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© Schattauer 2012
Original Article
Hereditary polyneuropathy
in the Alaskan Malamute*
K. Rentmeister1; T. Bilzer3; S. Petri2; G. Schanen4; M. Fehr1; O. Distl2; A. Tipold1
1Department
of Small Animal Medicine and Surgery, University of Veterinary Medicine Hannover (Foundation), Hannover, Germany; 2Institute for
Animal Breeding and Genetics, University of Veterinary Medicine Hannover (Foundation), Hannover, Germany; 3Institute of Neuropathology,
Heinrich Heine University, Düsseldorf, Germany; 4Small Animal Clinic Elmer-Kornberg – Schanen, Trier, Germany
Key words
Schlüsselwörter
Dog, electrodiagnosis, muscle- and nerve biopsy, segregation analysis
Hund, Elektrodiagnostik, Muskel- und Nervbiopsie, Segregationsanalyse
Summary
Zusammenfassung
Objective: To prove the hypothesis that a polyneuropathy in Alaskan
Malamutes has a genetic background. Material and methods: Pedigrees of 131 related Alaskan Malamutes were included in the current
study. Neurological examination, electrodiagnosis as well as muscle
and nerve biopsies could be performed in 10 dogs. Information about
the disease status of the other 121 Alaskan Malamutes were supplied
by referring veterinarians, breeders and owners. Segregation analysis
using four different models (monogenic, polygenic, mixed monogenicpolygenic and the phenotypic model) was performed on 71 dogs to test
the different mechanisms of genetic transmission. Results: In seven
clinically affected dogs abnormal electromyographic findings and reduced nerve conduction velocity were detected. Suspected diagnosis
of polyneuropathy was confirmed by nerve biopsy results, characterized by axonal degeneration and hypomyelination. Muscle specimens
revealed signs of neurogenic myopathy. Three related clinically normal
Alaskan Malamutes also displayed moderate neuromuscular changes
in histopathology. In the segregation analysis the polygenic model
proved as best suitable to explain the observed segregation pattern
among all other models tested. Conclusion: The current study could
demonstrate that polyneuropathy in Alaskan Malamutes is a hereditary disease with variable phenotypic expression ranging from severely
affected to subclinical forms, which has to be considered in future gene
analysis studies.
Ziel der Untersuchung war die Überprüfung der Hypothese, dass die
Polyneuropathie des Alaskan Malamute eine erbliche Komponente hat.
Material und Methoden: Um diese Hypothese zu bestätigen, wurden
Abstammungsnachweise von 131 verwandten Alaskan Malamutes herangezogen. Bei 10 Hunden waren eine neurologische Untersuchung,
Elektrodiagnostik und Beurteilung von Nerven- und Muskelbioptaten
möglich. Weitere Informationen über den Gesundheitsstatus von 121
Alaskan Malamutes konnten über Haustierärzte, Züchter und Besitzer
eingeholt werden. Zur Testung der verschiedenen Mechanismen eines
Erbgangs wurde bei 71 Hunden eine Segregationsanalyse durchgeführt, die vier Modelle (monogen, polygen, gemischt monogen-polygen
und phänotypische Verteilung) beinhaltete. Ergebnisse: Bei sieben erkrankten Alaskan Malamutes ließen sich Spontanaktivität im Elektromyogramm und eine erniedrigte Nervenleitgeschwindigkeit nachweisen. Die histologische Untersuchung von Muskel- und Nervbioptaten
bestätigte die Verdachtsdiagnose einer Polyneuropathie. Diagnostiziert wurden eine neurogene Muskelatrophie, Axondegeneration und
Hypomyelinogenese. Drei verwandte, zum Zeitpunkt der Untersuchung
klinisch unauffällige Hunde wiesen histopathologisch ebenfalls geringgradige neuromuskuläre Veränderungen auf. Das polygene Modell der
Segregationsanalyse konnte das beobachtete Segregationsmuster am
besten erklären. Schlussfolgerung: Es ließ sich zeigen, dass es sich
bei der Polyneuropathie des Alaskan Malamute um eine genetische Erkrankung handelt. Der Phänotyp ist sehr variabel und reicht von Tetraparesen bis zu subklinischen Formen. Diese subklinischen Formen
müssen bei zukünftigen Genanalysen berücksichtigt werden.
