

The pathway then branches according to cell type, gene dose (mono- vs biallelic), and environment:

BRANCH A — Bone (gain-of-function, environment-gated): 2A. Mutant p62 potentiates RANK→NF-κB signaling in osteoclast precursors (PMID: 16813535) → leads to osteoclast RANKL-hyperresponsiveness and increased osteoclast number. 3A. Alone this is insufficient (p62KI mice do not develop lesions) → a second hit (paramyxoviral MVNP; inferred from epidemiologic measles/livestock associations) acts via IL-6 (PMID: 24339057) → results in exuberant, disorganized bone remodeling. 4A. Leads to focal accelerated bone turnover → cortical thickening, medullary expansion, deformity, fractures, deafness → clinical Paget disease of bone. 5A. Rarely (<1%, often with a ZNF687 second hit) → malignant transformation to osteosarcoma (PMID: 32106343).

BRANCH B — Neuromuscular (loss-of-function / impaired autophagy): 2B. Mutant p62 blocks selective autophagy and impairs mitophagy (PMID: 40374720) → leads to accumulation of ubiquitinated cargo and damaged mitochondria. 3B. Results in cytoplasmic TDP-43 and p62 aggregation with rimmed vacuoles in myofibers and neurons (PMID: 39142003). 4B. Co-modifier alleles (e.g., TIA1 p.N357Ser) lower the threshold → digenic distal myopathy (PMID: 39142003); in motor/cortical neurons → ALS/FTD with accelerated progression (PMID: 25708934).

BRANCH C — Recessive childhood neurodegeneration (complete loss-of-function): 2C. Biallelic LoF → complete absence of p62 protein → defective early mitophagy response (PMID: 27545679) → leads to childhood-onset neurodegeneration with ataxia, dystonia, gaze palsy (DNECAS).

Branch comparison table

ASCII schematic

                 SQSTM1 / p62 germline variant
                 (UBA domain, e.g. P392L)
                            |
        +-------------------+--------------------+
        |                                        |
  GAIN-OF-FUNCTION                        LOSS-OF-FUNCTION
  (RANK -> NF-kB up)                      (autophagy/mitophagy down)
        |                                        |
  osteoclast hyper-                      ubiquitin + damaged
  responsiveness                         mitochondria accumulate
        |                                        |
  + MVNP / IL-6  <-- environmental        p62/TDP-43 aggregation
    SECOND HIT        trigger                    |
        |                             +----------+-----------+
  Paget disease of bone               |                      |
        |                        +TIA1 modifier         biallelic LoF
  <1% osteosarcoma               (digenic)                   |
  (+ZNF687)                           |                 childhood
                              distal myopathy /          neurodegeneration
                              IBM / ALS / FTD            (ataxia, gaze palsy)

Evidence Base

Challenges / nuances: The VCP-focused MSP therapeutic literature (rapamycin/chloroquine, exercise training; e.g. PMID: 24130765) is encouraging for the autophagy-targeting concept but is not SQSTM1-specific, and one key paper was retracted (PMID: 25884947) — it should not be over-extrapolated to SQSTM1. The livestock/measles environmental link is correlational (R²=0.236) and mechanistically supported only in mice, not proven in human SQSTM1 carriers.

Section-by-Section Knowledge-Base Content

1. Disease Information. Adult-onset dominant multisystem proteinopathy (MSP3, OMIM 616887, MONDO:0800464) plus a distinct recessive pediatric neurodegeneration (DNECAS). Synonyms: SQSTM1-related MSP; p62-related myopathy/ALS/FTD/PDB. Information is aggregated from case series, cohorts, and registries (disease-level), not single-patient EHR.

2. Etiology. Primary cause: germline SQSTM1 variants. Genetic risk: UBA-domain missense (P392L most common); modifier TIA1 p.N357Ser (muscle), ZNF687 P937R (malignant transformation). Environmental: paramyxovirus (measles/MVNP), rural/cattle exposure, age, male sex. Gene-environment interaction is proven in mice (p62KI requires MVNP/IL-6; PMID: 24339057).

3. Phenotypes. PDB (HP:0002661), distal amyotrophy (HP:0003693), rimmed vacuoles (HP:0003805), elevated CK (HP:0003236), ALS (HP:0007354), FTD (HP:0002145), ataxia (HP:0001251), dystonia (HP:0001332), gaze palsy (HP:0031605), hearing loss (HP:0000365), cognitive decline (HP:0100543). Onset mostly 3rd–5th decade (neuromuscular) or >55 (bone); childhood for recessive form. Progressive for neuro arm; treatable/stable for bone.

4. Genetic/Molecular. SQSTM1 (HGNC:11280, chr5q35, UniProt Q13501). Variants: missense, nonsense (K378X, E396X), frameshift. Classification per ACMG; P392L pathogenic. Modifier genes: TIA1, ZNF687. Rare in gnomAD. Germline origin. Functional consequence: gain-of-function (NF-κB/bone) and loss-of-function (autophagy/neuro).

