Canine-autosomal Recessive Muscular Dystrophy In Labrador Retrievers

Vol. 22, No. 2 February 2000

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FOCAL POINT ★ Autosomal recessive muscular dystrophy (ARMD) was identified in juvenile Labrador retrievers in the 1970s and is still prevalent among the breeding population in the United States.

KEY FACTS ■ Clinical signs of ARMD in Labrador retrievers typically occur between 4 and 6 months of age and may stabilize by 1 year. ■ ARMD is characterized by generalized weakness and severe muscle atrophy; hyporeflexia is a hallmark clinical sign. ■ Signalment, history, physical and neurologic examinations, and electromyography may support a diagnosis of ARMD; muscle biopsy is required for definitive diagnosis. ■ The pathophysiology of ARMD may represent a defect in the structural sarcoglycan proteins in muscle or may involve other enzyme dysfunctions of muscle. ■ There is no cure, efficacious treatment, or test to identify clinical carriers; relatives of affected puppies should not be used in breeding programs.

Autosomal Recessive Muscular Dystrophy in Labrador Retrievers University of Illinois

Lisa S. Klopp, DVM, MS Auburn University

Bruce F. Smith, VMD, PhD ABSTRACT: Autosomal recessive muscular dystrophy in Labrador retrievers was first reported in the mid-1970s. The method of inheritance follows a simple autosomal recessive pattern, but age at onset and severity of clinical signs may vary dramatically among affected littermates. A clinical diagnosis can be made from the history, signalment, and clinical signs, but a definitive diagnosis requires histopathologic evaluation of a muscle biopsy; histopathologic abnormalities typically occur as atrophy of type II muscle fibers. Although there is no cure or definitive treatment for this condition, the prognosis for animals with mild disease is favorable because clinical signs often stabilize by 1 year. To date, there is no clinical test to identify carrier dogs. Known genetic carriers should not be used in breeding programs.

A

n autosomal recessive hereditary myopathy in Labrador retrievers was first described by Kramer and colleagues in 1976.1 The disease was subsequently reported and studied by others in both the United States and the United Kingdom.2–8 It has been referred to in the literature by such names as hereditary myopathy in Labrador retrievers, muscular dystrophy in Labrador retrievers, type II muscle fiber deficiency, generalized muscle weakness, and myotonia. To date, autosomal recessive muscular dystrophy (ARMD) has been reported only in yellow- and black-coated Labrador retrievers of either sex.2,3 The disorder has been observed in field trial and champion show lines of Labrador retrievers.

CLINICAL SIGNS Onset of clinical signs usually occurs between 3 and 4 months of age (range, 6 weeks to 7 months).1–3,9–11 The age at onset and the severity of clinical signs can vary dramatically among affected littermates.3 Owners sometimes report that, in retrospect, an affected puppy seemed less active or playful compared with normal puppies of the same age.3 Weakness and exercise intolerance are insidious in onset and progress over the course of a few months.1,2,6,8,9,12 Generalized muscle atrophy, most prominently in the proximal limb (limb-girdle) muscles,1–3 is a predominant feature of the disease1,2,6,8,9,12 (Figure 1). Muscle atrophy may also be obvious in the paraspinal (cervical and thoracolumbar) and temporalis muscles.3

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Figure 1A

Figure 1B

Figure 1C

Figure 1D

Figure 1—A female Labrador retriever carrier of autosomal recessive muscular dystrophy (ARMD) and its three 5-month-old female puppies with ARMD. (A) The unaffected dam is well muscled and has no physical, electromyographic, or histopathologic evidence of ARMD. (B) The least clinically affected puppy; significant pathologic muscle changes and hyporeflexia are evident. (C and D) Severely affected siblings have limb-girdle atrophy, kyphosis, carpal hyperextension, and exercise intolerance.

