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Short Communication

Short Communication
Investigation of paralleland simultaneous
selection for collie eyeanomaly and ivermectin
toxicosis
K. Mizukami, A. Yabuki, D. Endoh,
H. S. Chang, K. W. Lee, M. Nakayama,
K. Kushida, M. Kohyama, J. I. Lee,O. Yamato
Collie eye anomaly (CEA) is an inherited canine ocular disordercausing regional choroidal hypoplasia and coloboma of the optic
disk or adjacent areas, and is associated with a 7.8-kilo base pair (bp)deletion in the canine NHEJ1 gene (Parker and others 2007). A 4-bp
deletion in the canine MDR1/ABCB1 gene (MDR1 gene defect) causes
neurotoxicosis after administration of P-glycoprotein substrates such as
ivermectin (Gramer and others 2011). These two mutations are common in collie-related and sighthound breeds, suggesting a possible parallel selection of the two defects, although these canine genes are located on different chromosomes: NHEJ1 on chromosome 37 and MDR1on chromosome 14 (Parker and others 2007, Gramer and ­others 2011).
Recently, a very high frequency of the CEA mutation has beenreported in the Hokkaido inu, a traditional Japanese breed; this is thefirst identification of a CEA-predisposed breed not classified as collies or sighthounds (Mizukami and others 2012a). A native Japanesebreed, the shiba inu, has been previously localised near collies in
multibreed cluster analyses (Parker and others 2007), suggesting thatcollies and such native Asian breeds share more genetic traits than
one may realise. However, prevalence of the MDR1 gene defect in theVeterinary Record (2014)
K. Mizukami, DVM, PhD,A. Yabuki, DVM, PhD,
H. S. Chang, DVM, PhD,K. Kushida, DVM,
M. Kohyama, DVM,
O. Yamato, DVM, PhD,
Laboratory of Clinical Pathology,
Department of Veterinary Medicine,
Joint Faculty of Veterinary Medicine,Kagoshima University, 1-21-24
Kohrimoto, Kagoshima 890-0065,
Japan
D. Endoh, DVM, PhD,
Department of Veterinary Radiology,
School of Veterinary Medicine, Rakuno
Gakuen University, 582 Midorimachi,
Bunkyodai, Ebetsu, Hokkaido 069-8501,
Japan
K. W. Lee, DVM, PhD,
Laboratory of Veterinary Internal
Medicine, College of Veterinary

doi: 10.1136/vr.102015
Medicine, Kyungpook National
University, 80 Daehakro, Bukgu, Daegu
702-701, Korea
M. Nakayama, DVM, PhD,
Nakayama Veterinary Hospital, 6-1
Minamifukuro, Nara, Nara 630-8342,
Japan
J. I. Lee, DVM, PhD,
Laboratory of Veterinary Public Health I,
College of Veterinary Medicine,
Chonnam National University, 300
Yonbongdong, Bukgu, Gwangju
500-757, Korea
E-mail for correspondence:
osam@vet.kagoshima-u.ac.jp
Provenance: not commissioned;
externally peer reviewed
Accepted May 26, 2014

Hokkaido inu is unknown. Furthermore, epidemiologic surveys targeting potential breeds likely to possess these two mutations have notbeen performed. This study investigated allele frequencies of these
multiple dog breeds and checked whether a parallel and simultaneous
selection for these two mutations has occurred.
In this study, 1331 DNA samples were collected from nine dog
breeds including two collie-related breeds (collie and border collie), onesighthound (Saluki), two non-collie European breeds (toy poodle andminiature dachshund), three Japanese breeds (Hokkaido, shiba, andRyukyu inu), and the Korean Jindo, via veterinary hospitals, a volunteer breeders association, and preservation associations. Although someof these samples belonged to the same kennels, nearly all of them wereunrelated to at least the grandparent level. These breeds were chosen for
the following reasons. Collie-related breeds are known to commonly
carry both mutations (Parker and others 2007, Gramer and others 2011).
The saluki is a sighthound that has not been demonstrated to carry themutations. The toy poodle and miniature dachshund possibly carry theCEA mutation, because sporadic cases of CEA-like ophthalmologicalsigns have occurred in these breeds (Parker and others 2007). Japanesebreeds are suspected to carry the mutations because the Hokkaido inu
has very high frequency of the CEA mutation (Mizukami and others 2012a). The Korean Jindo is also suspected to carry the mutations,because this breed has similar appearance and inherited traits to theJapanese breeds (Yamato and others 1999). The number of dogs in each
breed is shown in Table 1. The genotypes of the two mutations weredetermined using real-time PCR methods reported previously (Changand others 2010, Mizukami and others 2012b).
The results of this study are shown in Table 1. Collies had very
high frequency of both mutations. Border collies had high frequencyof the CEA mutation, but very low frequency of the MDR1 genedefect. Hokkaido inus did not show the MDR1 gene defect but displayed very high frequency of the CEA mutation. Ryukyu inus did
not show the CEA mutation, but 6 of 33 dogs carried the MDR1 genedefect. The other breeds did not carry either mutation. It should be
noted that the sample numbers of native breeds (e.g. Ryukyu inu andKorean Jindo) are low enough to allow for the possibility of misrepresenting the frequency, or entirely missing the presence, of thesemutant alleles. However, due to the rarity of these breeds even in Japanand Korea, these numbers are likely adequate to evaluate the frequencies of these alleles.
The results of the present study demonstrated that the CEA andMDR1 gene defects do not always coexist in canine breeds even if they
are predisposed toward one of these defects. This suggests that there isno parallel and simultaneous selection for the two defects, althoughdefinitive statistical analysis could not be performed because of thesmall number of breeds possessing one or both of the mutations. Collies
had markedly high frequencies of both mutations as previously reported (Parker and others 2007, Gramer and others 2011). Border collies also
had high frequency of the CEA mutation but very low frequency of theMDR1 gene defect, as first demonstrated in the same population previously (Mizukami and others 2012b). The saluki has long been considered an ‘ancient’ breed; however, a recent genetic study demonstrated
that ancient breeds do not resemble early domestic dogs more closely
but have rather avoided recent admixture with other breeds, likely facilitated by geographical and cultural isolation (Larson and others 2012).
Therefore, the reason why the saluki had neither of the mutations
might be that these mutations have not been introduced because of isolation in the course of establishment of this breed. The toy poodle andminiature dachshund had neither of the mutations, although sporadic
cases having ophthalmological lesions similar to those in CEA havebeen found in these breeds (Parker and others 2007). These lesions areobserved in non-CEA disorders including merle syndrome (Parker and­others 2007); therefore, disorders such as merle syndrome might cause
similar ocular lesions in these two breeds.
The results demonstrated negligible relationship between theCEA and MDR1 mutations in the Japanese and Korean breeds.
August 16–23, 2014 | Veterinary Record