Correspondence to
Kai Rentmeister
Tierärztliche Praxis für Neurologie
Mainfrankenpark 16b
97337 Dettelbach
E-Mail: [email protected]
Hereditäre Polyneuropathie beim Alaskan Malamute
Tierärztl Prax 2012; 40 (K): 26–34
Received: October 13, 2011
Accepted after revision: December 16, 2011
* Dedicated to the 60th birthday of Prof. Dr. Ingo Nolte.
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K. Rentmeister: Hereditary polyneuropathy in the Alaskan Malamute
Introduction
Clinical and neurological examination
Since the late 1970s various case reports about hereditary polyneuropathies in different canine breeds have been described such
as the giant axonal neuropathy in German shepherds (11, 12), the
progressive axonopathy in Boxers (16), the hypomyelinating polyneuropathy in the Golden retriever (3), the laryngeal paralysispolyneuropathy complex in Bouviers de Flandre (24), young Dalmatians (6), young Rottweilers (17) and Siberian Huskies (21), the
distal sensorimotor polyneuropathy in the Rottweiler (5), the hypertrophic neuropathy in the Tibetan terrier (8) and Mastiff (22),
the sensory neuropathies in the English pointer (9, 10) and Dachshund (13), and the distal symmetric polyneuropathy in the Great
Dane (2). A recent review was provided by Granger (15).
Also in Alaskan Malamutes a hereditary polyneuropathy has
been suspected. The first affected dogs were examined in Norway
between the years 1977 to 1979 by Moe et al. (18), Moe and Bjerkas
(19) and Moe (20). A total of 12 dogs could be traced back to one
common ancestor. Braund et al. (7) presented 11 cases of Alaskan
Malamutes with polyneuropathy from different litters. Although
the authors strongly suggested a genetic disease, several clinical,
electrodiagnostical and histopathological differences to the dogs
examined by Moe et al. (18) could be found. Using the term “idiopathic polyneuropathy in Alaskan Malamutes”, Braund et al. (7) distinguished this condition from the hereditary form previously
described in Norway.
The objective of the present study was to elucidate whether a
hereditary polyneuropathy exists in the Alaskan Malamute and to
substantiate the clinical findings by electrodiagnostical, histopathological examinations and segregation analyses.
General and neurological examination was performed on 10 Alaskan Malamutes (dogs 30, 31, 50, 51, 52, 53, 65, 66, 76 & the control
dog). Concerning the remaining littermates (dogs 54, 55, 56 &
57), the information about the clinical status is based upon telephone calls with the referring veterinarians, the breeders or the
owners.
Material and methods
Dogs
A total of 131 Alaskan Malamutes were included in the present
study (씰Fig. 1). Each dog received a number according to the information given by the breeders.
Initially, two Alaskan Malamutes were presented at the Department of Small Animal Medicine and Surgery, Veterinary School of
Hannover, Germany, with a history of abnormal vocalization characterized by hoarse barking since three months, progressively decreasing muscle tone, weakness and exercise intolerance (dogs 52
& 53). Their littermates (dogs 51, 54, 55, 56 & 57), their parents
(sire = dog 31, dam = dog 50) and four related dogs (dogs 30, 65, 66
& 76) were investigated. Another non-related and clinically healthy
Alaskan Malamute of the same breeder was added as control dog.
In collaboration with the breeders, a pedigree of 131 related dogs
could finally be established. The ancestors of the Alaskan Malamutes examined in Hannover, Germany, were directely related
with the Norwegian dogs (dogs 101–131) described by Moe et al.
(18) and Moe and Bjerkas (19).
Further examinations
Routine hematology and blood chemistry was performed on dogs
31, 50, 51, 52, 53 & 76. Additional examinations (T4, TSH, antiacetylcholine-receptor antibodies) and edrophonium chloride
testing (Tensilon®, Roche, 0.2 mg/kg BW) were performed on dog
52. Radiographs of thorax and abdomen were taken from dogs 31,
50, 51, 52 & 53.
