Nemaline Myopathy via the NEB Exon 55 Deletion

Variants in the nebulin gene (NEB, OMIM 161650) are one cause of autosomal recessive nemaline myopathy (NEM, OMIM 256030). NEM is a genetically and clinically heterogeneous disorder characterized by muscle weakness, hypotonia and the presence of nemaline bodies in skeletal muscle fibers. Muscle weakness is typically observed in affected neonates or infants, although later onset cases are reported (Ryan et al. Ann Neurol 50:312-320, 2001). The most severely affected muscle groups are proximal limb muscles, facial, bulbar, and respiratory muscles. Deep tendon reflexes are absent or depressed. Histologically, NEM is characterized by type 1 fiber predominance and the presence of rod-like structures called nemaline bodies upon Gomori trichrome staining of skeletal muscle (Ryan et al. Neurol 60:665-673, 2003). Six clinical types of NEM have been delineated based on age of onset, severity and distribution of weakness, and respiratory function (Ryan et al. 2001; North and Ryan. GeneReviews 2010). Nebulin gene variants more often cause typical neonatal-onset disease, although NEB variants have been found in every clinical form of NEM (Lehtokari et al. Hum Mut 27:946-956, 2006). Overlap among the six clinical groups is significant, and adults are sometimes diagnosed only after another family member has presented with typical signs.

To date, variants in six genes have been shown to cause nemaline myopathy. Variants in ACTA1 (NEM3) and NEB (NEM2) are the only relatively common causes (Ryan et al. 2001). All reported cases of NEB-associated NEM have demonstrated autosomal recessive inheritance (Lehtokari et al. 2006). The only common NEB variant is an exon 55 deletion found at a carrier frequency of about 1% among people of Ashkenazi Jewish ancestry (Anderson et al. Hum Genet 115:185-190, 2004). Much rarer causes of NEM are variants in TPM3 (NEM1), TPM2 (NEM4), TNNT1 (NEM5), and CFL2 (NEM7).

This test is intended to detect the exon 55 NEB deletion variant found in Ashkenazi Jews; it will not detect any other sequence variant.  In a world-wide cohort of 355 patients with nemaline myopathy, fourteen probands were found to have the exon 55 deletion variant and all shared the same ancestral haplotype (Lehtokari et al. Neuromusc Disord 19:179-181, 2009).  Sequencing the entire NEB coding region may be indicated in individuals who are clinically affected and have either one or no alleles with the exon 55 deletion.  Complete NEB gene sequencing is available from PreventionGenetics.  Because nemaline myopathy exhibits locus and allelic heterogeneity, a negative NEB test does not rule out this diagnosis when classic clinical and histological findings are present.

Testing is accomplished by amplifying patient and control DNAs with PCR primers that flank or lie within the 2,502 bp deletion, essentially as described by Anderson et al. (Hum Genet 115:185-190, 2004). This test permits the identification of patients with normal genotypes, patients who are homozygous for the deletion, and heterozygous carriers.