MYL2-associated congenital fiber-type disproportion and cardiomyopathy with variants in additional neuromuscular disease genes; the dilemma of panel testing

Next-generation sequencing has led to transformative advances in our ability to diagnose rare diseases by simultaneously sequencing dozens, hundreds, or even entire genomes worth of genes to efficiently identify pathogenic mutations. These studies amount to multiple hypothesis testing on a massive scale and not infrequently lead to discovery of multiple genetic variants whose relative contributions to a patient's disease are unclear. Panel testing, in particular, can be problematic because each of the many genes being sequenced might represent a plausible explanation for a given case. We performed targeted gene panel analysis of 43 established neuromuscular disease genes in a patient with congenital fiber-type disproportion (CFTD) and fatal infantile cardiomyopathy. Initial review of variants identified changes in four genes that could be considered relevant candidates to cause this child's disease. Further analysis revealed that two of these are likely benign, but a homozygous frameshift variant in the myosin light chain 2 gene, MYL2, and a heterozygous nonsense mutation in the nebulin gene, NEB, met criteria to be classified as likely pathogenic or pathogenic. Recessive MYL2 mutations are a rare cause of CFTD associated with both skeletal and cardiomyopathy, whereas recessive NEB mutations cause nemaline myopathy. Although the proband's phenotype is likely largely explained by the MYL2 variant, the heterozygous pathogenic NEB variant cannot be ruled out as a contributing factor. This case illustrates the complexity when analyzing large numbers of variants from targeted gene panels in which each of the genes might plausibly contribute to the patient's clinical presentation.

In-frame de novo mutation in BICD2 in two patients with muscular atrophy and arthrogryposis

We describe two unrelated patients, a 12-yr-old female and a 6-yr-old male, with congenital contractures and severe congenital muscular atrophy. Exome and genome sequencing of the probands and their unaffected parents revealed that they have the same de novo deletion in BICD2 (c.1636_1638delAAT). The variant, which has never been reported, results in an in-frame 3-bp deletion and is predicted to cause loss of an evolutionarily conserved asparagine residue at position 546 in the protein. Missense mutations in BICD2 cause autosomal dominant spinal muscular atrophy, lower-extremity predominant 2 (SMALED2), a disease characterized by muscle weakness and arthrogryposis of early onset and slow progression. The p.Asn546del clusters with four pathogenic missense variants in a region that likely binds molecular motor KIF5A. Protein modeling suggests that removing the highly conserved asparagine residue alters BICD2 protein structure. Our findings support a broader phenotypic spectrum of BICD2 mutations that may include severe manifestations such as cerebral atrophy, seizures, dysmorphic facial features, and profound muscular atrophy.

The importance of managing the patient and not the gene: expanded phenotype of GLE1-associated arthrogryposis

GLE1 encodes a protein important for mRNA export and appears to play roles in translation initiation and termination as well. Pathogenic variants in GLE1 mutations have been associated with lethal contracture syndrome and lethal arthrogryposis with anterior horn cell disease; phenotypes reported in individuals include fetal akinesia and a severe form of motor neuron disease, typically presenting with prenatal symptoms and perinatal lethality. In this article, we identified biallelic missense mutations in GLE1 by trio whole-exome sequencing in an individual affected with congenital motor weakness and contractures as well as feeding and respiratory difficulties. Muscle biopsy was consistent with anterior horn cell disease and supported the pathogenicity of the sequence variants. Importantly, this individual survived past the perinatal period with respiratory support and currently demonstrates age-appropriate cognition and slow but steady motor developmental progress. We propose that pathogenic variants in GLE1 can be associated with a nonperinatal lethal motor phenotype, and affected individuals can demonstrate motor skill progression, unlike prototypical anterior horn cell diseases such as spinal muscular atrophy.