For electrodiagnostic testing as well as muscle and nerve biopsies
all dogs received general anaesthesia, which was induced by intravenous injection of diazepam (1.0 mg/kg BW) and levomethadone
(0,6 mg/kg BW), and maintained by inhalation of isoflurane and
oxygen. Electrodiagnostic testing was performed using a Nicolet
Viking IV-D electrodiagnostic device (Würzburg-Höchberg, Germany). Electromyography (EMG) and motor nerve conduction
velocity (NCV) on common peroneus and radialis nerve was tested
on all 10 dogs previously undergoing neurological examination
(dogs 30, 31, 50, 51, 52, 53, 65, 66, 76 & the control dog).
Muscle and nerve biopsies of seven Alaskan Malamutes (dogs
30, 31, 50, 51, 52, 53 & 76) were taken at the proximal lateral stifle
level from the common peroneus nerve and the lateral head of the
gastrocnemius muscle according to established surgical techniques
(4). The biopsies were immediately stored in small plastic tubes
and cooled using gel freeze packs without adding any fixative. They
were transported to the Institute of Neuropathology of Heinrich
Heine University, Düsseldorf, by overnight express.
Muscle specimens were orientated in transverse and longitudinal directions, embedded in tragant, frozen in isopentane at
–135 °C, and stored in airtight plastic tubes at –80 °C. Cryostat
sections of muscle and nerve specimens of 6–10 μm were stained
with hematoxylin and eosin (H&E), and Gomori trichrome according to Engel; in addition, muscle specimens were treated with
reduced nicotinamide adenine dinucleotide tetrazolium reductase
(NADH-TR, pH 9.5), oil red O, myosine adenosine triphosphatase
(ATPase, pH 10.0 and 4.3), and acidic phosphatase. In parallel,
parts of the nerve specimens were fixed in a mixture of 2.5% formalin and 1% glutaraldehyde, buffered at pH 7.4, and processed
for paraffin-histology or embedded in Epon, respectively. Semithin transverse sections were stained with toluidine-blue.
Pedigrees
The relationship between all 131 Alaskan Malamutes is shown in
씰Figure 1. Information about the disease status of 121 Alaskan
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K. Rentmeister: Hereditary polyneuropathy in the Alaskan Malamute
Fig. 1 Pedigrees of 131 Alaskan Malamutes. A) complete pedigree, B) subpedigree-1, C) subpedigree-2.
■ = male dog, ● = female dog, ◊ = dog of unknown
sex, ■ = not examined, grey filling of the symbols:
clinically normal dog with positive histopathology,
black filling of the symbols: clinically affected dog with
positive electrodiagnostical and/or histopathological
examination.
Malamutes was supplied by referring veterinarians, breeders and
owners. This pedigree includes four nucleus families being paternal half sibs, which are connected to another affected pedigree
member (dog 96) via their sire. A direct relationship between the
four paternal half sib groups and the further affected members of
the pedigree could not be found. The litter with the dogs 51–57 was
inbred as well as the affected dogs 31, 50 and 76. Because information on the disease status of ancestors was often missing, the complex pedigree (씰Fig. 1A) was divided in two subpedigrees for
segregation analysis: subpedigree-1 (씰Fig. 1B) including a total of
29 dogs and subpedigree-2 (씰Fig. 1C) with a total of 42 dogs. Dogs
93–95 and 97–100 of subpedigree-1 were reported to perform well
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K. Rentmeister: Hereditary polyneuropathy in the Alaskan Malamute
on the sled team and although not examined were considered to be
clinically healthy.
In consequence, the segregation analysis included eight litters
with a total of 71 Alaskan Malamutes.
distributed. Degrees of freedom for the likelihood ratio test statistic are given by the difference of the number of independently
estimated parameters for the models compared.
Segregation analysis
Results
The mode of inheritance was tested by employing complex segregation analysis models developed by Bonney (1) and Elston (14).
