1. Diseases of Skeletal Muscle December 9 th , 2008

• Describe the morphologic effects of denervation (and reinnervation) on skeletal muscle.

• Apply the reading frame hypothesis to the pathogenesis of dystrophinopathies.

• Discuss the limitations of routine muscle biopsy in the diagnosis of:

• • Muscular dystrophies

• • Mitochondrial diseases

• Describe the pathologic features that distinguish dermatomyositis and inclusion body myositis from polymyositis.

2. Skeletal muscle: Histology

• Multinucleated syncitia

• Peripheral nuclei

• Minimal endomysial fibrous tissue

• Admixture of fiber types

3. Neurogenic or Myopathic?

• Neurogenic

• bimodal size distribution

• angulated fibers

• apparent increase in nuclei

• no necrosis, regeneration, or fibrosis*

• Myopathic

• random size variation

• round fibers

• centralization of nuclei

• necrosis, regeneration, +/- fibrosis, inflammation

*except in longstanding denervation and in elderly patients

4. In elderly patients with weakness, mildly elevated CPK does not equal (primary) myopathy!

5. Neurogenic V. Myopathic

6.

• Grouped atrophy involving both type 1 and type 2 fibers

• Fiber type grouping (secondary to reinnervation)

• Formation of target fibers

7. Endomysial fibrosis = Muscular dystrophy

8. Dystrophin Dystrophin Actin -Largest known gene, by far (the average gene consists of 3000 bases; the dystrophin gene comprises 2.4 million) -Short arm of X chromosome

9.

10. Duchenne muscular dystrophy

• Early childhood weakness

• Gower sign

• Calf hypertrophy

• Wheelchair dependence by age 12

• Death from cardiomyopathy with conduction defects, respiratory weakness, & pneumonia

11. Duchenne muscular dystrophy Dystrophin Actin

12. Duchenne muscular dystrophy Actin

13. Duchenne muscular dystrophy Actin

14. Duchenne muscular dystrophy Actin

15. Dystrophin: Duchenne muscular dystrophy Revertant fibers result from reading frame restoring mutations

16. Becker muscular dystrophy Dystrophin Actin -Dystrophin present, but abnormally short due to in-frame deletions -Variety of clinical presentations and progressions, most involving at least some degree of proximal weakness

17. Dystrophin: Becker muscular dystrophy Immunohistochemical staining is non-specific!

18. Dystrophinopathy: Diagnosis

• Deletions or duplications in DNA isolated from peripheral blood

• • ~70% of Duchenne

• • ~85% of Becker

• Muscle biopsy

• • Immunostaining for dystrophin

• • Immunblot analysis

• • Evaluation of related proteins

19. Limb-Girdle Muscular Dystrophies

• Autosomally determined face-sparing, proximally predominant, progressive muscular dystrophies

• • 10% autosomal dominant (6 subtypes, LGMD1A-F)

• • 90% autosomal recessive (11 subtypes, LGMD2A-K)

• Age at onset varies greatly (usually 1 st 3 rd decade)

• Defective proteins coded by mutant genes may be detected by immunohistochemistry or immunoblotting

20. Dysferlinopathy (LGMD-2B)

• Relatively sudden onset in late teens

• Early inability to tiptoe

• Calf pain and swelling

• Good prior muscular prowess

• Very high serum CK

• Inflammation may be prominent

21. How dysferlin works Dystrophin Actin

22. Dystrophin Actin

23. Dystrophin Actin

24. Dystrophin Actin

25. Polymyositis Invasion of non-necrotic fibers

26. Dermatomyositis

27. DM = Complement mediated, small vessel vasculitic myopathy

28. Primary myopathies with inflammation

• Limb-girdle muscular dystrophy, types 2A (calpainopathy) & 2B (dysferlinopathy)

• Duchenne muscular dystrophy

• Fascioscapulohumeral dystrophy

• Inclusion body myositis

29. Inclusion body myositis

• Time of onset generally after age 40

• Preference for quadriceps, especially in men, finger flexors, and pharyngeal muscles

• Highly variable rate of progression (older age of onset associated with faster progression)

• Serum CK usually less than 12x upper limit of normal, but higher values do not exclude IBM

30. IBM: Pathologic diagnosis

• H&E and /or Gomori trichrome staining to visualize vacuolated muscle fibers and mononuclear cell inflammation

• Immunohistochemical staining for paired helical filaments (SMI-31)

• Electron microscopy is not required

31. IBM: Pathologic diagnosis

32.

33.

34. IBM-accumulated proteins

• Myostatin

• Beta-amyloid & its precursor protein

• Phosphorylated tau protein

• Cellular prion protein

• Alpha-synuclein

• Parkin

• DJ-1

35. SMI-31 Immunostaining

36. Inclusion body myositis as a degenerative disease

• Strongly age-dependent/realted

• Biochemical evidence of oxidative stress

• Accumulation of mitochondrial mutations

37. Mitochondrial disorders

• Syndromes associated with defects in oxidative phosphorylation

• Genetics may be mitochondrial OR nuclear (mendelian)

• Adults usually demonstrate myopathy with variable CNS involvement

• Children usually manifest psychomotor delay, hypotonia, acidosis, and cardiorespiratory failure

38. Stochastic segregation during cell division results in tissue and cellular heteroplasmy

39. Heteroplasmy results in a threshold effect , both clinically and pathologically

40. Cellular heteroplasmy/threshold effect

41. Developmental heteroplasmy Skeletal muscle Heart Eye Liver Kidney Pancreas Blood Inner Ear GI Tract Brain A single mitochondrial mutation may demonstrate heterogenous phenotypes!

42. Diseases of Skeletal Muscle December 9 th , 2008

• Describe the morphologic effects of denervation (and reinnervation) on skeletal muscle.

• Apply the reading frame hypothesis to the pathogenesis of dystrophinopathies.

• Discuss the limitations of routine muscle biopsy in the diagnosis of:

• • Muscular dystrophies

• • Mitochondrial diseases

• Describe the pathologic features that distinguish dermatomyositis and inclusion body myositis from polymyositis.