Popliteal pterygium syndrome

AD; IRF6, allelic with Van der Woude syndrome. Popliteal pterygium syndrome is caused by dominant-negative missense variants, characteristically in the IRF6 DNA-binding domain, rather than the loss-of-function variants that cause Van der Woude syndrome.

IRF6 is a transcription factor that helps build the periderm, a transient layer covering the embryonic epithelium. Think of the periderm as the release liner on a sticker: an anti-adhesive that keeps the glue from bonding to the wrong thing, so a surface sticks only where and when it should.

Without a working periderm, epithelial surfaces fuse early and in the wrong places. This produces the popliteal pterygia (webbed knees), ankyloblepharon (webbed eyelids), and oral synechiae (webbing in the mouth). The cleft palate follows the same logic: without the anti-adhesive layer, the palatal shelves stick to the tongue or mandible early, get stuck there, and never elevate to reach the midline.

In popliteal pterygium syndrome, a dominant-negative missense variant in the DNA-binding domain makes a protein that still dimerizes but cannot bind DNA, so it drags the wild-type protein into nonfunctional complexes. That is why the phenotype is more severe: the Van der Woude features plus pterygia, syndactyly, and genital anomalies. Same gene, two mechanisms, one severity spectrum.

The whole program is switched on from above by p63 (encoded by TP63), which directly drives IRF6. That is why TP63-related AEC (Hay-Wells) syndrome pairs ankyloblepharon with clefting: whether the hit lands on p63 or on IRF6, the result is the same failure of the anti-adhesive periderm.

• Popliteal pterygia: webs behind the knees that can tether the leg. The sciatic nerve may run within the web, which matters for surgery.
• Pyramidal skin fold over the hallux nail: highly specific for popliteal pterygium syndrome and the cleanest discriminator from Van der Woude.
• Orofacial: cleft lip and/or palate, paramedian lower lip pits, and intraoral bands (oral synechiae, sometimes syngnathia).
• Other webbing: intercrural pterygia and, in some, filiform bands between the eyelids (ankyloblepharon).
• Syndactyly of the fingers or toes.
• Genital anomalies: hypoplastic labia majora, bifid scrotum, and cryptorchidism.

• Clinical recognition of pterygia and the hallux fold together with the orofacial features.
• Molecular confirmation of an IRF6 pathogenic variant. A DNA-binding-domain missense variant fits popliteal pterygium syndrome, whereas a loss-of-function variant fits Van der Woude.

• Van der Woude syndrome (allelic, also IRF6): the milder end of the same spectrum, with lip pits and clefting but no pterygia, hallux fold, or genital anomalies.
• Bartsocas-Papas syndrome (RIPK4): a more severe, often lethal popliteal pterygium phenotype, inherited autosomal recessive.
• Escobar (multiple pterygium) syndrome (CHRNG): multiple pterygia across joints from a fetal acetylcholine-receptor defect, autosomal recessive, and without the IRF6 orofacial pit pattern.

• Multidisciplinary cleft team care for cleft lip and/or palate.
• Orthopedic and plastic surgery for pterygia, with caution: the sciatic nerve can lie within the popliteal web.
• Release of oral synechiae and repair of genital anomalies as needed.
• Genetic counseling for autosomal dominant inheritance. Examine a mildly affected parent for lip pits, since the same variant can look like Van der Woude in one relative and popliteal pterygium in another.