The analysis was performed by using S.A.G.E., Version 3.0. The
joint likelihood function for each pedigree was maximized conditionally on the phenotypes of the probands (dog 52 in subpedigree-2 and dog 131 in subpedigree-1). Ascertainment correction
was applied as the pedigrees were non-randomly ascertained.
Clinically normal dogs with positive histopathology were coded
with “1” (grey coloured in 씰Fig. 1), clinically affected dogs with
positive histological or/and electromyographical findings were
coded with “2” (black coloured in 씰Fig. 1). Four different groups
of models were considered in the segregation analysis:
● Monogenic inheritance: This model assumes one gene locus
with two alleles in Hardy-Weinberg equilibrium. The allele effects can be recessive, dominant or arbitrary. The transmission
probabilities in this case are τAA = 1, τAB = 0.5 and τBB = 0. The
transmission parameter is the probability that a parent of genotype AA transmits allele A to offspring with genotype AA, AB or
BB. The transmission probabilities are used to construct the genotypic probability distributions of offspring in dependence on
the parental genotypes. Therefore, genotypes can be defined as
types that transmit to offspring in Mendelian fashion.
● Polygenic inheritance: The effects of parents on offspring were
regarded as logistic regressions. Three different submodels were
specified. Submodel 1 (type of familiar correlation = 2) included the effect of the affected parents and spouses, submodel
2 (type of familiar correlation = 4) the effect of unaffected parents and spouses as well as the effect of affected parents and
spouses. Submodel 3 (type of familiar correlation = 6) distinguished between affected sires, dams, and spouses as well as between unaffected sires, dams, and spouses.
● Mixed monogenic-polygenic inheritance (mixed major gene):
This model described the inheritance by using monogenic and
polygenic components as described above. The major gene effect was modelled as recessive, dominant or arbitrary effect in
each of the three submodels for polygenic inheritance.
● Only one phenotypic distribution (μ-model): In this model only
one phenotypic distribution with an arbitrary mean was used.
Results of neurological, electrodiagnostical and histological examinations are summarised in 씰Table 1.
Clinical and neurological examination
Clinical examination of all dogs was normal. At neurological
examination, all Alaskan Malamutes had normal consciousness,
behaviour and posture. All dogs were pacing except dog 76.
In dog 52, gait analysis revealed tetraparesis with exercise intolerance. The postural reactions were generally decreased. Cranial
nerve examination showed strong atrophy of the muscles innervated by the trigeminal nerve, and abnormal vocalization was
characterized by a hoarse bark. Spinal reflexes were absent or
markedly reduced. The muscle tone was reduced and a severe general muscle atrophy was noticed, especially on the hind limbs.
Superficial and deep pain seemed to be normal or only slightly depressed. Affection of the phrenic nerve was suggested because of
an abdominal type of respiration.
Dog 53 had similar, but less apparent signs. Dog 31 only displayed mild proprioceptive deficits in both front legs, dog 66 and
76 had slightly decreased postural reactions and spinal reflexes in
the rear (씰Table 1). The remaining Malamutes (dogs 30, 50, 51, 65
& the control dog) showed no neurological abnormalities.
Two owners informed us, that dogs 55 & 56 also displayed deficits in form of abnormal vocalization, exercise intolerance and
slight gait abnormalities, but refused any examinations.
According to the results of neurological examination, a generalised polyneuropathy was suspected.
Further examinations
Results of routine hematology and blood chemistry were normal.
Additional testing performed on dog 52 excluded hypothyroidism
(T4 = 1.5 μg/dl, TSH = 0.2 ng/ml) and ruled out myasthenia gravis by negative edrophonium chloride testing and absent antiacetylcholine-receptor antibodies. Plain radiographs of thorax
and abdomen (dogs 31, 50, 51, 52 & 53) were normal.
