Mutations in KLHL40 are a frequent cause of severe autosomal-recessive nemaline myopathy

Abstract

Nemaline myopathy (NEM) is a common congenital myopathy. At the very severe end of the NEM clinical spectrum are genetically unresolved cases of autosomal-recessive fetal akinesia sequence. We studied a multinational cohort of 143 severe-NEM-affected families lacking genetic diagnosis. We performed whole-exome sequencing of six families and targeted gene sequencing of additional families. We identified 19 mutations in KLHL40 (kelch-like family member 40) in 28 apparently unrelated NEM kindreds of various ethnicities. Accounting for up to 28% of the tested individuals in the Japanese cohort, KLHL40 mutations were found to be the most common cause of this severe form of NEM. Clinical features of affected individuals were severe and distinctive and included fetal akinesia or hypokinesia and contractures, fractures, respiratory failure, and swallowing difficulties at birth. Molecular modeling suggested that the missense substitutions would destabilize the protein. Protein studies showed that KLHL40 is a striated-muscle-specific protein that is absent in KLHL40-associated NEM skeletal muscle. In zebrafish, klhl40a and klhl40b expression is largely confined to the myotome and skeletal muscle, and knockdown of these isoforms results in disruption of muscle structure and loss of movement. We identified KLHL40 mutations as a frequent cause of severe autosomal-recessive NEM and showed that it plays a key role in muscle development and function. Screening of KLHL40 should be a priority in individuals who are affected by autosomal-recessive NEM and who present with prenatal symptoms and/or contractures and in all Japanese individuals with severe NEM.

Figure 1

Family Pedigrees and Light and Electron Microscopy of Muscle Biopsies (A) Modified Gomori trichrome (upper) and electron microscopy (lower) of muscle biopsies from affected individuals of families 15 (right) and 20 (left). Abnormal variation in fiber size, together with many small myofibers and sometimes increased connective tissue, and the presence of numerous red- or purple-stained nemaline bodies (arrows) can be seen (upper panels). Numerous nemaline bodies with varying sizes and shapes and a lack of normal myofibrils are visible by electron microscopy (arrows). Scale bars represent 20 μm for modified Gomori trichrome and 1 μm for electron microscopy. (B) Pedigrees for the families in which exome sequencing and analysis were performed on the probands. Asterisks indicate the individuals whose DNA was analyzed by exome sequencing. Segregation of the mutations identified in each pedigree is shown.

Figure 2

Mutations Identified in Our Cohort and the Structural Modeling of the Missense KLHL40 Substitutions (A) Schematic presentation of the genomic structure of KLHL40 (upper) and its encoded protein, KLHL40, with the BTB-BACK domain and kelch repeats (lower). The localization of mutations and substitutions identified is depicted with dots, and the number of dots for each mutation or substitution indicates the number of times it was found. Most substitutions occurred at conserved amino acids. The dots above KLHL40 indicate truncating mutations, and those below KLHL40 indicate missense mutations. (B and C) Structural modeling of the missense KLHL40 substitutions. The crystal structures of the (B) kelch domain of KLHL40 and the (C) BTB-BACK domain of KLHL11 and the location of the substitutions are shown. p.Pro397Leu, p.His455Arg, p.Glu469Cys, p.Thr506Pro, p.Glu528Lys, p.Ala538Pro, and p.Glu588Lys map to the kelch repeats (B), p.Asp34His and p.Leu86Pro map to the BTB domain, and p.Val194Lys and p.Trp201Leu map to the BACK domain (C). The side chains of the mutated residues are shown as sticks with space-filling spheres in red. α helices, β sheets, and loops are drawn as ribbons, arrows, and threads, respectively. Each kelch repeat (B) is color coded in the kelch domain, and the BTB and BACK domains (C) are colored pink and green, respectively. Molecular structures were drawn with PyMOL. (D) The calculated free-energy changes resulting from the missense substitutions in the kelch domain of human KLHL40 and the BTB-BACK domain of human KLHL11 were predicted by FoldX. Data are presented as the mean ± SD. Residue numbers used in (C) and (D) refer to human KLHL11, and those corresponding to human KLHL40 are in parentheses.

