Dominant mutations in KBTBD13, a member of the BTB/Kelch family, cause nemaline myopathy with cores

Abstract

We identified a member of the BTB/Kelch protein family that is mutated in nemaline myopathy type 6 (NEM6), an autosomal-dominant neuromuscular disorder characterized by the presence of nemaline rods and core lesions in the skeletal myofibers. Analysis of affected families allowed narrowing of the candidate region on chromosome 15q22.31, and mutation screening led to the identification of a previously uncharacterized gene, KBTBD13, coding for a hypothetical protein and containing missense mutations that perfectly cosegregate with nemaline myopathy in the studied families. KBTBD13 contains a BTB/POZ domain and five Kelch repeats and is expressed primarily in skeletal and cardiac muscle. The identified disease-associated mutations, C.742C>A (p.Arg248Ser), c.1170G>C (p.Lys390Asn), and c.1222C>T (p.Arg408Cys), located in conserved domains of Kelch repeats, are predicted to disrupt the molecule's beta-propeller blades. Previously identified BTB/POZ/Kelch-domain-containing proteins have been implicated in a broad variety of biological processes, including cytoskeleton modulation, regulation of gene transcription, ubiquitination, and myofibril assembly. The functional role of KBTBD13 in skeletal muscle and the pathogenesis of NEM6 are subjects for further studies.

Figure 1

Histochemical, Ultrastructural, and Immunohistochemical Findings in a Muscle Biopsy Sample from the Right Biceps Brachii Muscle Biopsy of the Spanish Family Index Patient (A) Clusters of nemaline rods on modified trichrome staining forming confluent inclusion in some areas under the sarcolemma and centrally within the cytoplasm (scale bar represents 50 μm). (B) Electron micrograph showing multiple nemaline rods located under the sarcolemma and near Z-disks (scale bar represents 2 μm). Muscle sample fixed in 2% glutaraldehyde, postfixed with 1% osmium tetroxide, was embedded in araldite. Ultrathin sections were stained with uranyl acetate and lead citrate and were viewed and photographed with a JEOL 1011electron microscope. (C) Cores devoid of oxidative enzyme activity appear on NADH reaction (scale bar represents 50 μm). (D) Double-labeling immunohistochemistry for slow (pale) and fast (dark) myosin demonstrating type 1 fiber predominance and hypertrophy versus type 2 fiber atrophy (scale bar represents 50 μm).

Figure 2

KBTBD13 Mutations in NEM6 Families (A) The NEM6 locus is a 6.7 Mb interval delimited by markers D15S155 and D15S125, which physically map between the 58.1 Mb and 68.4 Mb chromosomal positions. The region contains 28 genes expressed in striated muscle. (B) KBTBD13 mutations identified in NEM6 patients are located within the highly conserved second and fifth Kelch repeats. (C) Evolutionary conservation of KBTBD13 domains. The mutated residues are shown in bold.

Figure 3

Tissue-Specific Expression of the Mouse KBTBD13 Tissue source is indicated on top of loaded RT-PCR products. GAPDH was used as internal control for normalization of cDNA quantity. The sizes of cDNA fragments are 249 bp for KBTBD13 and 101 bp for GAPDH.

Figure 4

Localization of KBTBD13 Tagged with Myc-Flag in Embryonic Mouse Cardiomyocytes Top images: wild-type KBTBD13; bottom images: p.Arg408Cys mutant KBTBD13. KBTBD13 is stained in red and α-actinin in green; the nucleus is labeled in blue with DAPI. Inset: higher magnification of boxed areas shows no visible colocalization of the wild-type or mutant KBTBD13 (arrowheads) with α-actinin in striated premyofibrils (arrows). Scale bar represents 10 μm.