RAPSN

RAPSN encodes rapsyn, a postsynaptic scaffolding protein essential for clustering nicotinic acetylcholine receptors (AChRs) at the neuromuscular junction (PMID:34033754). Rapsyn drives postsynaptic differentiation by undergoing liquid-liquid phase separation through multivalent interactions of its tetratricopeptide repeat (TPR) domains, forming liquid-like condensates that recruit AChRs along with cytoskeletal and signaling proteins; this condensation is enhanced by MuSK signaling (PMID:34033754). Self-association and co-clustering depend on intact TPR architecture, and a mutation in the TPR6 domain disrupts these functions in CMS patients (PMID:34033754, PMID:22326364). RAPSN transcription in muscle is governed by E-box elements in its promoter that bind the myogenic transcription factors MyoD and myogenin; promoter mutations reduce transcriptional output (PMID:12651869). Loss-of-function mutations span the regulatory and coding sequence — promoter E-box, splice-site, missense, and frameshift alleles — and produce a severity spectrum in which incomplete loss of function causes congenital myasthenic syndrome while complete loss causes lethal fetal akinesia (PMID:12651869, PMID:16931511, PMID:18179903). Missense mutations such as R164C and L283P specifically impair co-clustering of AChR with rapsyn (PMID:16931511).

1. 1994 Medium

   Establishing the genomic structure of the gene was the first step in defining the protein's domain organization and enabling later mutation analysis.

   Evidence Genomic cloning, RNase protection, and genetic mapping of the mouse Rapsn gene

   PMID:7698761

   Open questions at the time

   • Did not address protein function or AChR clustering
   • Mouse gene only; human locus not yet defined
2. 1996 Medium

   Cloning the human cDNA and mapping the locus provided the reagents and chromosomal position needed to link the gene to human disease.

   Evidence cDNA cloning/sequencing and radiation hybrid mapping placing RAPSN at 11p11.2-p11.1

   PMID:8812503

   Open questions at the time

   • No functional assay of the human protein
   • No disease association established yet
3. 2003 High

   It was unknown how RAPSN transcription is controlled; this work showed E-box promoter elements bound by myogenic factors drive expression and that promoter mutations cause disease.

   Evidence EMSA, luciferase reporter assays in C2C12 myotubes/HEK with MyoD/myogenin, and patient allele transcription analysis

   PMID:12651869

   Open questions at the time

   • Did not quantify how reduced transcription maps to clinical severity
   • Other promoter/enhancer elements not excluded
4. 2006 Medium

   To connect coding mutations to molecular dysfunction, missense and splice mutations were shown to impair AChR co-clustering or generate frameshift transcripts.

   Evidence Minigene splicing analysis and AChR-rapsyn co-clustering assays of mutant constructs

   PMID:16931511

   Open questions at the time

   • Single-lab cellular assay
   • Mechanism of clustering loss at protein level not defined
5. 2008 Medium

   The relationship between mutation severity and phenotype was unresolved; a frameshift allele established that complete loss of function causes lethal fetal akinesia versus milder congenital myasthenia for partial loss.

   Evidence Direct sequencing, familial segregation, and patient-sample functional studies in a fetal akinesia family

   PMID:18179903

   Open questions at the time

   • Functional studies incompletely described
   • Quantitative genotype-phenotype boundary not defined
6. 2012 Low

   A TPR6-domain mutation in CMS patients implicated this domain in rapsyn self-association and AChR co-clustering.

   Evidence Genetic sequencing with functional role inferred from domain localization in compound heterozygosity

   PMID:22326364

   Open questions at the time

   • No direct in vitro or cellular assay of TPR6 function performed
   • Compound heterozygosity confounds attribution to single allele
7. 2021 High

   The physical mechanism of AChR clustering was unresolved; this work showed rapsyn forms phase-separated condensates via multivalent TPR interactions that recruit AChRs and that CMS mutations disrupt this LLPS in vitro and in mice.

   Evidence In vitro LLPS and co-condensation assays, TPR mutagenesis, MuSK signaling modulation, and a transgenic mouse carrying an LLPS-deficient CMS mutation

   PMID:34033754

   Open questions at the time

   • Stoichiometry and composition of in vivo condensates not fully defined
   • Mechanistic link between MuSK signaling and condensate enhancement incompletely mapped

• How rapsyn condensate composition and dynamics are regulated across development and how the full spectrum of CMS alleles quantitatively maps to LLPS defects remain open.
• No structural model of TPR-mediated multivalency reported in the corpus
• Quantitative coupling of LLPS impairment to clinical severity not established