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Genetic counseling: Becker Muscular Dystrophy

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Becker Muscular Dystrophy

General InformationEdit

  • Dystrophinopathies
    • Characterized by a spectrum of muscle disease that ranges from mild to severe
    • Duchenne/Becker muscular dystrophy is severe
      • Skeletal muscle is primarily affected in both
      • DMD is rapidly progressive and presents in early childhood.
        • Patients are often wheelchair-bound by age 12
      • Becker is characterized by later-onset skeletal muscle weakness
        • Patients remain ambulatory into their 20s
        • Despite milder skeletal muscle involvement, heart failure from dilated cardiomyopathy is a common cause of morbidity and the most common cause of death
        • Mean age of death is in the mid-40s
  • Dystrophin gene
    • Xp21.2
    • Encodes protein dystrophin
    • 85% of males with BMC have deletions or duplications involving exons of this gene
    • Molecular genetic testing is available clinically
  • Prevalence
    • 1 in 18,000 live male births

DiagnosisEdit

  • Clinical
    • Family history
      • Compatible with X-linked recessive inheritance
    • Creatine Phosphokinase (CK) concentration
      • Evaluated by blood test
      • Elevated serum CK concentration results from progressive elimination of dystrophic muscle fibers.
        • Can also result from strenuous exercise
      • CK concentration gradually decreases with advancing age due to the progressive elimination of these muscle fibers.
      • 100% of males with BMD have a serum CK concentration >5x normal
      • ~30% of female carriers have a concentration 2-10x normal
        • Some investigations have shown a wide variability in BMD carriers
        • Many have levels within the normal range, so other tests are necessary
        • CPK is not completely reliable
    • Clinical findings
      • Progressive symmetrical muscle weakness and atrophy, proximal greater then distal, often with calf hypertrophy
        • Weakness of quadriceps femoris may be the only sign
      • Activity-induced cramping in some patients
      • Flexion contractus of the elbows may occur late in the course
      • Wheelchari dependency, if present, after 16 years of age
      • Preservation of neck flexor muscle strength in BMD differentiates it from DMD
  • Molecular genetic testing
    • Deletions involving one or more exons of the DMD gene
      • ~85% of males with BMD have deletions
      • Testing is done by PCR or southern blot
      • Available on a clinical basis
    • The remaining ~15% have duplications of one or more exons of the DMD gene or other mutations such as small deletions or insertions, single base changes, or splicing mutations
    • Carrier testing may be performed but requires quantitative analysis for gene dosage which can be difficult to perform and interpret
    • Prenatal testing is available

Clinical DescriptionEdit

  • Males
    • Characterized by later-onset skeletal muscle weakness (as compared to DMD which presents ~ age 4 years)
      • Patients remain ambulatory into their 20s with BMD
    • Heart failure from dilated cardiomyopathy is a common cause of morbidity and the most common cause of death
    • The mild end of the spectrujm includes men with onset of symptoms after age 30 years who remain ambulatory even into their 60s
    • Cognitive impairment is not as common or as sever as in DMD
  • Females
    • Occasionally have clinical features as the result of X chromosome rearrangements involving the DMD locus (Xp21.2)
    • 81% of BMD carriers have no signs or symptoms
    • 14% have mild to moderate muscle weakness
    • 16% have left ventricle dilation
    • 5% have myalgia or cramps
    • No BMD carrier females have been found to have dilated cardiomyopathy
  • Genotype-Phenotype Correlations
    • In males, phenotypes are best correlated with the degree of expression of dystrophin, which is largely determine by the reading frame of the spliced messaged obtained from the deleted allele
    • The BMD phenotype occurs when there is some dystrophin, usually resulting from:
      • Deletions or duplications that juxtapose "in-frame" exons
      • Some splicing mutations
      • Most non-truncating single base changes that result in translation of a protein product with intact N or C termini
    • A dystrophin protein that retains partial function produces the milder BMD phenotype
  • Management
    • There is no treatment for BMD
    • Appropriate management can prolong survival and improve quality of life
      • Physical therapy to promote mobility
        • Range-of-motion exercises
      • Braces to delay the onset of contractures
      • Monitoring and surgical intervention for orthopedic complications
        • Scoliosis, kyphosis, or lordosis
      • Routine monitoring for evidence of cardiomyopathy
    • All carriers should have a complete cardiac evaluation at least once