Abnormal axial skeleton postures are often observed and include kyphosis of the thoracolumbar spine, low head carriage, and lordosis or kyphosis of the neck1,3,11 (Figure 1). Carpal hyperextension, carpus valgus, and adduction of the elbows are often observed in the thoracic limbs of severely affected dogs,3,11 but the joints do not seem to be painful.1,3 Postural abnormalities may reflect joint laxity caused by loss of muscle support. Clinical and radiographic evidence of orthopedic abnormalities have not been directly associated with ARMD.2 Affected dogs have a short-strided, stilted gait that can worsen markedly with exercise and cold weather1–3,11; animals may collapse if overworked.1–3,11 Moderate to severely affected animals may move the pelvic limbs in a “bunny-hopping” motion. 2,3,11 Adult dogs with ARMD occasionally develop megaesophagus.3,11

DIAGNOSIS Autosomal recessive muscular dystrophy is diagnosed based on signalment, history, and clinical signs. Other puppies from the same litter or puppies from relatives’ litters are often affected as well, which provides evidence that the disorder has a hereditary basis (Figure 1). Physical examination reveals a bright, alert, and responsive—although sometimes inactive—puppy. 3,11 Neurologic examination indicates severe muscle atrophy and generalized weakness. The disease is not associated with muscle pain1; muscle tone may be normal to decreased.3,11 Testing of postural reactions shows normal conscious proprioception.3 Wheelbarrowing and hopping reactions may be poorly performed by a weak animal,3,11 and the dog may be unable to lift its head during the wheelbarrow test.

KYPHOSIS ■ CARPAL HYPEREXTENSION ■ MUSCLE ATROPHY

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Hyporeflexia or areflexia is plex repetitive) discharges.3,4 a typical finding during myoTrue myotonic discharges are tatic reflex testing.3,7,10,11 Hyuncommon.4 The proximal poreflexia is a common cliniand distal thoracic limbs, cal feature of neuropathic proximal pelvic limb, and disease but, in myopathic cervical and thoracolumbar disease, is usually seen only paraspinal muscles as well as in conjunction with serious the muscles of mastication muscle atrophy and severely have been tested. 4 One of impaired ambulation. In the authors (L.K.) found fact, ARMD was once prospontaneous discharges in posed to be a neuropathic the intrinsic tongue muscles disease because of this conin two affected siblings evalfounding finding. 1–3,9,12,15,16 Figure 2A uated at Auburn University. The underlying defect reNerve conduction velocity sponsible for hyporeflexia is and repetitive nerve stimulanot known. It has been protion studies have revealed no posed that severe muscle atabnormalities.2,4 rophy may result in a loss of A definitive diagnosis is reflex activity; however, even obtained via muscle biopsy dogs with only mild atrophy and histopathologic evaluaand clinical signs can still be tion. Characteristic changes hyporeflexic.7,10 in muscle include fiber size The mediators or receptors variation and fiber type groupof the myotatic reflex are ing of both type I and type specialized muscle fibers II muscle fibers, prominent (muscle spindles) that are infiber atrophy, internalization nervated by type IA sensory Figure 2B of nuclei, fiber splitting, fibers and ϒ-motoneurons.13 fiber degeneration and reIt is possible that these spe- Figure 2—Adenosine triphosphatase staining (pH 10.2) of generation, and increases in cialized fibers are affected by muscle biopsies. (A) Biopsy from a normal Labrador retriever endomysial and perimysial the disease, but no studies showing the normal distribution of type I (light staining) and connective tissue and fat (fitype II (dark staining) fibers. (B) Biopsy from the puppy in have evaluated muscle spin- Figure 1D. Type II fibers are virtually nonexistent. brosis9; Figure 2). Similar to dles in ARMD. Results of the clinical abnormalities, the cranial nerve examinathese changes vary among tion are within normal limits, with the exception of temaffected animals and worsen over time.9 Changes in poralis muscle atrophy.2,3 Sensory and autonomic dysproximal limb muscles are typically more severe than functions do not occur.2 are changes in distal limb muscles.3,9 The most severe Serum creatine kinase levels can range from normal morphologic changes may occur in the temporalis musto severely elevated.2,3,7,11 Excessive activity can increase cle.5 Fiber size variation and grouping are typically sug1,3,11 Creatine kinase levels are not creatine kinase levels. gestive of neurogenic disease; however, these findings as persistently or severely elevated as they are in golden have also been observed in Becker-type and limb-girdle retrievers with Duchenne muscular dystrophy. 11,14 muscular dystrophies in humans.17–20 Histopathologic Hematologic and other serum biochemical abnormalifindings of internalized nuclei, disturbed architecture, ties are not found in ARMD.2,3 In two dogs (4 and 6 degeneration, regeneration, and fibrosis are usually asmonths of age) studied by one of the authors (L.K.), sociated with myopathic disorders.11,17,21 Normal fasciclactic acid and aspartate aminotransferase values were ular peripheral nerve and spinal cord evaluations have not elevated after exercise. not supported a neurogenic pathogenesis.3,5,9 Electrodiagnostic studies in dogs with ARMD reMyosin adenosine triphosphatase (ATPase) reactivity vealed abnormal spontaneous activity in affected adult under various acid–base conditions inactivates different and juvenile Labrador retrievers.2–4 Commonly reported ATPase systems and allows differentiation of type I, spontaneous discharges include fibrillation potentials, IIA, and IIC muscle fibers.22 Histochemical analysis of positive sharp waves, and bizarre high-frequency (comalkaline ATPase provides the clearest differentiation beHYPOREFLEXIA ■ MYOTATIC REFLEX ■ SERUM KINASE ■ HISTOPATHOLOGIC FINDINGS