Electrodiagnostic findings
The different models were compared by using likelihood ratio
tests. A saturated model (most general model) was fitted to the data
and the likelihood of this model was used in the denominator of
the likelihood ratio test. In the numerator of the likelihood ratio
test the likelihood of the model being tested has to be nested in the
model of the denominator. Then, the natural logarithm of the likelihood ratio multiplied by minus two is asymptotically chi-square
Electromyographic examination (EMG) revealed positive sharp
waves in dogs 53 and 66 in interosseous, cranial tibial and quadriceps muscles. In addition, dog 53 had reduced insertion potentials in the muscles of the front legs. In dog 52, EMG was characterized by reduced insertion potentials, fibrillation potentials and
positive sharp waves.
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K. Rentmeister: Hereditary polyneuropathy in the Alaskan Malamute
Pedigree-Nr. Age
Sex
Neurologic signs
Electrodiagnosis
Histopathology
30
11y 10ms f
clinically normal
EMG: normal; NCV = 88
m/s
moderate degenerative
polyneuropathy
31 (sire)
6y 6ms
m
postural reactions reduced
in both frontlegs
EMG: normal; NCV = 62
m/s
moderate degenerative
polyneuropathy
50 (dam)
3y 2ms
f
clinically normal
EMG: normal; NCV = 73
m/s
marginal degenerative
polyneuropathy
51
1y 2ms
f
clinically normal
EMG: normal; NCV = 60
m/s
marginal degenerative
polyneuropathy
52
1y 2ms
m
marked tetraparesis, exercise intolerance, pacing, depressed proprioception and
absent spinal reflexes, general muscle atrophy including
muscles innervated by the
trigeminal nerve, abnormal
vocalization
EMG: decreased insertion severe degenerative
potentials, fibrillation po- polyneuropathy
tentials and positive sharp
waves; NCV: no evaluation
due to flat evoked potentials
53
1y 2ms
f
tetraparesis, proprioception
and spinal reflexes reduced,
abnormal vocalization
EMG: positive sharp waves, moderate degenerative
reduced insertion potenpolyneuropathy
tials, positive decremental
response; NCV = 29 m/s
55
1y 2ms
f
abnormal vocalization, exer- not performed
cise intolerance and slight
gait abnormalities*
not performed
56
1y 2ms
f
abnormal vocalization, exer- not performed
cise intolerance and slight
gait abnormalities*
not performed
65
2y 9ms
m
clinically normal
not performed
66
2y 10ms
f
proprioception and spinal re- EMG: positive sharp
flexes reduced in the rear
waves; NCV = 72 m/s
not performed
76
7ms
m
proprioception and spinal re- EMG: normal;
flexes reduced in the rear
NCV = 65 m/s
marginal degenerative
polyneuropathy
control
4y 11ms
m
clinically normal
not performed
EMG: normal;
NCV = 83 m/s
EMG: normal;
NCV = 81 m/s
Table 1
Neurological, electrodiagnostic and histopathological results in
12 related Alaskan Malamutes. EMG: electromyogram; NCV: nerve
conduction velocity;
f: female; m: male;
ms: months; y: years.
* information provided by local veterinarian
Motor nerve conduction velocity (NCV) from the peroneal or
radialis nerve (씰Table 1) ranged from 60 to 93 m/sec in dogs 30,
31, 50, 51, 65, 66, 76 & the control dog and was reduced in dog 53
(29 m/sec). In dog 52, NCV could not be determined because of
flat evoked potential curves. Dog 53 showed a slight decremental
response on repetitive stimulation of the peroneal nerve.
Histopathological findings
Dogs with neurological deficits and positive electrodiagnostic
findings had moderately to severely affected muscles. Pathological
bimodal fibre size variation was the most prominent finding in
dogs 30 (moderate), 31 (moderate), 52 (severe) and 53 (moderate)
characterised by scattered angular atrophic as well as hypertrophic
fibres (씰Fig. 2A). Both fibre types were involved, whereby most of
the hypertrophic ones belonged to type 1, and most of the atrophic
fibres to type 2 (씰Fig. 2B). Occasionally there was evidence of fibre
type grouping. Some of the hypertrophic fibres had a slightly dystrophic shape. Myofibrils seemed to be normal, and no vacuoles or
storage material could be found. Internal nuclei were rare. Fibresplitting as well as clusters of myogenic nuclei as sign of regeneration occurred. Additionally, fibre degeneration and focal necrosis
with phagocytosis (씰Fig. 2C) was observed. There was no evidence of inflammation in any of the specimens. Muscles of three
clinically normal dogs (50, 51 & 76) were also affected: they had a
slightly altered fibre calibre spectrum with individually disseminated – mostly angular – fibre atrophies.
