Nemaline Myopathy 2
In 5 affected individuals from 5 Ashkenazi Jewish families with autosomal recessive typical nemaline myopathy (NEM2; 256030), Anderson et al. (2004) identified a homozygosity for a 2,502-bp deletion completely encompassing exon 55 and parts of introns 54 and 55 of the NEB gene, predicted to result in a transcript encoding 35 fewer amino acids. Screening for this mutation in a random sample of 4,090 Ashkenazi Jewish individuals revealed a carrier frequency of 1 in 108.
Lehtokari et al. (2009) identified the 2,502-bp deletion in 14 of 355 probands with nemaline myopathy from around the world; 2 of the probands had been reported by Anderson et al. (2004). Seven probands were homozygous for the deletion, and 7 carried the mutation in heterozygosity. Two of the families were not of known Ashkenazi Jewish descent, but carried the common haplotype identified in Ashkenazi Jews. The findings were consistent with a founder effect.
Ottenheijm et al. (2009) studied the muscular phenotype of nemaline myopathy patients with NEB exon 55 deletion (NM-NEB). SDS-PAGE and Western blot analysis revealed greatly reduced nebulin levels in skeletal muscle of NM-NEB patients, with the most prominent reduction at nebulin's N-terminal end. Muscle mechanical studies indicated an approximately 60% reduced force generating capacity of NM-NEB muscle and a leftward shift of the force-sarcomere length relation in NM-NEB muscle fibers. This indicates that the mechanism for the force reduction is likely to include shorter and nonuniform thin filament lengths in NM-NEB muscle compared with control muscle. The average thin filament length was reduced from approximately 1.3-micrometer in control muscle to approximately 0.75-micrometer in NM-NEB muscle. Ottenheijm et al. (2009) hypothesized that dysregulated thin filament length may contribute to muscle weakness in nemaline myopathy patients with nebulin mutations.
Arthrogryposis Multiplex Congenita 6
Yonath et al. (2012) reported 4 unrelated pregnancies with abnormal prenatal ultrasound findings in fetuses with arthrogryposis multiplex congenita-6 (AMC6; 619334). In each family, 1 or both of the parents was of Ashkenazi Jewish descent, and the common exon 55 deletion in the NEB gene was found in the heterozygous state in the patients and in unaffected parents. A second pathogenic NEB mutation was found in 3 of the patients; a second mutation could not be identified in 1 of the patients. Prenatal ultrasound showed polyhydramnios, decreased fetal movements, clubfoot, and clenched hands. All patients showed severe hypotonia after birth, and all died within the first months of life.
Feingold-Zadok et al. (2017) identified this mutation in 2 fetal sibs with AMC6 from an Ashkenazi Jewish family (family 1), in compound heterozygosity with a splice site mutation.
