FRAX

Question 1

Genetic mutation

Answer 1

Trinucleotide repeat expansion CGG tract in the 5’UTR of the FMR1 gene

Gene deletions and point mutations within the RNA binding domain account for small number of cases

Question 2

Incidence

Answer 2

1 in 4000-5000 males

Most common single gene cause of LD

Most common cause of autism

Question 3

The CGG repeat

Answer 3

Highly polymorphic in population 5-58 repeats

Interspersed with AGG interruptions, thought to confer stability

Expansion to full mutation only occurs by maternal transmission

Question 4

Mosaicism

Answer 4

15-20%

2 forms:

1. Repeat size mosaicism ( full and premutation in males)
2. Methylation mosaicism (full mutations variably methylated, milder phenotype)

Tissue mosaicism?

Question 5

Phenotype

Answer 5

ID
Social and behavioural problems
Autism
Subtle physical features:
-Large ears
-Elongated face
-Macroorchidism
-Joint hyper mobility
-Hypotonia

Question 6

Pathogenicity of FXS

Answer 6

Expansion to full mutation causes hypermethylation of gene promoter and gene silencing

Methylation of CGG cytosines
Deacetylation of tails of his tones H3 and H4
Reduced methylation of lysine 4 (H3K4)
Increased methylation lysine 9 (H3K9)

Results in heterochromatin configuration to exclude TF binding

Question 7

Role of FMRP

Answer 7

Expressed predominately in brain (differentiated neurons in hippocampus and granular layer of cerebellum) and testes

Role in down regulation of translation of specific target mRNAs

In mice binds and stabilises PSD-95 mRNA (regulates neuronal synaptic signalling and learning)

20 isoforms by alternative splicing

Question 8

FXTAS

Answer 8

Fragile X associated tremor/ataxia syndrome

Question 9

FXTAS pathogenicity

Answer 9

Premutation trinucleotide repeat expansion in FMR1 gene

Increase in gene transcription and reduced translation results in accumulation of FMR1 mRNA, which contributes to nuclear inclusions

Toxic gain of function effect

Translation of other proteins impaired due to increased interaction of trinucleotide binding proteins and translation factors with inclusions, impairing translation of proteins involved in fundamental cellular processes (model)

Brain atrophy

Question 10

FXTAS phenotype

Answer 10

Late onset
Age of onset and severity correlate with CGG repeat length
Progressive intentional tremor
Parkinsonism
Ataxia
May progress to dementia
Peripheral neuropathy

Males and Females (females lower risk)
1/3 males with PM affected
5% men with sporadic ataxia may have FXTAS

Question 11

FXPOI

Answer 11

Fragile X associated primary ovarian insufficiency

Question 12

FXPOI phenotype

Answer 12

Amenorrhea before age 40 for 4 or more months

Varying degree of ovarian function

Elevated FSH within menopausal range

5-10% with FXPOI can conceive

Question 13

FXPOI pathophysiology

Answer 13

Carriers of FMR1 premutation (20% affected compared to 1% pop)

Risk increased for repeat lengths 80-100

Molecular mechanism unknown

May be due to over production of mRNA

Question 14

Other causes of POI

Answer 14

Turner syndrome
X Chr rearrangements
Chemotherapy

Question 15

What proportion of women with POI have a FMR1 premutation?

Answer 15

6.5%

Question 16

FMR1 testing referral reasons

Answer 16

Diagnosis/exclusion with presenting symptoms of FXS FTAS or FXPOI

Carrier testing for FH

Prenatal

Question 17

Prenatal diagnosis

Answer 17

Females with premutation or full mutation

Amplified CGG PCR on amino or CVS

Urgent TAT 3 days

Test for MCC

Methylation pattern may not be representative (20% CVS DNA partially or unmethylated)

Question 18

Describe the different repeat classes and associated phenotype

Answer 18

under 45 (Normal)

45-54 (Intermediate)

55-200 (Premutation) Risk of FXTAS or FXPOI (females)

over 200 (Full mutation) Males affected, females variable (~50% affected with milder phenotype)

Question 19

Molecular pathogenesis FXS

Answer 19

Reduction in FMRP

Thought to have role in down-regulation of translation of target mRNAs by forming part of a messenger ribonucleoprotein complex that in turn interacts with members of the RNA-induced silencing complex and ribosomes to regulate translation in neurons