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. 2014 Aug 28;9(8):e106425.
doi: 10.1371/journal.pone.0106425. eCollection 2014.

A COLQ missense mutation in Labrador Retrievers having congenital myasthenic syndrome

Caitlin J Rinz  1 , Jonathan Levine  2 , Katie M Minor  3 , Hammon D Humphries  4 , Renee Lara  5 , Alison N Starr-Moss  1 , Ling T Guo  4 , D Colette Williams  6 , G Diane Shelton  4 , Leigh Anne Clark  1
Affiliations

A COLQ missense mutation in Labrador Retrievers having congenital myasthenic syndrome

Caitlin J Rinz et al. PLoS One. .

Abstract

Congenital myasthenic syndromes (CMSs) are heterogeneous neuromuscular disorders characterized by skeletal muscle weakness caused by disruption of signal transmission across the neuromuscular junction (NMJ). CMSs are rarely encountered in veterinary medicine, and causative mutations have only been identified in Old Danish Pointing Dogs and Brahman cattle to date. Herein, we characterize a novel CMS in 2 Labrador Retriever littermates with an early onset of marked generalized muscle weakness. Because the sire and dam share 2 recent common ancestors, CMS is likely the result of recessive alleles inherited identical by descent (IBD). Genome-wide SNP profiles generated from the Illumina HD array for 9 nuclear family members were used to determine genomic inheritance patterns in chromosomal regions encompassing 18 functional candidate genes. SNP haplotypes spanning 3 genes were consistent with autosomal recessive transmission, and microsatellite data showed that only the segment encompassing COLQ was inherited IBD. COLQ encodes the collagenous tail of acetylcholinesterase, the enzyme responsible for termination of signal transduction in the NMJ. Sequences from COLQ revealed a variant in exon 14 (c.1010T>C) that results in the substitution of a conserved amino acid (I337T) within the C-terminal domain. Both affected puppies were homozygous for this variant, and 16 relatives were heterozygous, while 288 unrelated Labrador Retrievers and 112 dogs of other breeds were wild-type. A recent study in which 2 human CMS patients were found to be homozygous for an identical COLQ mutation (c.1010T>C; I337T) provides further evidence that this mutation is pathogenic. This report describes the first COLQ mutation in canine CMS and demonstrates the utility of SNP profiles from nuclear family members for the identification of private mutations.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Multigenerational pedigree of Labrador Retrievers.
Filled individual icons denote affected dogs and semi-filled icons denote obligate carriers. Asterisks mark individuals selected for whole-genome SNP profiling.
Figure 2
Figure 2. Peroneal MNCV of an affected Labrador Retriever recorded at the extensor digitorum brevis muscle with stimulation at the level of the hock, stifle, and hip.
Although the CV was normal considering the dog’s age, the amplitude of the CMAP was uniformly diminished. The timebase between vertical columns on the x-axis is 2 msec and the voltage measured between adjacent rows on the y-axis is 2 mV. Control tracings are from the peroneal nerve of a healthy 5 month old Beagle. Note: labeling varies with the control amplitudes measured from baseline to peak, peak-to-peak amplitudes are within normal limits (≥8 mV). Latency 2 markings also vary.
Figure 3
Figure 3. Repetitive stimulation of the peroneal motor nerve of an affected Labrador Retriever at 2 Hz (A), 5 Hz (B), and 50 Hz (C).
Decrement of the CMAP was observed at all tested frequencies. Sweep speed and sensitivity settings are identical to those in Figure 2. Control tracings are from the peroneal nerve of a healthy 5 month old Beagle with no decrement seen at low frequency stimulation and normal pseudofaciliation (CMAP gets taller and narrower) with tetanic stimulation.
Figure 4
Figure 4. Cryosections (8 µm) of intercostal muscle from a normal dog, a Labrador Retriever with CMS (end-plate AChE deficiency), and a Jack Russell Terrier with CMS due to AChR deficiency (neuromuscular disease control) are illustrated.
For each dog, histochemical staining for esterase activity (brown stain) is shown along with a serial section demonstrating immunofluorescent localization of α-bungarotoxin for AChR and end-plate localization (red color). Muscle nuclei are blue (Dapi stain). There is a good correlation between esterase staining (brown) and α-bungarotoxin localization (red) in the control dog muscle. Although esterase staining is present in the Labrador Retriever muscle (arrows), the localization correlates poorly with that of AChRs. In the CMS Jack Russell Terrier esterase staining was present; however, staining for AChR was markedly decreased or absent, consistent with a markedly decreased AChR content. Bar = 50 µm for all images.
Figure 5
Figure 5. Microsatellite and SNP haplotypes (color-coded bars below individuals) are shown for 3 candidate genes.
Positions (in Mb) are according to CanFam 2. Filled individual icons denote affected dogs and semi-filled icons denote obligate carriers. Chromosome 23 haplotypes (blue) are inherited IBD in both affected dogs.
Figure 6
Figure 6. (A) Sequence from the 5′ end of the C-terminal domain of ColQ in mammals.
Identical residues are denoted by an asterisk, conserved substitutions by a colon, and semi-conserved substitutions by a period. Residues altered in human CMS cases are highlighted in yellow , , , . (B) BtsI digest results for the Labrador Retriever family. PCR amplicons from COLQ exon 14 are 470 bp in size and cleaved into 204 and 266 bp fragments in the presence of c.1010T>C. Three clinically normal littermates were identified as carriers, denoted by semi-filled icons.
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