Cilia & Ciliopathies

Cilia are surface organelles built around a microtubule core (the axoneme) anchored to a basal body (modified centriole). Almost every vertebrate cell has at least one: most as a stationary "antenna" (the primary cilium), some as an actively beating motile cilium. When cilia fail, the resulting diseases (the ciliopathies) span an apparently unrelated set of organs: kidney, retina, liver, brain, skeleton, gonads, and respiratory tract. Recognizing the shared organellar mechanism is one of the most useful moves in dysmorphology because a presentation of "retinal degeneration + cystic kidneys + polydactyly + obesity" or "chronic sinopulmonary infections + situs inversus" maps to a single concept: the cilium broke.

The embryonic node is the exception that proves the rule: nodal cilia are 9+0 but rotate, generating leftward fluid flow that establishes left-right asymmetry. Loss of nodal motility produces randomized situs (heterotaxy / situs inversus).

Cilia have no ribosomes; every protein has to be transported in. IFT trains are kinesin- and dynein-driven cargo trains that shuttle along the axoneme:

• IFT-B + kinesin-2 = anterograde (base → tip)
• IFT-A + cytoplasmic dynein-2 = retrograde (tip → base)

Mutations in IFT-A or IFT-B subunits, in the kinesin/dynein motors, or in the transition zone (the gatekeeper at the cilium base: Joubert/Meckel-Gruber proteins MKS1, TMEM67, CC2D2A, CEP290) all produce ciliopathies. The phenotype depends on which tissue's cilium is most affected.

The primary cilium concentrates receptors for several developmental pathways:

• Hedgehog (SHH/IHH): Smoothened and the GLI transcription factors traffic through the cilium. Cilium dysfunction = abnormal Hedgehog signaling, explaining why ciliopathies often have polydactyly, holoprosencephaly-spectrum features, and skeletal dysplasias (the same axes Hedgehog patterns).
• Wnt: both canonical and non-canonical (planar cell polarity) Wnt require ciliary input.
• PDGF / Hippo / Notch: also concentrated at the cilium.
• In the kidney tubule, the primary cilium senses fluid flow via PC1/PC2 (polycystin-1 and -2). Loss → cyst formation. This is the mechanism behind ADPKD and ARPKD.

Primary Ciliary Dyskinesia (PCD) / Kartagener syndrome

• Genes: dynein arm components (DNAH5, DNAI1, DNAAF1, plus dozens of others). Autosomal recessive.
• Phenotype:
  • Chronic sinopulmonary disease (otitis media, sinusitis, bronchiectasis)
  • Situs inversus in ~50% (Kartagener triad = bronchiectasis + situs inversus + chronic sinusitis)
  • Male infertility (immotile sperm flagella, same axoneme)
  • Female subfertility (fallopian tube cilia)
• Diagnosis: nasal nitric oxide (low), high-speed video microscopy of beating cilia, electron microscopy, genetic testing.
• See the dedicated Primary Ciliary Dyskinesia leaf for the full clinical picture.

Heterotaxy / Situs Anomalies

• Defective nodal cilia → randomized left-right axis. Outcomes range from benign isolated dextrocardia to complex CHD with asplenia/polysplenia (Ivemark syndromes).

A constellation of phenotypes that overlap dramatically because they all hit the same organelle in different tissues:

A handful of features keep showing up because each one reflects ciliary failure in a specific tissue:

When 3+ of these co-occur, ciliopathy is the unifying diagnosis until proven otherwise.

• Most ciliopathies are AR. ADPKD is the major exception; OFD1 is X-linked dominant; some RP forms are X-linked or AD.
• Allelic heterogeneity is profound. The same gene (TMEM67, CEP290, CC2D2A) can produce Joubert, Meckel-Gruber, BBS, or NPHP depending on residual function. Genotype-phenotype correlation is weak; clinical phenotype + multigene panel is the practical approach.
• Carrier frequency in some founder populations is meaningful (BBS in Bedouin populations, ARPKD in some isolates).
• Recurrence risk for AR ciliopathies is 25% per pregnancy. Reproductive options include preimplantation genetic testing once the family's variant pair is known.