Module 8: Disease & Therapeutics

Metastasis is the cause of 90% of cancer deaths. The first rate-limiting step is epithelial-mesenchymal transition (EMT): a regulated cell-biology programme (Thiery, Acloque, Huang, & Nieto 2009 Cell) in which an epithelial cell loses its E-cadherin adherens junctions, its apical-basal polarity, and its cortical cytokeratin network and acquires a mesenchymal cytoskeleton with vimentin intermediate filaments, front-back polarity, and motility.

Key molecular hallmarks of EMT:

• Loss of E-cadherin (CDH1): the gatekeeper switch. Repressed by Snail, Slug, Twist, ZEB1/2 transcription factors.
• Gain of N-cadherin (CDH2) and vimentin (VIM): mesenchymal intermediate-filament system.
• Arp2/3 + WAVE complex upregulation drives lamellipodia-based migration.
• MMP-9, MMP-2 matrix metalloproteinases degrade basement membrane, allowing invasion.
• CXCR4/CXCL12 chemokine axis guides circulating tumour cells (CTCs) to distant organs expressing CXCL12 (bone marrow, lung, liver).

Mechanically, metastatic cells are typically softer than their benign counterparts (Cross 2007 Nat. Nanotechnol., see Module 6) — apparent Young modulus drops by \(\sim 70\%\). They form invadopodia (Weaver 2006): F-actin + Arp2/3 + cortactin + MMP14 protrusions that concentrate matrix proteolysis at the leading edge. Inhibition of Arp2/3 (CK-666), cortactin, or N-WASP reduces invadopodial activity and metastatic burden in murine models.

Duchenne muscular dystrophy (DMD)is an X-linked recessive disorder affecting \(\sim 1/3500\) live male births. It is caused by loss-of-function mutations in the DMD gene encoding dystrophin, a 427 kDa rod-shaped cytoskeletal protein linking subsarcolemmal F-actin to the dystrophin-associated protein complex (DAPC) that spans the sarcolemma and binds extracellular laminin-211 (merosin) via \(\alpha\)-dystroglycan.

Without dystrophin the DAPC disassembles and the sarcolemma cannot transduce contractile force to the ECM; repeated sarcolemmal tears during eccentric contraction trigger Ca²⁺ influx, calpain activation, mitochondrial dysfunction, myofibre necrosis, fibrosis, and eventual replacement of muscle by fatty-connective tissue. Natural history: proximal weakness by age 3, loss of ambulation around age 10–13, respiratory/cardiac failure by the late 20s.

Becker muscular dystrophy (BMD)arises from in-frame deletions in DMD that produce a truncated but partially functional dystrophin. Phenotype is milder, with LOA typically after age 16 and survival into middle age. The dichotomy DMD vs BMD mirrors the reading-frame rule (Monaco 1988): out-of-frame = DMD, in-frame = BMD, with very few exceptions.

Therapeutic strategies:

• Corticosteroids (prednisone, deflazacort): anti-inflammatory, delay LOA by 2–3 years; still standard of care.
• Antisense oligonucleotide (AO) exon-skipping: force the spliceosome to skip a disrupted exon, restoring the reading frame and converting DMD to a BMD-like phenotype. FDA-approved: eteplirsen (exon 51, 2016), golodirsen (exon 53, 2019), viltolarsen (exon 53, 2020), casimersen (exon 45, 2021). Restores ~5–12% of normal dystrophin.
• Gene therapy with microdystrophin: AAV9-delivered truncated construct (SRP-9001 / delandistrogene moxeparvovec, FDA-approved 2023 for ages 4–5). Delivers ~30% of normal levels of a functional mini-dystrophin.
• CRISPR/Cas9 exon excision: preclinical, exon-51/exon-45 excision in DMD mouse (mdx) and dog models.
• Utrophin upregulation (ezutromid trials): utrophin is a paralogue that can partially substitute for dystrophin.