Figure 3

KLHL40 Expression in Human and Mouse Tissues (A) Taqman quantitative real-time PCR analysis of cDNA from adult or fetal human tissues. Error bars represent the SD. The following abbreviation is used: Sk, skeletal. (B) KLHL40 levels in C2C12 cells and mouse tissues (HPA, top panel) and immunoblotting for sarcomeric α-actin (clone 5C5, middle panel) and GAPDH (lower panel). Lanes are as follows: myo, C212 myoblasts; D2, myotubes on day 2 of differentiation; D4, myotubes on day 4 of differentiation; D6, myotubes on day 6 of differentiation; Gastroc (left), C57BL/6 postnatal day 2 (d2) gastrocnemius; Gastroc (right), C57BL/6 8-week-old gastrocnemius; and EDL (extensor digitorum longus) to liver, C57BL/6 8-week-old tissues. For all mouse tissue lysates, samples were pooled from three different mice. (C) On the left is KLHL40 expression in human skeletal muscle (HPA, top panel), immunoblotting for α-actinin (clone EA-53, middle panel), and Coomassie staining of MHC band (bottom panel). Lanes are as follows: F:19w, 19-week-old fetus; F:23w, 23-week-old fetus; F:31w, 31-week-old fetus; 11d, 11-day-old neonate; 6mo, 6-month-old baby; and C1–C4, healthy adult controls of 19–42 years of age. On the right, KLHL40 intensity normalized to MHC for fetal muscle is 3.34 ± 0.92 (n = 3) versus 1.37 ± 0.21 (n = 6) for postnatal skeletal muscle. ^∗p = 0.023, unpaired two-tailed t test. Error bars represent the SEM. (D) Single Z-plane confocal microscopy showing localization of KLHL40 (green) and α-actinin (red) in a longitudinal section of skeletal muscle from a 31-week-old fetus; costaining with Hoechst (blue) is also shown (Merge). Scale bars represent 5 μm. (E) Immunoblotting shows that KLHL40 is absent in KLHL40-associated NEM muscle (II-1 from family 10 [F10] and V-2 from family 17 [F17]) compared with healthy control muscle (C1, C3, and C4). Coomassie staining of the MHC band (bottom panel) and immunoblotting for sarcomeric α-actin (clone 5C5, middle panel) indicate similar or greater loading for the KLHL40-associated NEM samples compared with control samples. (F) Immunofluorescence for KLHL40 in a human 23-week-old fetal skeletal muscle sample (F:23w), an adult healthy control (C4), and KLHL40-associated NEM muscle biopsies (II-1 from family 10 [F10], V-2 from family 17 [F17], family 21 [F21], and family 26 [F26]). Scale bars represent 50 μm.

Figure 4

Expression and Function of klhl40 in Zebrafish (A) In situ hybridization demonstrates that expression of both klhl40a and klhl40b is restricted to the skeletal muscle at 16 and 24 hpf. (B) Gross morphology of uninjected embryos (WT) and embryos injected with klhl40a-MO, klhl40b-MO, and klhl40a-MO/40b-MO. Lateral views of MO-injected embryos (4 ng) at 48 hpf are shown. Scale bars represent 500 μm. (C) Percentage of embryos categorized in phenotypic classes after injection with the 5mis-MO control, klhl40a-MO, klhl40b-MO, or klhl40a-MO/40b-MO. We categorized the phenotypes at 48 hpf into normal (normal appearance), mild (curved trunk), and severe (tail defect and severe development delay) (n = 111–130). (D) Knockdown of klhl40a, klhl40b, or both resulted in severe disruption of the skeletal muscle: fibers appeared wavy, and there were extensive gaps between fibers in contrast to the densely packed and aligned fibers of the controls. Maximum-intensity projection images from a confocal image series followed immunolabeling with a myosin antibody (F59, upper panels) at 36 hpf and F-actin (lower panels) at 72 hpf. (E) Embryos injected with 5mis-MO, klhl40a-MO, klhl40b-MO, or klhl40a-MO/40b-MO were categorized phenotypically on the basis of the presence of myofiber detachment affecting one to two somites (mild) or multiple (three or more) somites (severe) (n = 25–44). (F) Double-labeled immunofluorescence was performed on isolated myofibers from 72 hpf embryos with the use of phalloidin (green) and α-actinin (red). Frequent areas of aberrant α-actinin accumulation were detected in klhl40a-MO/40b-MO myofibers (arrowheads). (G) Electron microscopy of 72 hpf myofibers. A 5mis-MO-injected embryo shows correctly aligned sarcomeres and T-tubules (upper panel). A klhl40a-MO/40b-MO-injected embryo (lower panel) shows disarranged myofibrils with widened Z-disks (arrow), but thin filament lengths are unchanged. The scale bar represents 0.7 μm.

Figure 5

Correlation between the c.1582G>A (p.Glu528Lys) Mutation and Clinical Features The clinical characteristics of NEM are shown for the two groups of affected individuals (32 total), either with the c.1582G>A (p.Glu528Lys) mutation (as group A) or without it (as group G). The numbers of total affected individuals with clinical records regarding either the presence or the absence of each characteristic are indicated below the bars, and the numbers of affected individuals in each group are indicated above the respective bars. Labels on the x axis are as follows: prenatal symptoms, individuals demonstrating either fetal akinesia or hypokinesia, polyhydramnios, or fetal edema or effusion; ventilator required, individuals with respiratory failure requiring ventilation; artificial feeding, dysphagia-affected persons requiring tube feeding or gastrostomy; ophthalmoparesis, individuals with ophthalmoparesis along with facial weakness; muscle weakness, individuals with the most severe form of muscle weakness and demonstrating no antigravitory movement; and deceased, individuals who were deceased at the time of study. Asterisks indicate that statistical significance was observed.