Differential DiagnosisEdit

  • Limb-girdle muscular dystrophy
    • A group of disorders clinically similar to DMD
    • Occurs in both sexes
    • Caused by mutations I genes that encode sarcoglycans and other proteins that interact with dystrophin
  • Emery-Dreifuss muscular dystrophy
    • Associated with limb contractures and cardiac arrhythmia
  • X-linked recessive, autosomal dominant, and autosomal recessive forms
    • Caused by mutations in the LMNA gene
  • Spinal muscular atrophy
    • Caused by mutations in the SMN gene
    • Characterized by:
      • Poor muscle tone
      • Symmetric muscle weakness that spares the face and ocular muscles
      • Evidence of anterior horn cell involvement
        • Includes fasciculations of the tongue and absence of deep tendon reflexes
  • Dilated cardiomyopathy
    • Can be sporadic or familial
  • No other phenotypes are associated with mutations in the DMD gene

InheritanceEdit

  • X-linked recessive
    • Carrier females have a 50% chance of transmitting the BMD mutation in each pregnancy
    • With each pregnancy, a carrier has a 25% chance of having an affected child
  • Risk to family members
    • A woman with an affected son and one other affected relative in the maternal line is an obligate heterozygote
    • A woman with more than one affected son and no other family history can have:
      • A germline mutation
        • DMD disease-causing mutation present in every cell
      • Germline mosaicism
        • Mosaicism for a DMD disease-causing mutation which includes the germline
        • The frequency of germline mosaicism in DMD is estimated at 12% to 20%
    • If proband is only affected family member, must determine if mother and other females are carriers
      • The proband may have a de novo DMD disease-causing mutation
        • The mutation could have occurred in the egg at the time of conception
        • The mutation could have occurred after conception and therefore is present in some but not all cells of the proband's body.
        • The likelihood that the mother is a carrier is low
      • The proband's mother may have a de novo mutation
        • 2/3 of mothers of sporadically occurring males with DMD are carriers
        • Could have occurred if:
          • Mutation occurred in the egg or sperm at the time of her conception and is present in every cell of her body (germline mutation)
          • Mutation is present in some but not all cells of her body (somatic mosaicism)
          • Mutation is present only in her egg cells (germline mosaicism) and is not detected in a blood sample.
      • The proband's mother may have inherited a DMD mutation from her mother who is a carrier, her mother or father who has somatic mosaicism, or her mother or father who has germline mosaicism.

Risk AssessmentEdit

  • All daughters of a male affected with BMD are carriers; none of the sons will inherit the mutation

Ordering the testEdit

  • Patient must sign consent form for DNA analysis
    • Use blue ink so it is obvious which is the original
    • Give the patient a copy
    • Put the original in the chart
  • Use special specimen processing request form
    • Make a copy for the chart
    • Give a copy to Lori Martineek at E352
  • Blood is drawn at Test Referral Center
    • Pt must have consent form, 2-ply specimen processing request form, and DNA analysis requisition
    • Tell pt to make sure her name is on the tube of blood
    • TRC will FedEx to Dr. Prior's lab
  • Contact Dr. Thomas W. Prior to inform him that you are sending the sample
    • Prior-1@medctr.osu.edu
    • Give him clinical history and lab report # from prior testing

ResourcesEdit

  • Muscular Dystrophy Association
    • 800-572-1717
    • [1]

ReferencesEdit

  • Emery and Rimoin's Principles and Practice of Medical Genetics. Third Edition, 1996. pp 2337-2354
  • National Organization for Rare Disorders (NORD)
  • Nelson's Textbook of Pediatrics. 15th Edition. pp1745-1748
  • Online Mendelian Inheritance in Man (OMIM). BMD#300376
  • [www.geneclinics.org] (GeneReviews)

NotesEdit

The information in this outline was last updated in 2002.


Material obtained under GFDL Licence from http://en.wikibooks.org/wiki/Handbook_of_Genetic_Counseling

Heckert GNU white Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License."

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