5. Environmental. Paramyxovirus/measles nucleocapsid (via IL-6), livestock/rural exposure; secular decline implies modifiable environmental component.

6. Mechanism. See causal chain above. Pathways: RANK–NF-κB (KEGG), selective autophagy/mitophagy (GO:0000423), KEAP1-NFE2L2. Cell types: osteoclast (CL:0000092), motor neuron (CL:0000100), skeletal myofiber.

7. Anatomy. Bone (UBERON:0001474; skull, spine, pelvis, long bones), skeletal muscle (UBERON:0001134; distal > proximal), spinal cord/motor neurons, frontotemporal cortex, cerebellum. Subcellular: autophagosome (GO:0005776), lysosome, mitochondrion.

8. Temporal. Adult/late onset (bone, >55; neuromuscular, 3rd–5th decade), childhood for recessive. Insidious, chronic, progressive (neuro) vs. focal/stable (bone).

9. Inheritance/Population. AD (dominant arms), AR (DNECAS), often digenic. Incomplete, age-dependent penetrance. Variable expressivity. PDB ~1–2% in high-prevalence regions, declining; male-predominant; NW England clustering.

10. Diagnostics. ALP + bone-turnover markers + radiography/bone scan (bone); EMG + muscle MRI + biopsy with p62/TDP-43 (muscle); molecular SQSTM1 sequencing / NGS panels (definitive).

11. Prognosis. Bone arm benign/treatable, <1% osteosarcoma (poor, <50% 5-yr survival). Neuro arm progressive, life-limiting; ALS carriers have shortened survival.

12. Treatment. Zoledronic acid (NCIT:C1699) for PDB — including genotype-guided prophylaxis; dexamethasone for APR; supportive care (PT/OT, respiratory) for neuromuscular arms. No disease-modifying neuro therapy.

13. Prevention. Primary/secondary: genetic testing + prophylactic zoledronic acid in SQSTM1 carriers (proven in ZiPP RCT); genetic counseling; cascade testing. Tertiary: bisphosphonates to prevent deformity/complications.

14. Other species. Mouse (NCBI Taxon 10090) ortholog Sqstm1; p62(P394L) knock-in and p62-null models. No major naturally occurring animal disease documented.

15. Model organisms. p62(P394L) knock-in (bone), humanized p62-R321C (CNS), Sqstm1-null (mitophagy/recessive), patient fibroblasts/iPSC, RAW264.7 osteoclast cultures.

Limitations and Knowledge Gaps

1. No dedicated SQSTM1-MSP primary dataset was analyzed — this report synthesizes published literature; all statistics are extracted from cited studies.
2. Penetrance and genotype-phenotype rules are imprecise. Why one carrier develops PDB, another ALS, and another nothing is unexplained beyond modifier alleles and environmental hits.
3. The environmental "second hit" is proven only in mice. The measles/MVNP-IL-6 axis and livestock correlation (R²=0.236) are not mechanistically confirmed in human SQSTM1 carriers.
4. No disease-modifying therapy for the neuromuscular arms. Autophagy-modulating strategies remain extrapolations from VCP models (one key paper retracted).
5. Cohorts are small and geographically skewed (Europe, East Asia); carrier frequency, incidence of the full MSP syndrome, and sex ratios for the neuromuscular arms are poorly quantified.
6. Prognostic biomarkers are lacking for predicting which carriers will convert to neurodegeneration.

Proposed Follow-up Experiments / Actions

1. Genotype-phenotype registry: Pool international SQSTM1 carriers to estimate arm-specific penetrance, modifier burden (TIA1, ZNF687), and environmental covariates.
2. Test the viral second-hit hypothesis in humans: Serological/PCR studies for paramyxovirus exposure stratified by SQSTM1 genotype and PDB status.
3. iPSC-derived osteoclast + motor-neuron isogenic panels carrying P392L, R321C, and biallelic LoF to dissect the gain- vs loss-of-function branch point quantitatively.
4. Biomarker discovery: Longitudinal serum/CSF neurofilament, TDP-43, and bone-turnover markers to predict neuromuscular conversion in asymptomatic carriers.
5. Therapeutic trials of autophagy enhancers specifically in SQSTM1 muscle/neuron models before clinical translation.
6. Extend prophylaxis evidence: Longer-term ZiPP follow-up and health-economic analysis to inform guidelines on genotype-guided zoledronic acid.
7. Screen for ZNF687 co-mutation in polyostotic SQSTM1-PDB patients as a malignant-transformation risk stratifier.

Report compiled from 10 confirmed findings across 5 investigation iterations and 60 reviewed papers. Evidence types span human clinical cohorts, human genetics, in vitro functional assays, mouse models, and epidemiology.