Related diseases: limb-girdle muscular dystrophies (mutations in sarcoglycans, dystroglycan, caveolin-3, calpain-3); Emery-Dreifuss(emerin, lamin A/C — see laminopathies); congenital muscular dystrophies (laminin-\(\alpha\)2, POMGnT1); facioscapulohumeral(FSHD, D4Z4 macrosatellite contraction).

The nuclear lamina is an intermediate-filament meshwork of A-type (lamin A, lamin C from alternative splicing of LMNA) and B-type (lamin B1 from LMNB1, lamin B2 from LMNB2) lamins lining the inner nuclear membrane. Lamins provide nuclear shape, tether heterochromatin, couple mechanical signals from the cytoskeleton via the LINC complex (SUN-nesprin-KASH), and regulate transcription.

Mutations in LMNA cause a spectacularly diverse set of laminopathies, most famously:

• Hutchinson-Gilford progeria syndrome (HGPS): de novo C1824T (p.G608G) silent mutation in exon 11 of LMNA that activates a cryptic splice donor, deleting 50 amino acids from prelamin A and leaving a permanently farnesylated protein (progerin). Children age ~7\(\times\) faster than normal (Eriksson et al. 2003 Nature; De Sandre-Giovannoli 2003 Science). Phenotype: failure to thrive, alopecia, lipodystrophy, osteolysis, atherosclerosis; death from myocardial infarction or stroke by age 13.
• Emery-Dreifuss muscular dystrophy (EDMD): LMNA or EMD (emerin) mutations; scapulo- humeroperoneal weakness + joint contractures + cardiac conduction defects.
• Dilated cardiomyopathy (DCM) with conduction defect: LMNA truncation/missense; one of the most lethal forms of DCM, with high risk of sudden cardiac death.
• Familial partial lipodystrophy type 2 (FPLD2), Charcot-Marie- Tooth type 2B1, mandibuloacral dysplasia, restrictive dermopathy (ZMPSTE24/lamin A processing defect).

Therapeutic approach for progeria: lonafarnib (FDA-approved 2020), a farnesyltransferase inhibitor that reduces progerin farnesylation and modestly extends lifespan. Under investigation: antisense-mediated exon-skipping of the cryptic splice, CRISPR base editing of the G608G mutation.

Hypertrophic cardiomyopathy (HCM)affects 1 in 500 adults and is the leading cause of sudden cardiac death in young athletes. Over 70% of familial HCM is caused by mutations in genes encoding sarcomeric proteins:

• MYH7 (\(\beta\)-myosin heavy chain): ~40% of HCM.
• MYBPC3 (myosin-binding protein C): ~40%.
• TNNT2 (troponin T): ~5%.
• TNNI3 (troponin I), TPM1 (tropomyosin), MYL2/MYL3 (myosin light chains), ACTC1 (cardiac actin).

Most HCM mutations are gain-of-function, producing hypercontractile sarcomeres with reduced superrelaxed myosin fraction (Alamo 2017; Spudich 2015). The clinical phenotype is left-ventricular hypertrophy, diastolic dysfunction, outflow-tract obstruction, and arrhythmia.

Mavacamten (FDA-approved 2022, Camzyos) is a first-in-class small-molecule myosin ATPase inhibitor that selectively stabilizes the super-relaxed (SRX) state of \(\beta\)-cardiac myosin (Green 2016 Science). It reduces the fraction of active, force-generating myosin heads, lowering cardiac contractility toward normal. The EXPLORER-HCM Phase 3 trial (Olivotto 2020 Lancet) demonstrated improved exercise capacity, NYHA class, and outflow gradient reduction. This is the first disease-modifying therapy for HCM, and a proof-of-principle for direct-to-sarcomere drug design.