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tween type I and II fibers 22; such analysis has shown that fiber atrophy predominantly involves type II fibers in all muscles except the cranial tibial muscle.2,3,5,12 Hemodynamics of the gracilis muscle have been studied in dogs with ARMD.6 Basic hemodynamic factors (e.g., precapillary, postcapillary, and venous pressures; total vascular resistance) were not statistically different from those of normal controls.6 In addition, changes in microvascular function with ischemia and reperfusion were similar in diseased and normal muscles.6 Figure 3—A young man with fascioscapulohumeral muscular dystrophy, a slowly progressive Biochemical analyses have been myopathy that affects particular muscles of the face, torso, and appendages. The disease is performed on the muscle tissue inherited as an autosomal dominant trait with 95% penetrance. (From Dubowitz V: Muscle from affected Labrador retriev- Disorders in Childhood. London, WB Saunders Co, 1995; with permission.) ers and normal dogs.5 Water, sodium, chloride, zinc, copper, were below normal.5 These changes are seen in other calcium, and neutral lipid contents were higher in most forms of hereditary myopathies (muscular dystrophy).5 muscle tissue from dogs with ARMD than in normal 5 Whether such imbalances are primary defects associatdogs. Concentrations of magnesium and potassium

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ed with the disease or secondary to abnormal membrane integrity is not known. Regardless, these lipids, electrolytes, and metals are important for metabolism, electrical function in the muscle cells, and integrity of the membrane. Derangement may lead to further injury of the muscle cells.5 Isoelectric-focusing protein gel electrophoresis has been performed on type I predominant (anconeus) and type II predominant (biceps femoris) muscles to compare healthy dogs with those that have ARMD.23 Affected dogs were shown to have less musclespecific protein and other protein bands compared with healthy controls,23 which provides evidence of a biochemical abnormality. In addition, erythrocyte membranes have been used to study dynamic and static measures of membrane fluidity as a measure of muscle lipid function and content.24 Erythrocyte membranes from affected dogs were shown to have altered fatty acid constituents and less fluidity than those of normal dogs.24

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Figure 4—Structural relationships of the dystrophin and sarcoglycan proteins

and their relationship to the muscle fiber cytoskeleton. Dystrophin proteins bind cytoskeletal actin units and the transmembrane component of dystroglycan proteins (β-subunits). The extracelluar component of dystroglycan (α-subunit) binds extracellular laminin. The sarcoglycan subunits (α, β, γ, and δ) form a transmembrane sarcoglycan complex. These protein complexes are important for muscle fiber membrane integrity and have been implicated in some forms of human muscular dystrophy.

PATHOGENESIS The biochemical cause of ARMD in Labrador retrievers is not known. Muscular dystrophies in humans are a heterogeneous group of genetic disorders characterized by progressive muscular weakness.17,21,25,26 Typical findings in humans are muscle fiber degeneration and necrosis.21 Various animal models have been used to study the pathogenesis of the human disorder21 (Figure 3), but whether animal muscular dystrophies are adequate models for the human disease has been questioned. Molecular research has, however, indicated similar genetic and protein defects in the disease in both animal and human models.21 Studies of limb-girdle muscular dystrophy in humans, to which ARMD has been compared, have found some shared characteristics,9 including fiber atrophy, angulated fibers, necrosis, regeneration, internalization of nuclei, fiber splitting, and increased endomysial and perimysial connective tissue.9,14,17 These characteristics are not specific to limb-girdle muscular dystrophies and are found in muscular dystrophies of different underlying defects as well as in endocrine and paraneoplastic myopathies.12,27–29 They most likely represent the fact that muscle has limited types of pathologic responses to disease. One study showed that muscle changes were more severe in the autosomal recessive dystrophies than in the autosomal dominant limb-girdle forms.17 Of interest, muscle fiber deficiency does not always occur in type II fibers as it does in ARMD of Labrador retrievers.17 The same study showed

