Download Fragile X Laboratory Testing - Association for Molecular Pathology

Fragile X
Laboratory Testing:
Background and Quality
Improvement Opportunities
(part 1 of 2)
Elaine Lyon, Ph.D.
University of Utah/ARUP Laboratories
Association for Molecular Pathology,
Clinical Practice Committee
1
Chair,
Outline
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Clinical Features of Fragile X
Molecular Basis of Disease
Molecular Testing/Interpretation
Opportunities for Improvement
Quality control material
 Interlaboratory study (AMP, CDC, NIST)
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2
Fragile X Syndrome
3
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Most common inherited form of mental retardation.
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Incidence 1:4000 males and 1:8000 females.
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Affected males have mental retardation, characteristic
physical features and behavior.
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Affected females exhibit a less severe phenotype.
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Found in all populations.
Features
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Mental impairment
Attention deficit/autistic-like
Long, thin face - prominent forehead
Large ears
Flexible joints
Low muscle tone
Enlarged testicles
Jones KL. Smith’s Recognizable Patterns of Human Malformation, 4 th Ed.
Chromosome Level
Folate-sensitive fragile site at Xq27.3 (FRAXA).
Other sites: FRAXD, FRAXE, & FRAXF.
5
Molecular Level

Tri Nucleotide Repeat (CGG) at the 5'
Untranslated Region (UTR).
A small expansion (pre-mutation) associated with
increased mRNA
 A large expansion associated with methylation,
inactivating gene expression.
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Molecular Schematic
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Protein Level
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Normal – protein widely expressed (nerve,
brain, etc.)
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Pre-mutation – normal protein, increased
mRNA
Full mutation – no protein produced
Protein expression by immunohistochemistry
(IHC)
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RNA binding protein
suspected deletions/point mutations in males
Transmission
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Female pre-mutation carriers
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50/50 chance of transmitting unstable allele
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Male pre-mutation carriers
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Will transmit pre-mutation to all daughters
Unlikely to expand
Intermediate
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May stay within pre-mutation range
May expand to full mutation (higher pre-mutations more likely to
fully expand in one generation)
May expand to pre-mutation, but not full mutation, in one
generation
Risk of Expansion by
Pre-mutation Size
Number of Maternal Pre-Mutation
CGG Repeats
Approximate % Risk that a Son Will be
Affected with Fragile X Syndrome
56-59
7%
60-69
10%
70-79
29%
80-89
36%
90-99
47%
> 100
50%
Adapted originally from Warren & Nelson 1994; modified according to Nolin et al. 1996.
GENEReviews at www.genetests.org, FMR1-Related Disorders.
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Fragile X-associated Tremor/Ataxia Syndrome
(FXTAS)
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Symptoms
 Late-onset, progressive
cerebellar ataxia/intention
tremor
 Short-term memory loss,
executive function deficits,
cognitive decline
 Lower-limb proximal muscle
weakness, and autonomic
dysfunction
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Genetics
 FMR1 pre-mutation
 mRNA accumulation
Penetrance is age related
Jacquemont, JAMA 2004
Premature Ovarian Failure (POF)
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Cessation of
menses before
age 40
21% of females
with premutations
180
160
140
120
100
80
60
40
20
0
Full Mutation
Premutation
Control
18 - 29
30 - 39
40 - 49
50+
Age at Interview
Proportion of Subjects Experiencing POF
0.3
Proportion of Subjects
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Number of Subjects
Distribution of Subjects
0.26
0.25
0.22
0.2
0.15
0.11
0.1
0.05
0
0.05
0.02
0
0
0
0
0
0
0
18 - 29
30 - 39
40 - 49
Age at Interview
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Allingham-Hawkins, AJMG, 1999
50+