Dilated cardiomyopathy (DCM)(1 in 2500) is caused by a broader genetic spectrum: TTN (titin truncations, ~25% of familial DCM), LMNA (10%), MYH7, TNNT2, DES (desmin), BAG3, RBM20. Phenotype is chamber dilation and systolic failure. Omecamtiv mecarbil (a cardiac myosin activator) was developed to increase systolic force in DCM but did not meet primary outcomes in GALACTIC-HF (Teerlink 2021 NEJM). The opposite-direction logic — activate in DCM, inhibit in HCM — illustrates the central role of cross-bridge kinetics in cardiac disease.

Cilia are microtubule-based projections with a 9+2 (motile) or 9+0 (primary) axoneme. Dysfunction of cilia causes a diverse group of diseases called ciliopathies:

• Primary ciliary dyskinesia (PCD) / Kartagener syndrome: mutations in outer or inner dynein arm components (DNAI1, DNAH5, DNAH11) immobilize motile cilia of the airway, middle ear, sinuses, Fallopian tubes, and sperm flagellum. Phenotype: chronic bronchiectasis, rhino-sinusitis, otitis media, infertility. Kartagener triad adds situs inversus (50% of PCD patients) because embryonic node cilia direct left-right asymmetry via rotational beating (Nonaka 1998 Cell).
• Autosomal dominant polycystic kidney disease (ADPKD): mutations in PKD1 (polycystin-1, 85%) or PKD2 (polycystin-2, 15%), both localized to the primary cilium of kidney tubule cells. Failure of polycystin flow-sensing drives cyst expansion; 1 in 1000 adults; leading genetic cause of end-stage renal disease. Tolvaptan (V2 receptor antagonist) slows progression.
• Bardet-Biedl syndrome (BBS): mutations in any of >20 BBS genes encoding the BBSome, an intraflagellar transport adapter. Phenotype: retinal dystrophy, polydactyly, obesity, hypogonadism, renal anomalies, intellectual disability.
• Joubert syndrome, Meckel syndrome, Nephronophthisis: other primary-cilium disorders with CNS, skeletal, and renal phenotypes.

The common mechanism is disruption of ciliary signalling (Hedgehog in development, Ca²⁺ flow sensing in kidney, photoreceptor-outer- segment transport in retina). Because primary cilia are required across so many organ systems, ciliopathies present with multisystem phenotypes.

Many bacterial and viral pathogens have evolved to manipulate host actin dynamics for invasion, intracellular motility, and cell-to-cell spread. Two classic case studies:

• Listeria monocytogenesexpresses the surface protein ActA, a WASP-family nucleation promoting factor that directly activates host Arp2/3 (Welch & Mitchison 1998). A polymerizing “comet tail” of actin pushes the bacterium through the cytoplasm at \(\sim 0.1\!-\!0.5\) μm/s and propels it into neighbouring cells via protrusions that fuse with an adjacent plasma membrane. Shigella (IcsA/VirG) and Rickettsia use analogous mechanisms.
• Salmonella entericauses the SPI1 type-III secretion system to inject effectors into epithelial cells: SipC directly nucleates actin; SipA bundles and stabilizes filaments; SopE/SopE2 activate host Rac1/Cdc42 by mimicking a GEF. The net result is massive membrane ruffling and bacterial engulfment, bypassing normal phagocytic regulation.
• Vaccinia virus drives Arp2/3-dependent actin tail formation via A36R-phosphorylated Nck/WIP recruitment to N-WASP.
• Enteropathogenic E. coli(EPEC) forms pedestals by translocating Tir (translocated intimin receptor) into the host membrane, where Tir-phosphorylation recruits Nck → N-WASP → Arp2/3.

The study of these pathogens has, circularly, been one of the most productive sources of discovery in basic cell biology: ActA was the first discovered nucleation promoting factor and motivated much of the early work on the Arp2/3 complex.

