Loss-of-Function Mutations in LGI4, a Secreted Ligand Involved in Schwann Cell Myelination, Are Responsible for Arthrogryposis Multiplex Congenita

Abstract

Arthrogryposis multiplex congenita (AMC) is a developmental condition characterized by multiple joint contractures resulting from reduced or absent fetal movements. Through genetic mapping of disease loci and whole-exome sequencing in four unrelated multiplex families presenting with severe AMC, we identified biallelic loss-of-function mutations in LGI4 (leucine-rich glioma-inactivated 4). LGI4 is a ligand secreted by Schwann cells that regulates peripheral nerve myelination via its cognate receptor ADAM22 expressed by neurons. Immunolabeling experiments and transmission electron microscopy of the sciatic nerve from one of the affected individuals revealed a lack of myelin. Functional tests using affected individual-derived iPSCs showed that these germline mutations caused aberrant splicing of the endogenous LGI4 transcript and in a cell-based assay impaired the secretion of truncated LGI4 protein. This is consistent with previous studies reporting arthrogryposis in Lgi4-deficient mice due to peripheral hypomyelination. This study adds to the recent reports implicating defective axoglial function as a key cause of AMC.

Keywords: ADAM22; LGI4; Schwann cells; arthrogryposis multiplex congenital; hypomyelination; secreted ligand; whole-exome sequencing.

Figure 1

Germline Mutations in LGI4 in Four AMC-Affected Families and Endogenous Transcript Analysis (A) Pedigrees for AMC-affected families 1, 2, 3, and 4. Arrows indicate mutant nucleotide positions. The affected individuals carry either compound heterozygous (family 2 and 4) or homozygous (families 1 and 3) mutations. The nucleotide and amino acid changes are indicated with respect to the reference sequences (GenBank: NM_139284.2 and GenBank: NP_644813.1, respectively). Open symbols: unaffected; filled symbols: affected. (B) cDNA analysis of LGI4 in AMC-affected families. RT-PCR analysis was performed from RNA after reverse transcription by using random hexamers. Gel electrophoresis of RT-PCR products revealed abnormal fragments in affected individuals when compared to control (Ctrl) as the consequence of the spliced mutations. Sanger sequencing was performed from the RT-PCR products in control and affected individuals (Figure S2). (C) Location of mutations in LGI4. (D) Phylogenetic conservation of mutated residues in LGI4 vertebrate homologs. Accession numbers are from UniprotKB. Abbreviations are as follows: M. domestica, Monodelphis domestica; P. sinensis, Pelodiscus sinensis; asterisk (^∗) represents fully conserved residue; colon (:) represents residues with strongly similar properties; period (.) represents residues with weakly similar properties.

Figure 2

Morphological Characteristics of Nerve Lesions in the Affected Individual II:1 of Family 2 Carrying Deleterious LGI4 Mutations Tissue samples were fixed in a 2% glutaraldehyde fixative solution, post-fixed with osmium tetroxide, and embedded in resin epoxy. Semi-thin sections were stained with toluidine blue (A and B). Ultra-thin sections were contrasted with uranyl acetate and lead citrate and examined under a PHILIPS CM10 transmission electron microscope (C and D). (A and B) Transverse semi-thin sections of the sciatic nerve of the affected fetus (A) displays no myelinated fibers when compared with an age-matched control case (B), where numerous myelinated fibers are observed (black arrows; original magnification 1,000). (C and D) TEM transverse section of sciatic nerve of the affected fetus (C) confirms complete lack of myelinated fibers with normal endoneural cellularity mainly corresponding to Schwann cells (arrow), when compared to the control case, where numerous myelinated fibers (arrow) are observed (D). Scale bars represent 1 μm.

Figure 3

LGI4 Splice Mutations Reduce Endogenous mRNA Levels and Impair Secretion of Mutant LGI4 (A) Primary dermal fibroblasts from control subject and affected individual II:3 of family 3 were reprogrammed to iPSCs and then differentiated to NCSCs using indicated transcription factors and small molecule inhibitors. (B) Quantitative RT-PCR (qRT-PCR) analysis in fibroblasts, iPSCs, and NCSCs shows an upregulation of LGI4 expression in NCSCs. (C) qRT-PCR analysis shows a decrease in endogenous LGI4 mRNA levels in affected individual-derived cells relative to control (Ctrl), suggestive of nonsense-mediated decay. (D) RT-PCR analysis of NCSCs using primers spanning exons 8 and 9 (lanes 1, 2) and primers spanning exons 7 and 9 (lanes 3, 4, 5) shows that affected individual cells produce aberrantly spliced LGI4 transcripts labeled F3a, F3b, and F3c. (E) Schematic representation of the mRNA products from affected individual cells detected in (D) and Figure 1B. Frameshifted exons are colored orange and premature stop codons are denoted by red lines. (F) qRT-PCR using primers that detect correctly spliced transcripts only (WT) shows a dramatic reduction of WT LGI4 transcripts in the affected individual cells of family 3. (G) Different variants of LGI4 from (E) were tagged with HA and transfected into HEK293T cells. Western blot using HA-HRP antibody (Sigma, H6533) shows that LGI4 mutants accumulate in the cell lysate whereas WT LGI4 is enriched in the conditioned media. This suggests that truncated LGI4 proteins fail to be properly secreted. Calnexin (Abcam, ab31290) was used as a loading control. (H) LGI4 is secreted by Schwann cells and signals through its cognate receptor ADAM22 expressed on the surface of peripheral neurons. This signal transduction is believed to signal back to Schwann cells to induce myelination. ^∗∗∗p < 0.001.