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K. Rentmeister: Hereditary polyneuropathy in the Alaskan Malamute
In endomysial nerve fascicles (씰Fig. 3A), as well as in peripheral
nerve cross sections (씰Fig. 3B) of dogs 31, 52 & 53, alterations of
the myelin-axon-relation were present. This was characterized
either by axonal swelling, degeneration and/or demyelination,
complete loss of myelinated fibres, Schwann cell proliferation, infiltrating macrophages and mild to moderate endoneural fibrosis.
Signs of remyelination were reflected by very thin myelin sheaths,
being inappropriate for the axonal diameter. In dogs 30 and 52, evidence of axonal necrosis was visible by myelin ovoids and balls
(씰Fig. 3C). In the clinically normal dogs 50, 51 & 76, several degenerated and hypomyelinated axons in peripheral nerve fascicles
were found.
Pedigrees
The prevalence of polyneuropathy among the analysed families
was 29.6%. In total 21 affected animals could be either examined in
our clinic or by the referring veterinarian. These puppies displayed
weakness and reduced spinal reflexes. Among the affected puppies
three were males, nine were females, and in nine affected animals
the sex was unknown. The differences in prevalence of polyneuropathy among sexes were not significant (p = 0.153).
All families with affected puppies were related to each other.
Inbreeding coefficients among litters varied from 0% to 20.0%.
However, no significant differences in prevalences for polyneuropathy due to the inbreeding coefficient could be observed
(p = 0.11).
Segregation analysis
The model with only one phenotypic distribution was compared
with the monogenic, polygenic and mixed major gene models.
The Mendelian models and mixed major gene models were not
significantly different from the environmental model (μ-model,
씰Table 2). The polygenic model with the logistic regression of offspring on affected parents and spouses explained the segregation
significantly better than the environmental model. The further
likelihood ratio tests compared the polygenic model with the Mendelian and mixed major gene models. The polygenic model fitted
the data significantly better than the Mendelian models applied
(씰Table 3). Also, the mixed major gene model could not improve
the estimates of the polygenic model significantly, as was shown by
the likelihood ratio test between these models. The likelihood ratio
test of the polygenic model against the most general model was not
significant indicating that the polygenic model fitted the data sufficiently well (씰Table 4).
Summarising the results of the segregation analysis the polygenic model proved as best suitable to explain the observed segregation pattern in the available pedigree. Thus, the hypothesis of a
strong genetic background for polyneuropathy in this pedigree of
Alaskan Malamutes was supported by the segregation analyses performed. Allowing the transmission probability for τAB to vary in
the mixed major gene model with arbitrary gene effects led to a
Fig. 2 A) Cross-section of the quadriceps muscle of dog 53 showing bimodal fibre size variation with atrophic fibres – predominately angular (arrows)
as well as large fibres (hematoxylin-eosin, cryostat-section, ×400). B) Fibre
size variations with hypertrophic type 1 muscle fibres (blue) and atrophic
type 2 muscle fibres (brown). Normal myofibrils, no vacuoles or storage
materials (NADH-TR, pH 9.5; cryostat-section, ×400). C) Occasional fibre
degeneration and focal necrosis with phagocytosis (arrows), no evidence of
inflammation (Gomori-trichome/Engel, cryostat-section, ×400).
