Breed distribution and history of Breed distribution and history of canine canine mdr1-1∆mdr1-1∆, a pharmacogenetic , a pharmacogenetic mutation that marks the emergence mutation that marks the emergence

of breeds from the collie lineageof breeds from the collie lineage

Author: Neff, M.W. et.al.

Presented by Katheryn McDonald and Todd Mercier

IntroductionIntroduction Goal of study: To identify breeds at risk for

multi-drug sensitivity and level of susceptibility for each breed

Ivermectin• 1980’s, parasiticide• Causes a charge in the ligand-gated chloride

ion channels of the peripheral nervous system, resulting in an influx that paralyzes nematode and arthropod parasites

• Generally safe in domestic animals due to the blood-brain barrier

IntroductionIntroduction

Ivermectin• Key component of the blood-brain barrier is P-

glycoprotein– Large protein complex that pumps drugs out of the

brain and into bloodstream– Encoded by the multiple drug resistance gene

(MDR1)– Lack of P-glycoprotein may lead to neurotoxicosis

IntroductionIntroduction MDR1

• mdr1-1∆–Causes frameshift with multiple premature

stop codons, drastically shortening the P-glycoprotein

–Allele probably results in a complete loss of P-glycoprotein function

–First discovered in the collie breed

ObjectiveObjective

Determination of genetic linkage of the mdr1-1∆ allele in non-herding breeds to find other at risk canine populations

Determination of ancestral lineage in other breeds not previously thought to be related to the collie

MethodsMethods

Four classes of dogs were tested• Breeds from the collie lineage based on breed

history• European herding breeds not thought to be

related to the collie• Sighthounds and miscellaneous breeds that

exhibited drug sensitivities• Multibreed panel of over a thousand samples

from more than 90 breeds

MethodsMethods DNA samples taken from buccal cells

collected by oral swab

Genotyping for MDR1 accomplished with PCR (rapid duplication of DNA)

Genome location mapped for MDR1 using a canine/hamster radiation hybrid panel

MethodsMethods

Haplotype analysis• Loci were selected based on proximity to

MDR1• Primers were produced for 4 markers and

MDR1 for PCR• Haplotypes reconstructed with PHASE

program (assumes all haplotypes from a pop. were created in that population)

MethodsMethods Statistical analysis

• Deviation from Hardy-Weinberg equilibrium in each test was assessed by a χ2 test in each breed

• Allele age determined by linkage disequilibrium (LD) at mdr1-1∆

• Distance between mdr1-1∆ and marker loci was 1cM or greater, suggests mutation is a weaker force than recombination

• However, mutation may result in rare haplotypes

ResultsResults 4000 samples surveyed, the mdr1-1∆ allele

was present in nine breeds

Allele % Genotype %

Breed # of dogs mdr1-1∆ mdr1-1∆/ mdr1-1∆ mdr1-1∆/MDR1 MDR1/MDR1

Austr. Shepherd 178 16.6 1.7 29.8 68.5

Austr. Shepherd min. 56 25.9 3.6 44.6 51.8

Collie 263 54.6 31.2 46.8 22.0

English Sheepdog 91 7.1 0 14.3 85.7

Longhaired Whippet 89 41.6 15.7 51.7 32.6

McNab 35 17.1 2.8 28.6 68.6

Old English Sheepdog 151 3.6 0 7.3 92.7

Shetland Sheepdog 190 8.4 1.1 14.7 84.2

Silken Windhound 84 17.9 1.2 33.3 65.5

ResultsResults mdr1-1∆ present in predicted collie

breeds

mdr1-1∆ not found in herding breeds traced back to Europe

mdr1-1∆ also found in two non-herding breeds, the Longhaired Whippet and the Silken Windhound (sighthounds)

ResultsResults

High frequency of the mdr1-1∆ allele in the Collie and Longhaired Whippet breeds suggests a founder effect for those breeds

So, separate mutation or are they related?

ResultsResults Palindrome located 9 base pairs upstream of

mdr1-1∆ could have served as mutational hotspot~ suggest possible unrelated mutations

MDR1 tested with radiation hybrid analysis, revealed marker alleles strongly associated with mdr1-1∆ were identical at 3 of the 4 loci for herding breeds and sighthounds

Identical allele by descent~ all dogs carrying the mdr1-1∆ allele are descendants of a dog from Great Britain before the genetic isolation of breeds

DiscussionDiscussion

mdr1-1∆ was not present in the Border collie, Bearded collie or the Austr. Cattle dog, yet do exhibit ivermectin sensitivity

• Possible reasons – mdr1-1∆ present at a lower frequency – Another mutation is responsible

DiscussionDiscussion Within breeds, genotype frequency is

consistent with Hardy-Weinberg equil. No evidence linking mdr1-1∆ with

selection or non-random mating Allele differences most likely arose from

genetic drift and expansion of formal breeds since the 1800’s

Ultimately genetic drift and artificial selection have played a major role in genetic composition of breeds

Questions? Questions?