Recurrent Muscle Weakness with Rhabdomyolysis, Metabolic Crises, and Cardiac Arrhythmia Due to Bi-allelic TANGO2 Mutations

Abstract

The underlying genetic etiology of rhabdomyolysis remains elusive in a significant fraction of individuals presenting with recurrent metabolic crises and muscle weakness. Using exome sequencing, we identified bi-allelic mutations in TANGO2 encoding transport and Golgi organization 2 homolog (Drosophila) in 12 subjects with episodic rhabdomyolysis, hypoglycemia, hyperammonemia, and susceptibility to life-threatening cardiac tachyarrhythmias. A recurrent homozygous c.460G>A (p.Gly154Arg) mutation was found in four unrelated individuals of Hispanic/Latino origin, and a homozygous ∼34 kb deletion affecting exons 3-9 was observed in two families of European ancestry. One individual of mixed Hispanic/European descent was found to be compound heterozygous for c.460G>A (p.Gly154Arg) and the deletion of exons 3-9. Additionally, a homozygous exons 4-6 deletion was identified in a consanguineous Middle Eastern Arab family. No homozygotes have been reported for these changes in control databases. Fibroblasts derived from a subject with the recurrent c.460G>A (p.Gly154Arg) mutation showed evidence of increased endoplasmic reticulum stress and a reduction in Golgi volume density in comparison to control. Our results show that the c.460G>A (p.Gly154Arg) mutation and the exons 3-9 heterozygous deletion in TANGO2 are recurrent pathogenic alleles present in the Latino/Hispanic and European populations, respectively, causing considerable morbidity in the homozygotes in these populations.

Figure 1

Nine Pedigrees with TANGO2 Mutations (A) Chromatograms are shown for families of Hispanic ancestry with the recurrent c.460G>A (p.Gly154Arg) mutation and c.605+1G>A mutation. (B) Early childhood photos of subjects 1 and 3. (C) Long-range PCR data confirm the exons 3–9 deletion in carrier parents and in subjects of European descent. Data are also shown for the consanguineous family 9 of Middle Eastern origin, having a smaller exons 4–6 deletion. In both gel pictures, the upper band represents the presence of the breakpoint junction for the deletion and the lower band (1 kb) represents the non-deleted allele. (D) Breakpoint junction sequences for the exons 3–9 deletion and the exons 4–6 deletion are depicted under each gel picture, respectively. The breakpoint junction sequence is aligned with reference sequences with the red/blue color change indicating transition between proximal and distal reference sequences.

Figure 2

TANGO2-Specific CNV Detection from the Clinical Exome Data and the Spectrum of Mutations in Nine Unrelated Families (A) Target Z-score of PCA-normalized read depths for exon targets of TANGO2. Samples with heterozygous or homozygous deletions are shown in various colors on the left and right, respectively. All samples with deletions are depicted with read depths (RD) well below the other samples (gray) (n = 4,334 total samples). Samples with homozygous deletions have RD lower than heterozygous deletions and contain exons with reads per kilobase of transcript per million mapped reads (RPKM) values equal or near 0. Individuals with exons 3–9 homozygous deletions (∼34 kb) and heterozygous deletions are shown in this figure, as subjects 2, 7, and 8. Subject 11 with the smaller exons 4–6 homozygous deletion is represented in red. (B) TANGO2 transcript (GenBank: NM_152906.5) with mutations and deletions found in our study cohort. Light blue regions represent the coding exons, dark blue regions represent 5′ and 3′ untranslated regions, and black bars represent introns (not to scale). The locations of c.460G>A (p.Gly154Arg) and c.605+1G>A variants are shown. The ∼34 kb deletion that encompasses exons 3 through 9 and the ∼9 kb deletion that includes exons 4 through 6 are shown.

Figure 3

c.460G>A (p.Gly154Arg) TANGO2 Mutant Cells Show Decreased ER and Golgi Area and Increased ER Stress (A) Fibroblasts from healthy control and subject 5 were intracellularly stained with wheat germ agglutinin (WGA), a marker of trans Golgi and anti-TANGO2. The volumes of the resulting structures of both WGA (34.4 ± 7.5 μm³ p.Gly154Arg, 246 ± 60 μm³ control, p < 0.01) and TANGO2 (34.9 ± 7.8 μm³ p.Gly154Arg, 294 ± 55 μm³ control, p < 0.01) were found to be significantly lower in the subject fibroblasts by Student’s t test. (B) BLCLs from healthy control and subject 4 were intracellularly stained with WGA and anti-GM130. The area of the resulting structures was found to be significantly smaller in affected BLCLs than from healthy controls (Student’s t test p < 0.0001) for both WGA and GM130. (C) Fibroblasts from healthy control and subject 5 were intracellularly stained with ER Tracker red and anti-BiP and treated either with DMSO or Brefeldin A (BFA), an inhibitor of the ER Ca²⁺ ATPase. The volume of the resulting structure of the ER was found to be lower in the subject fibroblasts by Student’s t test (14.4 ± 1.2 μm² p.Gly154Arg, 65 ± 3.5 μm² control, p < 0.0001), whereas the level of BiP within the ER was significantly increased when normalized to ER after treatment with brefeldin A (1.5 ± 0.4 μm² p.Gly154Arg, 0.4 ± 0.05 μm² control, p < 0.0001).