Dyer, KR

Dyer, RM

Moore, GA

Sponenberg, DP

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GA Moore

KR Dyer

RM Dyer

DP Sponenberg

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Autosomal recessive myotonia congenita in sheep

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Autosomal recessive myotonia congenita  in sheep

Sponenberg DP

Dyer RM

Dyer KR

Moore GA

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Original article
Autosomal recessive myotonia congenita
in sheep
GA Moore KR Dyer2 RM Dyer DP Sponenberg4
1 
Department of Large Animal Clinical Sciences;
2 
Department of Small Animal Clinical Sciences;
4 
3 
Department of Clinical Sciences, Maryland; ’-
 Department of BioSciences and Pathobiology, Virginia-Maryland Regional College
of Veterinary Medicine, Virginia Polytechnic
Institute and State University, Blacksburg, VA 24061, USA
(Received 6 August 1996; accepted 10 March 1997)
Summary - Myotonia congenita in sheep causes episodes of muscular rigor following
various stimuli. Episodes are transitory and do not harm the animal. Incidence in the
flock of origin included three ewe lambs and three ram lambs. Two of the affected animals
resulted from the mating of one specific unaffected ram to the unaffected daughters of a
second unaffected ram. This mating produced 25 normal animals in addition to the two
myotonic animals, which is consistent with the hypothesis of an autosomal recessive gene
responsible for the condition, assuming the sire and maternal grandsire were the only
ancestors carrying the gene (x2 = 0.64, 1 df, P > 0.25). The four other affected animals
had these two introduced rams in their pedigrees on both maternal and paternal sides,
suggesting that these two rams might indeed have introduced the gene into this flock.
sheep / myotonia congenita / genetics
R,ésumé - Myotonie congénitale autosomale récessive chez le mouton. La myotonie
congénitale chez le mouton cause des épisodes de raideur musculaire suite à divers stimuli.
Ces épisodes sont passagers et ne sont pas nuisibles à l’animal. Elle concernait trois
agneaux mâles et trois agneaux femelles dans le troupeau d’origine. Deux des animaux
a,!‘’éctës résultaient de l’accouplement d’un bélier non affecté avec les filles non affectées
d’un second bélier non affecté. Cet accouplement a produit 25 animaux normaux en plus
des animaux myotoniques, ce qui est cohérent avec l’hypothèse d’un gène autosomal récessif
responsable du syndrome, en supposant que le père et le grand-père maternel étaient les
seuls ancêtres portant le gène (x2 = 0,64, 1 df, p > 0,25). Les quatre autres animaux
* 
Correspondence and reprints.
Correspondence to be sent to Dr DP Sponenberg, Veterinary College, Virginia Tech
Blacksburg, VA 24061.
affectés avaient ces deux béliers dans leurs pedigrees, tant du côté paternel que du côté
ntaterael, ce qui suggère que ces deux béliers pourraient avoir introduit le gène dans le
troupeau.
ovin / myotonie congénitale / génétique
INTRODUCTION
Myotonia congenita is a condition of skeletal muscles, which is characterized by
rigid contraction of the muscles, generally following a startling stimulus. Myotonia
congenita has been described in humans, goats and other species (Bryant, 1979;
Ptacek et al, 1993). Myotonia congenita is distinct from hyperkalemic periodic
paralysis, paramyotonia, as well as from myotonic dystrophy. Myotonia congenita
in humans has two major forms. One is due to an autosomal dominant gene and
the other is due to an autosomal recessive gene (Ptacek et al, 1993).
Myotonic animals are consistently heavily muscled, and this development of
muscle is related to the myotonia. The heavy muscling of myotonic animals may
offer some advantages to meat production systems. Myotonic sheep occurred in a
single flock, and flock records were studied to determine the mode of inheritance
within this flock.
MATERIALS AND METHODS
Myotonia congenita in sheep was detected in a flock that was used for the production
of show lambs. Two affected sheep from the flock were evaluated by routine blood
and urine analyses. These two sheep were also evaluated by electromyography.
Samples from the biceps femoris muscle were evaluated histologically following
routine processing for hematoxylin- and eosin-stained tissue sections. Flock records
of pedigrees, production data, and evaluation of lambs as affected and unaffected,
were evaluated for evidence of mode of genetic transmission of the myotonia
congenita.
RESULTS
Clinical investigation of two affected animals from the flock revealed normal com-
plete blood counts, normal biochemical profiles, and normal levels of creatine kinase.
Urine analysis was also normal. The only histological change was hypertrophy of
both type I and type II myofibers in the biceps femoris muscles.
Several muscles of each lamb were investigated electromyographically, and all
of these exhibited classic myotonic potentials (Kimura, 1983). The myotonic dis-
charges occurred as sustained runs of positive or negative waves up to 1 mV. These
ranged about 50-100 impulses per second, which produced a diminishing buzz over
the speaker of the electromyograph. This diminishing buzz is characteristic of my-
otonia congenita.
Evaluation of herd records revealed that myotonia congenita was only expressed
