Pedigrees of Described Families with CHRNA1, CHRND, or RAPSN Mutations Indicate Recessive Inheritance and Increased Intrauterine Lethality
Two families with CHRNA1 mutations, two families with CHRND mutations causing lethal multiple pterygium syndrome, and one family with recessive RAPSN mutations resulting in fetal akinesia syndrome with inborn contractures are shown. Arrow: index patient. Small symbols represent abortions, intrauterine death, or termination of pregnancy because of severe affection.

Evolutionary Conservation of CHRNA1, CHRND, and RAPSN Missense Mutations
For all missense mutations, interspecies comparison reveals conservation in homolog positions in all mammals tested. CHRNA1 mutation R234L (A) and both novel RAPSN missense mutations (C, D) affect residues that show evolutionary conservation even beyond mammals. In addition, residue of CHRNA1 mutation R234L (A) is also conserved in other human AChR subunits β1, δ, γ, and ɛ as well as other α-type subunits α2 and α3 in nonmuscular AChR.
(A) CHRNA1 R234L.
(B) CHRND F74L.
(C) RAPSN F139S.
(D) RAPSN A189V.

Prenatal Ultrasound Revealed Fetal Hydrops and Contractures in Family CHRND-F1
Extensive generalized edema and flexion contractures of upper and lower limbs in individual V-3 from family CHRND-F1 were identified on prenatal ultrasound at gestational week 13+4 days. Edema separating skin from underlying structures extended from head and neck to trunk and is marked by an asterisk.
(A) Pathologic nuchal edema extending down the back.
(B) Planum frontooccipitale with noticeable hygroma.
(C) Cross section through shoulder region reveals flexion contractures with adducted forearms and hands.
(D) Upper abdominal cross section shows extensive edema.
(E) Flexion contracture of lower limb with pes equines deformity.

Schematic View of AChR Complex
AChR at the postsynaptic membrane in muscle cells consist of 5 subunits. 11 Two α1, one β1, one δ subunit are always present. The fifth subunit is the fetally expressed γ subunit. By around 33 weeks of gestation in humans, γ subunit expression is stopped switching to ɛ, thereby replacing fetal-type AChR by adult-type AChR. 10 Rapsyn is a cytoskeletal membrane protein colocalizing with AChR. Functionally, rapsyn is considered to transduce signals from the agrin-activated MUSK. Not shown here, rapsyn connects the receptor with the cytoskeletal dystrophin-glycoprotein-complex (DGC) and stabilizes AChR clusters by interaction with calpain. 12–15

CHRNA1 and CHRND Mutation-Positive Fetuses Presented with Massive Hydrops, Pterygia, and Contractures on Postmortem Examination whereas Novel Recessive Missense RAPSN Mutations Caused Congenital Arthrogryposis and Life-Threatening Respiratory Distress
(A–F) Individual II-2 from family CHRNA1-F2 shows generalized edema most extreme at neck and head (gray arrowheads in [A] and [B]), pterygia at elbows and knees (black arrowheads in [A] and [D]), and severe joint contractures (A, D–F) after induced abortion at 17 gestational weeks. X-ray reveals scoliosis, malformed head, and soft tissue swelling by nuchal hygroma (E, F).
(G–J) Individual V-3 from family CHRND-F1 is the same patient as in ultrasound Figure 3 and shown after induced abortion at gestational age 14+6. Generalized edema (gray arrowheads in [G] and [I]), pterygia at elbows and knees (black arrowheads in [G] and [H]), and severe joint contractures (G–J) were detected on postmortem inspection. The fetogram (I, J) showed relatively broad clavicles, ribs, and left metatarsal bone I, but did not reveal any severe skeletal anomalies.
(K–N) In contrast, patients with RAPSN mutations were born at term (family RAPSN-F1). Patient II-5 (K–M) was born with severe respiratory problems and inborn contractures (L, M). Her affected brother II-4 (N) had similar clinical manifestations and deceased because of respiratory insufficiency at the age of 10 months. Both patients had down-slanting palpebral fissures, mild hypertelorism, a wide nasal bridge, low-set ears, micrognathia, and small mouth with tented lips (K, N).

