Cell Biology, Collagen & Connective Tissue — USMLE Step 1 Notes

Organelle & protein-trafficking disorders

• I-cell disease (inclusion-cell disease): failure of N-acetylglucosaminyl-1-phosphotransferase to add the mannose-6-phosphate 'address tag', so lysosomal enzymes are mis-secreted into the blood instead of reaching the lysosome. Enzymes accumulate outside the cell; lysosomes fill with debris ('inclusions'). Coarse facial features, gingival hyperplasia, clouded corneas, skeletal changes and early death.
• Signal-tag principle: mannose-6-phosphate in the Golgi routes enzymes to lysosomes; the signal recognition particle routes ribosomes to the rough ER for secretory/membrane proteins.

Peroxisomal disorders

• Peroxisome roles: very-long-chain fatty-acid beta-oxidation, branched-chain (phytanic acid) alpha-oxidation, and plasmalogen synthesis.
• Zellweger syndrome: defective peroxisome biogenesis (PEX genes) → failure to make peroxisomes; hypotonia, seizures, hepatomegaly and craniofacial abnormalities in the neonate; very-long-chain fatty acids accumulate; usually fatal early.
• Refsum disease: defective phytanic acid alpha-oxidation; phytanic acid accumulates → retinitis pigmentosa, peripheral neuropathy, ataxia and anosmia; treat by dietary restriction of phytanic acid (avoid dairy/ruminant fat) and plasmapheresis.
• X-linked adrenoleukodystrophy: defective very-long-chain fatty-acid transport into peroxisomes (ABCD1); VLCFAs accumulate in the CNS white matter and adrenal cortex → a boy with behavioural/cognitive decline, a progressive leukodystrophy and adrenal insufficiency.

Li-Fraumeni syndrome

• Germline mutation of the TP53 tumour-suppressor ('guardian of the genome') → a strong, early, multi-cancer predisposition (SBLA: Sarcoma, Breast, Leukaemia/brain, Adrenal cortical carcinoma). Autosomal dominant; intensive surveillance and avoidance of unnecessary radiation are key.

Microtubules & cilia

• Microtubules: polymers of alpha/beta-tubulin (grow from the centrosome/MTOC) that drive mitosis, intracellular transport (kinesin moves toward the +/periphery, dynein toward the -/centre), and form cilia/flagella (a 9+2 axoneme).
• Drugs on microtubules: vinca alkaloids prevent assembly; taxanes stabilise/hyperpolymerise them; griseofulvin, colchicine and mebendazole also act on microtubules.
• Primary ciliary dyskinesia (Kartagener): a defect of the ciliary dynein arm impairs ciliary/flagellar beating → the triad of situs inversus, chronic sinusitis and bronchiectasis, plus infertility (immotile sperm/dysfunctional fallopian cilia) and recurrent otitis media.

The sodium-potassium pump (Na+/K+-ATPase)

• Function: uses ATP to pump 3 Na+ out and 2 K+ in per cycle, maintaining the resting membrane potential and the sodium gradient that powers secondary active transport (e.g. the Na/Ca exchanger, glucose/amino-acid cotransport).
• Pharmacology: cardiac glycosides (digoxin/ouabain) inhibit the pump; rising intracellular sodium reduces Na/Ca exchange, so intracellular calcium rises and cardiac contractility increases (positive inotropy).

Collagen — types & synthesis

• Key types: Type I — bone, skin, tendon (most abundant; defective in osteogenesis imperfecta). Type II — cartilage. Type III — reticulin/blood vessels/granulation tissue (defective in vascular Ehlers-Danlos). Type IV — basement membrane (defective in Alport; targeted in Goodpasture).
• Synthesis steps: translation of pre-pro-collagen → hydroxylation of proline and lysine (needs vitamin C) → glycosylation and triple-helix formation → exocytosis of procollagen → cleavage of propeptides to tropocollagen → cross-linking by lysyl oxidase (a copper-dependent enzyme) to form mature fibrils.

Collagen & connective-tissue disorders

• Osteogenesis imperfecta: defective type I collagen (usually autosomal dominant) → recurrent fractures with minimal trauma, blue sclerae, hearing loss (otosclerosis) and dental problems; may be mistaken for non-accidental injury. Bisphosphonates reduce fracture rate.
• Ehlers-Danlos syndrome: defective collagen/processing → hyperextensible skin, hypermobile joints and easy bruising; the dangerous vascular type (type III collagen, COL3A1) risks arterial, bowel and uterine rupture.
• Menkes disease: X-linked defect of a copper-transporting ATPase (ATP7A) causing copper deficiency, so lysyl oxidase fails → brittle 'kinky' hair, hypotonia, arterial tortuosity/aneurysm and neurodegeneration. (Contrast Wilson disease = copper overload, ATP7B.)
• Marfan syndrome: defective fibrillin-1 (FBN1), a scaffold for elastic fibres → tall stature, arachnodactyly, upward lens dislocation and aortic-root dilation/dissection; manage with beta-blockers/ARBs and aortic surveillance.
• Homocystinuria: most often cystathionine synthase deficiency → a marfanoid habitus but with downward lens dislocation, thrombosis and intellectual disability; some respond to vitamin B6, plus methionine restriction and cysteine/folate/B12.

Connecting the dots

• Collagen cofactors: vitamin C (hydroxylation) and copper (lysyl-oxidase cross-linking) — so scurvy and Menkes both weaken collagen despite normal genes.
• Marfan vs homocystinuria: both marfanoid; the lens direction (up vs down) and thrombosis separate them.
• Peroxisomal vs lysosomal: peroxisomal disorders accumulate very-long-chain/branched fatty acids; lysosomal disorders accumulate sphingolipids/mucopolysaccharides.

High-yield revision notes; verify against a primary source before clinical use.