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K. Rentmeister: Hereditary polyneuropathy in the Alaskan Malamute
Fig. 3 A) Intramuscular nerve fascicle of the quadriceps muscle of dog 52
with varying axonal diameters, partial demyelination and beginning endoneural fibrosis (Gomori-trichome/Engel, cryostat section, ×800). B) Branch of
the ischiadic nerve of dog 52, with severe axonal degeneration and demyelination (paraffin section, hematoxylin-eosin, ×600). C) Myelin ovoids and
balls within the same nerve branch (paraffin-section, hematoxylin-eosin,
×300).
value of 15.66 for the –2log-likelihood with an estimate for
τAB = 0.91. This result also showed that mixed major gene models with transmission probabilities for Mendelian inheritance
could not be fitted to this data set. A similar result was also
found for monogenic models, when τAB was not fixed to a value
of 0.5. The reason, why the monogenic and mixed monogenicpolygenic inheritance models could not sufficiently well explain
the data was that the distributions of types in the pedigrees deviated significantly from genotypic distributions for Mendelian
inheritance.
Discussion
The purpose of this prospective study was to evaluate, if the polyneuropathy found in the Alaskan Malamute breed has a genetic
background.
In dogs 52 and 53, a sensorimotor polyneuropathy with severe
clinical expression was suspected after neurological examination.
Some of their relatives also had neurological deficits corresponding to a peripheral nerve lesion. However, these dogs were only
moderately or subclinically affected, implying a polyneuropathy
characterized by a wide range of clinical expressions. The observed
neurological deficits (i. e. tetraparesis progressing to tetraplegia,
exercise intolerance, inability to jump and walk up stairs, proprioceptive deficits, general hyporeflexia and severe muscle atrophy)
were almost identical to those reported by Braund et al. (7). In
contrast to Moe et al. (18) and Moe and Bjerkas (19) the Malamutes participating in our study displayed no signs of visceral
nerve impairment (coughing, dyspnoea, regurgitation and megaoesophagus) except abnormal vocalization.
Electrodiagnostic testing had an excellent correlation to the
clinical severeness of neurological deficits, however in contrast
to histopathology electrodiagnostic work up was not sensitive
enough to detect subclinical stages. This can be explained by the
fact, that moderate demyelination and slight axonal degeneration
does not necessarily influence NCV if a sufficient number of large
and fast conducting nerve fibres are still present (23).
The histopathological changes resemble those described both
by Moe et al. (18) and Braund et al. (7). According to the findings
of Braund et al. (7), the peripheral neuropathy observed in our
dogs is a primary axonopathy, characterised by axon swelling and
degeneration, loss of myelinated nerve fibres as well as demyelination and remyelination in parallel. Neurogenic muscular
atrophy of varying degree was present in all dogs. A new finding,
however, is that the muscle fibre calibre spectrum was slightly to
moderately irregular in three clinically normal Alaskan Malamutes. These dogs with slight but distinctive neurogenic myopathy
and axonal changes are considered to be subclinically affected, an
aspect we were able to describe for the first time in hereditary poly-
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K. Rentmeister: Hereditary polyneuropathy in the Alaskan Malamute
neuropathy in the Alaskan Malamute. In correlation with Braund
et al. (7), histopathology revealed no signs of metabolic/toxic or
immunopathological disease. Interestingly, at least two of our affected dogs were older than the previously reported (7) age of onset
Table 2
Comparisons between
the μ-model and the
other models under hypothesis using regressive
logistic models for complex segregation analysis
of polyneuropathy in
Alaskan Malamutes.
for polyneuropathy in Alaskan Malamutes (10–18 months): their
age was 6.5 years (dog 31) and almost 12 years (dog 30). Due to
their minor neuromuscular changes these dogs probably were able
to functionally compensate their subclinical disease.
–2 log likelihood χ2
df-χ2
P
1
34.28
–
–
–
5
26.34
7.94
4
0.094
Mendelian recessive
–
5
27.50
6.78
4
0.148
Mendelian arbitrary
–
7
26.69
7.59
6
0.270
polygenic model
2
6
22.33
11.95
5
0.036
4
8
22.33
11.95
7
0.102
6
8
22.33
11.95
7
0.102
2
9
20.35
13.93
8
0.084
4
11
20.35
13.93
10
0.176
6
11
20.35
13.93
10
0.176
2
9
21.72
12.56
8
0.128
4
11
21.72
12.56
10
0.249
6
11
21.72
12.56
10
0.249
2
11
22.59
11.69
10
0.306
4
13
21.50
12.78
12
0.385
6
13
21.50
12.78
12
0.385
Tested hypothesis
Familiar
correlation
μ-model
–
Mendelian dominant
mixed model with
dominant major gene
mixed model with
recessive major gene
mixed model with
arbitrary major gene
df
–
χ : difference of –2log likelihood between the model under hypothesis and the μ-model.
df-χ2: difference of the degrees of freedom between the model under hypothesis and the μ-model.