in lambs descended from two specific rams that were themselves unaffected, and
not from two other unaffected rams that were also used in the flock. The two rams
capable of producing affected lambs produced six affected lambs, three ram lambs
and three ewe lambs. Two of the affected lambs resulted from the mating of one
specific ram to the unaffected daughters of the other ram, this specific combination
also producing 25 unaffected lambs. The other four affected lambs were the progeny
of animals that were offspring or paternal grandoffspring of one of the rams, and
maternal grandoffspring of the other. No affected lambs were produced from any
mating that did not have at least one of these rams in both the maternal and
paternal portion of the pedigree. These two rams were both brought into the flock
from other sources, neither was born into the flock.
DISCUSSION
The clinical investigation was consistent with this disease being myotonia congenita.
The electromyography was consistent with this, and the hypertrophy of the skeletal
muscles was consistent with myotonia congenita and not with dystrophic diseases
characterized by myotonia.
If an allele is recessive and is newly introduced into a population by an
heterozygous male, then the mating of this heterozygous male to daughters of an
heterozygous male (himself or another) should produce a 1:7 ratio, which fits the
production of 25 unaffected and two affected lambs by one ram from daughters of
another (x2 = 0.64, 1 df, p > 0.25). This logic is correct only if the original flock
lacked animals heterozygous for this gene, in which case the original frequency of
the gene is zero. The pedigree information behing the four myotonic lambs that
were not produced by the specific sire x maternal grandsire matings are consistent
with the gene being introduced by the two rams and not existing in the flock before
their use.
The only instances of affected lambs in this flock followed matings in which
these two rams appeared in both the maternal and paternal pedigrees. In no other
instance were affected lambs produced, which is evidence against this gene existing
in the flock before these two rams were introduced. The overall production of three
ram and three ewe lambs affected with myotonia congenita further indicates a lack
of sex linkage for the gene responsible for this trait in sheep. The name Myotonia
congenita is suggested for the locus of this allele, with the symbol Mc. The allele
name myotonic is suggested for the recessive allele, with the symbol Mc7&dquo;!. This
allele was unfortunately not documented prior to the publication of Mendelian
Inheritance in Sheep (MIS 196) (Lauvergne et al, 1996), and the authors suggest
that it be included in the next catalogue with the proposed names and symbols.
Myotonia congenita in sheep is similar to that in goats (Bryant, 1979) and
humans (Ptaced et al, 1993). Clinical signs in the lambs were first noted at a
few weeks to a few months of age. The first clinical sign was that the affected
lambs had dirty fleeces. This is unusual in normal sheep since they rarely if ever
recline laterally or roll, which generally keeps fleeces clean and free of soil. The
affected lambs were then observed to exhibit extensor rigidity in the face of various
stimuli, as is common with myotonia congenita in other species. This rigidity can
occasionally cause them to fall over, and this accounts for the dirty fleeces as the
lambs begin to express the trait.
The affected lambs were subjectively considered by the flock owner to have
heavier muscling than the unaffected lambs. Further study of this aspect of myotonia
congenita was not studied since only affected lambs were donated for evaluation. If
further studies indicate that myotonic lambs are more heavily muscled than non-
myotonic lambs, then this allele might have applications in meat producing systems.
Another recently documented gene with large effect on muscling is the callipyge
gene (Jackson and Green, 1993), which unfortunately also results in toughness of
some portions of the carcass. The non-expression of the callipyge allele in animals
inheriting it from the dam also may limit its usefulness in production systems
(Cockett et al, 1996).
REFERENCES
Bryant SH (1979) Myotonia in the goat. Anns N Y Acad Sci 317, 314-325
Cockett NE, Jackson SP, Shay TL, Farnir F, Berghmans S, Snowder GD, Nielsen DM,
Georges M (1996) Polar overdominance at the ovine callipyge locus. Science 273, 236-
238
Jackson SP, Green RD (1993) Muscle trait inheritance, growth performance, and feed
efficiency of sheep exhibiting a muscle hypertrophy phenotype. Proc West Sect Amer
Soc Anim Sci 44, 364-366
Kimura J (1983) Electrodiagnosis in Diseases of Nerve and Muscle. Principles and
Practice. EA Davis Co, Philadelphia, 259-288
Lauvergne JJ, Dolling CHS, Renieri C (1996) Mendelian Inheritance in Sheep 1996 (MIS
96). COGOVICA/COGNOSAG and University of Camerino, Clamart, France and
Camerino, Italy
Ptacek LJ, Johnson KJ, Griggs RC (1993) Genetics and physiology of the myotonic muscle
disorders. N Engl J Med 328, 482-488


Myotonia in the goat.

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Autosomal recessive myotonia congenita in sheep

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