that inflammatory infiltrates were present in about 25% of muscle biopsies from patients with autosomal recessive limb-girdle muscular dystrophy.17 This finding is not typical in either the human or Labrador form of the disease; however, it is sporadically found in both.9,17 Several different gene defects have recently been implicated in human limb-girdle muscular dystrophies.25,26 Among these are defects in a group of membrane proteins known as sarcoglycans.25 The sarcoglycans are a transmembrane component of the dystrophin–glycoprotein complex, which links the cytoskeleton to the extracellular matrix in adult muscle fibers.25,30 This complex is important for the integrity of muscle fiber membranes.25,26 Components of the dystrophin–glycoprotein complex include cytoplasmic proteins (e.g., dystrophin and syntrophins) and two transmembrane glycoprotein complexes25,26 (Figure 4). The transmembrane glycoprotein complexes include the dystroglycans (α and β) and the sarcoglycan complex (α, β, γ, and δ).25,26 Dystrophin, which is the underlying defective protein in Duchenne muscular dystrophy, binds cytoskeletal actin.25,26,31 The dystroglycans act as a transmembrane bridge by binding dystrophin intracellularly and laminin extracellularly.25,32 The actual functions of the sarcoglycan unit are not definitively known, but these proteins may mediate interaction of muscle cells with basal lamina. 25 Four

ELECTROPHORESIS ■ SARCOGLYCANS ■ DYSTROPHIN–GLYCOPROTEIN COMPLEX

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sarcoglycan subunits (α, β, γ, and δ) make up the sarcoglycan complex.21,25,26 These proteins are predominantly or exclusively expressed in striated skeletal muscle,25 and mutations have been found in all four genes in human forms of limb-girdle muscular dystrophy (types 2C to 2F).21,25,26,33–36 A unique ε-sarcoglycan that is broadly distributed in adult and embryonic muscle and nonmuscle tissue has been recently described.25 Its role in muscle development, integrity, and disease is not known. Additional genes involved in human limb-girdle muscular dystrophy types 1A and 2B have not yet been described. Finally, limb-girdle muscular dystrophy type 2A has been linked to a defect in calpain 3, a muscle-specific, calcium-activated neutral protease.36–38 Complete loss of calpain 3 has been shown in limb-girdle muscular dystrophy type 2A, which suggests an autosomal recessive method of inheritance.36 Unlike the proteins in the dystrophin–glycoprotein complex, calpain 3 is not a structural membrane protein, which suggests that the defect in the muscle results from enzymatic rather than structural dysfunction.36 Using immunohistochemical analysis, one of the authors (B.S.) and a colleague found dystrophin expression, which rules out a defect in the dystrophin gene in ARMD. They also showed, through such analysis in several of the sarcoglycans, that these proteins are present and correctly localized to the membrane. The presence of dystrophin and dystrophin-associated glycoproteins in the expected pericytoplasmic location is an indicator that the mutation does not occur in one of the dystrophin-associated glycoproteins. This observation and the specific loss of type II muscle fibers in dogs with ARMD suggest that calpain 3 may be a good candidate for study. Thus, current efforts at Auburn University to identify the mutation are focused on comparing the DNA sequence of normal canine calpain 3 to the DNA sequence from affected animals.

TREATMENT AND MANAGEMENT There is no treatment for ARMD. Clinical signs in affected dogs usually stabilize by 12 months of age.3,4,9,11 Clinical signs and dysfunction do not completely resolve.3 Affected dogs can often be acceptable house pets.3,11 Because disease severity varies, some owners believe that their pet has a poor quality of life. Cold, excessive exercise, and stress can exacerbate the disease; therefore, dogs affected with ARMD do not make good working dogs.3,4,11 Dogs with megaesophagus have a poor prognosis for survival because of the predisposition for aspiration pneumonia.3,11 Otherwise, the disease does not seem to affect an animal’s life span.3,11 Supportive care requires a warm environment in the

winter and low stress. Edrophonium chloride and diphenylhydantoin have been tried without success.4 Diazepam (10 mg orally twice per day) may help during exacerbations.4,11 Why this drug works is not entirely clear, but its effects probably result from relaxation of physiologically overloaded muscle fibers.