2
Table 3
Comparisons between
the polygenic model and
the other models under
hypothesis using regressive logistic models
for complex segregation
analysis of polyneuropathy in Alaskan Malamutes.
–2 log likelihood
χ2
df-χ2
P
6
22.33
–
–
–
–
5
26.34
4.01
1
0.045
Mendelian recessive
–
5
27.50
5.17
1
0.023
Mendelian arbitrary
–
7
26.69
4.36
1
0.042
mixed model with
dominant major gene
2
9
20.35
1.98
3
0.576
4
11
20.35
1.98
5
0.852
6
11
20.35
1.98
5
0.852
2
9
21.72
0.61
3
0.894
4
11
21.72
0.61
5
0.988
6
11
21.72
0.61
5
0.988
2
11
22.59
0.26
5
0.995
4
13
21.50
0.83
7
0.997
6
13
21.50
0.83
7
0.997
Tested hypothesis
Familiar
correlation
polygenic model
2
Mendelian dominant
mixed model with
recessive major gene
mixed model with
arbitrary major gene
df
χ : difference of –2log likelihood between the model under hypothesis and the polygenic model.
df-χ2: difference of the degrees of freedom between the model under hypothesis and the polygenic model.
2
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33
34
K. Rentmeister: Hereditary polyneuropathy in the Alaskan Malamute
Table 4 Comparisons between the most general model and the polygenic
model using regressive logistic models for complex segregation analysis of
polyneuropathy in Alaskan Malamutes.
Tested
Familiar
df
hypothesis correlation
most general 6
model
polygenic
model
–2 log like- χ2
lihood
–
df-χ2 P
17
8.77
–
–
2
6
22.33
13.56 11
0.258
4
8
22.33
13.56
9
0.139
6
8
22.33
13.56
9
0.139
χ: difference of –2log likelihood between the model under hypothesis
and the most general model.
df-χ2: difference of the degrees of freedom between the model under
hypothesis and the most general model.
The present study could prove that the Alaskan Malamute polyneuropathy definitely has a hereditary nature. The hypothesis of
a monogenic recessive inheritance for this polyneuropathy as
suggested by Moe et al. (18) and Moe and Bjerkas (19) had to be
rejected, since the distribution of affected dogs did not fit to the
Mendelian ratios. This could be shown by two different tests in our
genetic analysis. The most probable hypothesis that was accepted
by the models used was a polygenic transmission from parents to
offspring. This polygenic mode of inheritance may also explain the
various phenotypic clinical expressions ranging from severe tetraplegia to subclinical stages. The number of genes responsible cannot be determined using complex segregation analyses as the
number of possible models increases exponentially with the
number of gene loci and alleles considered. A mixed model with a
dominant major gene could not completely be ruled out, since
some of the ancestors with unknown disease status could have
been subclinically affected. Subclinical affected animals, which
were not examined histopathologically, might be the cause that the
mixed model could not be rejected and are a limitation to the
study. Further investigations are warranted to clarify the genetic
model and to identify possible mutations causing the Alaskan
Malamute polyneuropathy.
Conflict of interest
The authors confirm that they do not have any conflict of interest.
Conclusion for practice
The current study could demonstrate that polyneuropathy in Alaskan
Malamutes is a hereditary disease with variable phenotypic expression ranging from severely affected (weakness, tetraparesis, abnormal vocalization, abnormal performance as a sledge dog) to subclinical forms. Especially these subclinical forms have to be considered for breeding programs and future gene analysis studies trying to
detect possible gene mutations.
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