Week 9 Part 2 - Laboratory Diagnosis of Fragile X Flashcards
Fragile X Syndrome
Most common form of inherited intellectual disability in males & females
FRAXA is most common cause of autism and ASD
- ~30% of people with FRAX have autism
- 2-6% of people with autism have FRAX
FRAXA mutation testing is part of genetic assessment in males & females who present with development delay, mental disability and behavioural problems
Fragile X - Comparing Male and Female Presentation
~50% of females with full mutations have mild to moderate disability
Males have mild to severe mental retardation, delay in language, behavioural problems & cognitive deficits
Fragile X Clinical Features - Physical
Large ears
Long, narrow face with prominent forehead
Mitral valve prolapse
Seizures
Eye problems
Fragile X Clinical Features - Cognitive
FGX mutations affect brain development
Developmental delays
Mental impairment
Learning disabilities
Prevalence of Fragile X
Full Mutations; 1 in 7,000 males
Full Mutations; 1 in 11,000 females
Premutations; 1 in 850 males
Premutations; 1 in 300 women
What Causes Fragile X Disorders?
FMR1 gene - 17 exons encode fragile X mental retardation protein (FMRP)
Over 99% of FXS disorders are due to an expansion mutation in the FMR1 gene
CGGn repeat region in the 5’-UTR of the FMR1 promoter
CGG repeat may be ‘pure’ or interspersed with 9 or 10 AGGs
- FM with no AGG repeats are at risk of full expansion in the next generation
- those with AGG repeats are more stable
CGG expansion >200 repeats
- leads to hypermethylation of the cytosine residues of CGG repeats & upstream CpG island
- causing deactivation of the FMR1 gene
- no FMRP is produced causing Fragile X syndrome
AGG Interruptions
AGG repeats anchor the region during replication and prevent strand slippage.
Premutation alleles are less likely to contain AGGs and have long stretches of uninterrupted CGGs at their 3’ end
AGG interruptions predict the risk of expansion from premutations to full mutations in the next generation
Alleles with AGG interruptions have greater intergenerational stability of the repeat
Instability is strongly influenced by female and number of repeats and location of the AGG interruptions
The size of repeats and AGG interruption is relevant for prenatal counselling for women with premutations
Smaller repeat lengths and at least two AGGs decrease the risk of expansion in the next generation
Testing of AGG triplets can be done by TP-PCR or long-read single-molecule sequencing
Laboratory Testing for Diagnosis of Fragile X
(CGG)n Expansion
- conventional PCR
- Southern Blot
- repeat-primed PCR & capillary separation -Asuragen Assay / In house
-real time PCR
Deletions
- microarray
- MLPA
Fragile X PCR Results
Mosaicism
Mosaicism is the presence of two or more genetically different sets of cells in the body.
In adult tissues, the full mutation is mitotically stable when methylated
But in the early embryo, before methylation, fully expanded repeats experience mitotic instability frequently leading to size mosaicism
Mosaicism associating full mutation and premutation alleles (MoMP) is also not uncommon
Mosaicism in Males can have:
Mosaicism for full Mutation and Normal alleles (MoMN)
Mosaicism for full Mutation, Premutation, and Normal alleles (MoMPN)
Some individuals carry with >200 repeats that are completely or partially unmethylated
Functions of FMRP
Binds more than 1,000 mRNAs in the brain involved in neural processes, cell-cell communication, cytoskeleton and microtubule modulators and metabolic regulators
Also binds to mRNAs encoding transcription factors and epigenetic
chromatin modulators, especially in immature cells
Treatment of Fragile X
There is no cure and no specific treatment available for FXS
Management of this condition is generally supportive
Medications for behavioral issues that affect social interaction
Routine medical management of strabismus, ear infections, reflux, seizures, mitral valve prolapse, and/or high blood pressure
Stimulants and antipsychotics