Microtubules are among the most validated drug targets in oncology. Four mechanistic classes of MT-targeting agents (MTAs) cover thousands of clinical-grade compounds:

• Taxanes (paclitaxel/Taxol, docetaxel, cabazitaxel): bind the luminal taxane-binding pocket of \(\beta\)-tubulin, stabilize lateral contacts, suppress catastrophe, and produce kinetically frozen MTs at high dose. Horwitz and colleagues discovered the mechanism in 1979 (Nature); taxanes were originally extracted from Pacific yew (Taxus brevifolia). Indications: breast, ovary, lung, prostate, head and neck.
• Vinca alkaloids (vinblastine, vincristine, vinorelbine, vindesine): from Catharanthus roseus(Madagascar periwinkle). Bind the vinca domain at the \(\alpha\beta\)-interface, induce curved protofilaments, and depolymerize MTs. At low dose (nM) they suppress dynamics without depolymerization, which is the clinically relevant mechanism. Indications: Hodgkin lymphoma (vinblastine), ALL/NHL (vincristine), NSCLC (vinorelbine).
• Halichondrins(eribulin / Halaven): MT-dynamics inhibitor with a distinct binding site; approved for metastatic breast cancer and liposarcoma (Yu 2005, FDA 2010).
• Combretastatins / tubulysins / colchicine- site binders: vascular-disrupting agents (combretastatin A4 phosphate, CA4P) collapse tumour neo-vasculature by depolymerizing MTs in endothelial cells. Colchicine itself is used for gout but toxic as antitumour agent. Nocodazole is the research-grade colchicine-site drug.

Resistance mechanisms include P-glycoprotein overexpression, class III \(\beta\)-tubulin (TUBB3) expression, MT-associated protein alterations (survivin, stathmin), and point mutations in the drug-binding site. Second-generation taxanes (nab-paclitaxel, Abraxane) avoid the toxic Cremophor solvent and use albumin-bound nanoparticles for improved delivery.

Actin-targeting drugs are research-grade but not chemotherapeutic, due to toxicity to nearly every cell type:

• Latrunculin A/B (from Red-Sea sponge): sequesters G-actin, depolymerizes F-actin.
• Cytochalasin D: binds the barbed end and caps it.
• Jasplakinolide: stabilizes F-actin and enhances nucleation.
• Phalloidin (from death-cap mushroom): F-actin stabilizer, widely used as a fluorescent stain.
• Arp2/3 inhibitors (CK-666, CK-869; Nolen 2009): in development for breast-cancer metastasis.

Myosin inhibitors include blebbistatin (non-muscle myosin II), and the first FDA-approved mavacamten (Camzyos, 2022) for obstructive HCM (see Section 4). Para-nitro- blebbistatin and MYK-461are research-grade and clinical tools respectively.

The modern era of cytoskeletal pharmacology is increasingly molecularly targeted:

• Formin inhibitors: SMIFH2 (research grade); clinical leads in breast-cancer metastasis.
• ROCK inhibitors: Y-27632 (research), fasudil (approved in Japan for cerebral vasospasm), netarsudil (FDA 2017, Rhopressa) for open-angle glaucoma.
• LIMK inhibitors: in trials for fragile-X syndrome (cofilin hyperphosphorylation) and metastatic cancer.
• KIF11 / Eg5 kinesin inhibitors(ispinesib, monastrol): mitotic-kinesin inhibitors tested as taxane alternatives; most failed late-stage trials due to narrow therapeutic index.
• Myosin modulators: aficamten (second-generation HCM drug, Phase 3 SEQUOIA trial); EDG-7500 (next-gen in development).
• Gene therapy for DMD: delandistrogene moxeparvovec (Elevidys, FDA 2023, ages 4–5); CRISPR trials in preclinical dog models.
• Protein-degrader (PROTAC)strategies: targeted degradation of tubulin isotypes (preclinical).

A recurring theme is that nearly every cytoskeletal protein has a human disease associated with its dysfunction, and the therapies that emerge are increasingly sophisticated: gene replacement (AAV microdystrophin), splicing modulation (exon-skipping AOs), direct-to-sarcomere small molecules (mavacamten), and lipid-nanoparticle-delivered mRNA that restore a missing protein in muscle or kidney. The cytoskeleton will remain one of the most productive drug-target families for the foreseeable future.