Transcript mdr1-1 - Salamander Genome Project

Breed distribution and history of
canine mdr1-1∆, a pharmacogenetic
mutation that marks the emergence
of breeds from the collie lineage
Author: Neff, M.W. et.al.
Presented by Katheryn McDonald
and Todd Mercier
Introduction
 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
Introduction
 Ivermectin
• Key component of the blood-brain barrier is Pglycoprotein
– 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
Introduction
 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
Objective
 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
Methods
 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
Methods
 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
Methods
 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)
Methods
 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
Results
 4000 samples surveyed, the mdr1-1∆ allele
was present in nine breeds
Allele %
Breed
Genotype %
# 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
Results
 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)
Results
 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?
Results
 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
Discussion
 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
Discussion
 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?