FUTURE To date, there is no clinical test to determine carriers. The muscular dystrophy of Labrador retrievers has an autosomal recessive inheritance pattern,1 and both the sire and dam must carry the defective gene for offspring to be affected. According to simple Mendelian genetics, approximately 25% of the puppies in the litter will be affected and 50% will be carriers. Parents and siblings of affected puppies should not be used for breeding, even if mated with noncarriers, because this propagates carriers. (In a carrier and noncarrier mating, 50% of the offspring will be carriers). This recommendation may be met with resistance from breeders who own a champion dog that is a carrier of the affected gene. However, even if previous litters have not been affected, carrier dogs should not be used in breeding programs. Although gene prevalence in the Labrador retriever population is difficult to estimate with current testing procedures, the length of time that this disease has been recognized, increased clinical recognition in recent years, and presence of the disease in some of the top breeding stock indicate that the gene is in fact widespread within this breed. The future availability of a genetic test for ARMD would allow carrier dogs to be identified and bred with normal animals under the provision that puppies are tested and only normal, noncarrier puppies be used for breeding lines. ACKNOWLEDGMENTS

The authors thank Dr. Kyle Braund for providing Figure 2 and Kerry Helms for providing Figure 4.

REFERENCES 1. Kramer JW, Hegreberg GA, Hamilton MJ: Inheritance of a neuromuscular disorder of Labrador retriever dogs. JAVMA 179:380–381, 1981. 2. Kramer JW, Hegreberg GA, Bryan GM, et al: A muscle disorder of Labrador retrievers characterized by deficiency of type II muscle fibers. JAVMA 169:817–820, 1976. 3. McKerrell RE, Braund KG: Hereditary myopathy in Labrador retrievers: Clinical variations. J Small Anim Pract 28: 479–489, 1987. 4. Moore MP, Reed SM, Hegreberg GA, et al: Electromyographic evaluation of adult Labrador retrievers with type-II muscle fiber deficiency. Am J Vet Res 48:1332–1336, 1987. 5. Mehta JR, Braund KG, McKerrell RE, Toivio-Kinnucan M: Analysis of muscle elements, water, and total lipids from healthy dogs and Labrador retrievers with hereditary muscular dystrophy. Am J Vet Res 50:640–644, 1989.

CALPAIN 3 ■ IMMUNOHISTOCHEMICAL ANALYSIS ■ MEGAESOPHAGUS

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6. Amann JF, Laughlin MH, Korthuis RJ: Muscle hemodynamics in hereditary myopathy of Labrador retrievers. Am J Vet Res 49:1127–1130, 1988. 7. Simpson ST, Braund KG, Sorjonen DC: Muscular dystrophy of Labrador retrievers. Proc 2nd ACVIM:78, 1982. 8. Hoskins JD, Root CR: Myopathy in a Labrador retriever. Vet Med Small Anim Clin 78:1387–1390, 1983. 9. McKerrell RE, Braund KG: Hereditary myopathy of Labrador retrievers: A morphologic study. Vet Pathol 23:411– 417, 1986. 10. McKerrell RE, Anderson JR, Herrtage ME, et al: Generalized muscle weakness in the Labrador retriever. Vet Rec 115:276, 1984. 11. Sharp NJH, Kornegay JN, Lane SB: The muscular dystrophies. Semin Vet Med Surg 4:133–140, 1989. 12. Cardinet GH III, Holliday TA: Neuromuscular diseases of domestic animals: A summary of muscle biopsies from 159 cases. Ann NY Acad Sci 317:290–313, 1979. 13. Amann JF: Congenital and acquired neuromuscular disease of young dogs and cats. Vet Clin North Am Small Anim Pract 17:617–639, 1987. 14. Dubowitz V: Histochemistry of muscle disease, in Walton J (ed): Disorders of Voluntary Muscle, ed 4. Edinburgh, Churchill Livingstone, 1981, p 284. 15. Bear MF, Connors BW, Paradiso MA: Neuroscience—Exploring the Brain. Baltimore, Williams & Wilkins, 1996, pp 362–369. 16. Kornegay JN: Golden retriever myopathy. Proc 4th ACVIM: 193–196, 1984. 17. van der Kooi AJ, Ginjaar HB, Busch HFM, et al: Limb girdle muscular dystrophy: A pathological and immunohistochemical reevaluation. Muscle Nerve 21:584–590, 1998. 18. Ringel SP, Carroll JE, Schold SC: The spectrum of mild X–linked recessive muscular dystrophy. Arch Neurol 34: 408–416, 1977. 19. Ten Houten R, De Visser M: Histopathological findings in Becker-type muscular dystrophy. Arch Neurol 41:729–733, 1984. 20. Bradley WG, Jones MZ, Mussini JM, Fawcett PR: Beckertype muscular dystrophy. Muscle Nerve 1:111–132, 1978. 21. Nonaka I: Animal models of muscular dystrophies. Lab Anim Sci 48:8–17, 1998. 22. Braund KG: Clinical Syndromes in Veterinary Neurology. St. Louis, Mosby, 1994, pp 396–397. 23. Mehta JR, Braund KG, McKerrell RE, Toivio-Kinnucan M: Isoelectric focusing under dissociating conditions for analysis of muscle protein from clinically normal dogs and Labrador retrievers with hereditary myopathy. Am J Vet Res 50:633– 639, 1989. 24. Mehta JR, Braund KG, Hegreberg GA, Thukral V: Lipid fluidity and comparison of the erythrocyte membrane from healthy dogs and Labrador retrievers with hereditary muscular dystrophy. Neurochemical Research 16:129–135, 1991.