Downloaded from veterinaryrecord.bmj.com on August 29, 2014 - Published by group.bmj.com

Short Communication
TABLE 1: The number of dogs with each genotype (carrier and affected) and allele frequency for the collie eye anomaly and MDR1 mutations
in various dog breeds
Collie eye anomaly

MDR1 gene defect

Breed

NumberCarrier

Affected

Frequency

Carrier

Affected

Frequency

CollieBorder collie
SalukiToy poodle
Miniature Dachshund
Hokkaido inu
Ryukyu inu
Shiba inu
Jindo

9437
29229
232
1833
208
136

0128
000
12000

9
9000
6000

10.1670000.667†
000320000
6006000000000.833
0.002*
00000.091
00*These data were reported previously and updated in this study (Mizukami and others 2012b)†These data were reported previously (Mizukami and others 2012a)

Hokkaido inus had very high frequency of the CEA mutation as previously reported (Mizukami and others 2012a), but no MDR1 genedefect, in this study. However, the pattern seen in the Hokkaido inu
breed was not observed in other Japanese and Korean breeds. Neither
mutation was found in the Japanese shiba inu and Korean Jindo dogs.
However, in the Ryukyu inu breed, 6 of 33 dogs had the MDR1 genedefect, although none carried the CEA mutation.
The MDR1 gene defect has also been found in Old English sheepdogs, German shepherd dogs and Wällers as well as collie-related dogsand sighthounds (Gramer and others 2011). There are several possible
origins of the MDR1 gene defect. All the dogs carrying this mutation are thought to be descendants of a dog that lived in Great Britain
before genetic isolation of pure breeds by registered breeding (Neffand others 2004). However, the ancestors of Ryukyu and Hokkaido
inus are believed to have been brought to the Japanese archipelago
from the Asian Continent by ancient people 10 to 12 thousand years
ago (Tanabe 2006). These two breeds have been isolated in southern(Okinawa, alias Ryukyu) and northern (Hokkaido) islands, respectively. In view of the postulated migration route and the isolated habitatsof these dogs, the origin of the CEA and MDR1 mutations might exist
in more primitive dogs than once thought. Further studies are needed
to verify this hypothesis.
In conclusion, no parallel and simultaneous selection was foundbetween the CEA and MDR1 mutations in various dog breeds. Thesetwo mutations existed independently in breeds that are considered
distinctly different: the collie and Japanese breeds, which supports theearlier data (Parker and others 2007) demonstrating that these breeds
are genetically close to one another. It is possible that these mutations
originated in ancient dogs and resultant phenotypes had neutral or
only weak effects on survival and selection. Veterinarians should beaware that predispositions toward these two defects differ considerably depending on breed.

Acknowledgements

The authors are grateful to Mr Yoshio Arakaki of the Ryukyu Inu
Hozonkai and the Health & Breeding Center for Ryukyu Inus forcollecting samples of Ryukyu inus. The authors are also grateful toMr Kawamichi of the Japan Border Collie Health Network for collecting samples of border collies. This study was supported financially bygrants (nos. 25292181, OY and 25-4919, KM) from the Ministry ofEducation, Culture, Sports, Science and Technology of Japan.

Veterinary Record | August 16–23, 2014

Correction notice This article has been corrected since it was published Online First.
In the title, ‘An investigation’ has been amended to ‘Investigation’ to match house style.

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