In Situ Hybridization in Mouse Embryos Reveals Early Expression of AChR Subunit Genes and Rapsn in Somites and Later in Skeletal Muscle as well as in Hygroma-Relevant Regions
(A) Expression of Chrna1, Chrnb1, Chrnd, Chrng, and Rapsn in mouse development. Probes and embryonic stages are as indicated. E10.5, E11.5, and E12.5 are shown as whole-mount in situ hybridization (ISH), E14.5 and E15.5 are shown as section-ISH. Chrna1, Chrnb1, Chrnd, Chrng, and Rapsn are distinctly expressed in early somites as early as E10.5 (arrows), corresponding to human developmental age of 32 days (46 gestational day, Carnegie stage 14). At E11.5, expression of Chrna1, Chrnb1, Chrnd, Chrng, and Rapsn starts in upper developing limb and seems to further proceed from proximal into the developing muscle bulks at E12.5 days. At E14.5, expression corresponds to muscle anlagen at trunk, neck, limbs, and diaphragm. Whereas Chrna1, Chrnb1, Chrng, and Rapsn are stably expressed throughout the embryonic stages analyzed, Chrnd exhibits a more dynamic expression pattern. Chrnd shows strong expression in the posterior (newly formed) somites at E10.5 and E11.5. Expression apparently decreases around E12.5, to barely detectable levels in whole-mount and section in situ hybridization in somites (not shown). At E14.5, the muscles still show relatively weak expression. However, robust expression is reappearing in differentiated muscles at E15.5.
Comparative developmental stages adapted from Wessels and Markwald.
Mouse E10.5 ≈ Carnegie stage 14 ≈ human developmental age of 32 days (post conception) ≈ 46 days gestation (post menstruation)
Mouse E11.5 ≈ Carnegie stage 16 ≈ human developmental age of 37 days (post conception) ≈ 51 days gestation (post menstruation)
Mouse E12.5 ≈ Carnegie stage 18 ≈ human developmental age of 44 days (post conception) ≈ 58 days gestation (post menstruation)
Mouse E14.5 ≈ Carnegie stage 23 ≈ human developmental age of 54–56 days (post conception) ≈ 68–70 days gestation (post menstruation)
(B and C) Prenatal expression of AChR subunit genes and Rapsn in hygroma-relevant regions. Expression of α1 at E14.5 is representative also for β1, δ, and γ (data not shown). Note strong expression in nuchal area, close proximity to jugular lymphatic sac (marked as “jls” in higher magnification in [B]), and subcutaneous muscle layers (marked by an arrow in [C]). Mutations might contribute to edema by, for example, decreased or absent muscle contractures and subsequently impaired transport of lymphatic fluid. The lymphatic system develops from two structures. Deep parts of the jugular lymphatic sacs derive from tissue around jugular veins. Superficial parts of the jugular lymphatic sacs and peripheral lymphatic vessels develop from local lymphangioblasts originating from mesodermal anlagen. 31–34 As long as no connection is made between the jugular lymphatic sacs and the jugular veins, transient nuchal edema is physiological. If the lymphojugular junction connection is delayed and/or volume increases, abnormal nuchal edema or cystic hygroma occurs. A possible AChR-related pathomechanism for fetal edema is that defective neuromuscular signal transduction causes lack of muscle contractions and thus impairs lymphatic fluid movement. Other potential mechanisms include altered muscle development affecting subsequent development and differentiation of lymphatic vessels. Finally, both muscle and lymphatic anlagen may depend on similar AChR-related signals or components. AChR and rapsyn expression in premuscular and muscular tissues and close anatomical proximity to lymphatic structures both in embryonic development and fetal differentiation is consistent with this view.