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25. Ettinger AJ, Guoping F, Sanes JR: ∈-Sarcoglycan, a broadly expressed homologue of the gene mutated in limb-girdle muscular dystrophy 2D. J Clin Invest 272:32534–32538, 1997. 26. Straub V, Campbell KP: Muscular dystrophies and the dystrophin-glycoprotein complex. Curr Opin Neurol 10:168– 175, 1997. 27. Sorjonen DC, Braund KG, Hoff EJ: Paraplegia and subclinical neuromyopathy associated with a primary lung tumor in a dog. JAVMA 180:1209–1211, 1982. 28. Braund KG, Dillon AR, Mikeal RL: Experimental investigation of glucocorticoid-induced myopathy in the dog. Exp Neurol 68:50–71, 1980. 29. Braund KG, Dillon AR, Mikeal RL, August JR: Subclinical myopathy associated with hyperadrenocorticism in the dog. Vet Pathol 17:134–148, 1980. 30. Campbell KP: Three muscular dystrophies: Loss of cytoskeleton-extracellular matrix linkage. Cell 80:675–679, 1995. 31. Campbell KP, Kahl SD: Association of dystrophin and an integral membrane glycoprotein. Nature 338:259–262, 1989. 32. Henry MD, Campbell KP: Dystroglycan: An extracellular matrix receptor linked to the cytoskeleton. Curr Opin Cell Biol 8:625–631, 1996. 33. Roberds SL, Leturcq F, Allamand V, et al: Missense mutations in the adhalin gene linked to autosomal recessive muscular dystrophy. Cell 78:625–633, 1994. 34. Lim LE, Duclos F, Broux O, et al: β-Sarcoglycan: Characterization and role in limb-girdle muscular dystrophy linked to 4q12. Nature Genet 11:257–265, 1995. 35. Nigro V, de Sû Moreira E, Piluso G, et al: Autosomal recessive limb-girdle muscular dystrophy, LGMD2F, is caused by a mutation in the δ-sarcoglycan gene. Nature Genet 14: 195–198, 1996. 36. Richard I, Broux O, Allamand V, et al: Mutations in the proteolytic enzyme calpain 3 cause limb-girdle muscular dystrophy type 2A. Cell 81:27–40, 1995. 37. Tomè FMS, Evangelista T, Leclerc A, et al: Congenital muscular dystrophy with merosin deficiency. CR Acad Sci (Paris) 317:351–357, 1994. 38. Matsumara K, Nonaka I, Campbell KP: Abnormal expression of dystrophin-associated proteins in Fukuyama-type congenital muscular dystrophy. Lancet 341:521–522, 1993.

About the Authors Dr. Klopp, who is a Diplomate of the American College of Veterinary Internal Medicine (Neurology), is affiliated with the Department of Veterinary Clinical Medicine, College of Veterinary Medicine, University of Illinois, Urbana, Illinois. Dr. Smith is affiliated with the Scott-Ritchey Research Center, College of Veterinary Medicine, Auburn University, Alabama.