20.03.09 Fragile X and FMR1 related disorders

Question 1

Examples of FMR1-related disorders

Answer 1

Fragile X, Fragile X-associated tremor/ataxia syndrome (FXTAS) and FMR1-related primary ovarian insufficiency (POI)

Question 2

What does FMR1 gene encode

Answer 2

• FMRP (Fragile X mental retardation 1 protein)
• FMRP is an RNA binding protein. Also
• Present in many tissues like brain, testes, ovaries

Question 3

Other cellular functions of FMRP

Answer 3

• Regulates production of other proteins, role in the development of synapses
• Acts as a shuttle, transporting mRNA from nucleus to areas of cell where proteins are assembled.

Question 4

Disease mechanism for FRAX

Answer 4

• X-linked dominant inheritance.
• 5’UTR CGG repeat expansion
• Expansion of over 200 repeats leads to gene silencing.
• Hypermethylation of a CpG island 250bp downstream of CGG repeat causes Fragile X in males, curtails protein expression.

Question 5

Do FRAX premutation carriers have altered FMR1 expression

Answer 5

• Higher levels of FMR1 mRNA

- Normal levels of FMRP

Question 6

Disease mechanism in FXTAS and POI

Answer 6

• Unlikely to be due to lack of FMRP as these symptoms are not seen in FRAX patients with full mutations.
• Could be excess of FMR1 mRNA leads to a toxic gain of function phenotype

Question 7

Review of Fragile X

Answer 7

• Most common single-gene cause of learning difficulty in humans
• 1 in 4000 males, 1 in 5-8,000 females
• Fragile site (FRAXA) is expressible in around 2-40% blood cells in affected males

Question 8

Phenotype of males with full expansion

Answer 8

moderate to severe intellectual and social impairment, characteristic appearance (large head, long face, large ears, prominent forehead and chin), joint laxity, large testis.
-Dependent on size of expansion

Question 9

Phenotype of females with full expansion

Answer 9

• Apparently normal (50%)
• Mild to moderate mental and social impairment, with or without fragile site expression.
• Dependent on size of expansion and XCI

Question 10

What size are premutation alleles

Answer 10

50-200 repeats. Don’t cause FRAX but could expand to full mutations over one or more generations

Question 11

What is the smallest allele to have expanded to a full expansion in a single generation

Answer 11

56 repeats

Question 12

What causes variable phenotype in females.

Answer 12

Skewed X in activation

Question 13

What is POI

Answer 13

Cessation of menses in women before the age of 40years. Age of menopause is ~5years earlier in premutation carriers to non-carriers.

Question 14

What % women with FMR1 premutation will develop POI

Answer 14

20%

Question 15

Are women with a full mutation at increased risk of POI

Answer 15

No

Question 16

What proportion of POI is due to FMR1

Answer 16

4%

Question 17

What is FXTAS

Answer 17

Late onset (60-65yrs) progressive cerebellar ataxia and intention tremor. Males and some females with premutations.

Question 18

Is there a correlation between repeat size and age of onset/severity with FXTAS

Answer 18

Yes. Large the expansion the earlier the onset and more severe the symptoms.

Question 19

In people over 50, is penetrance of FXTAS higher in men or women

Answer 19

Higher in men- 45.5% (women is 16.5%)

Question 20

What is FRAXE

Answer 20

• Mutations in FMR2 gene. Distal to FMR1
• Expansions of GCC tract in 5’UTR of FMR2
• Phenotype is less severe than FRAXA. Lacks syndromic features.
• Lower prevalence
• No phenotype attributed to premutations.

Question 21

What is a fragile site

Answer 21

a specific heritable point on a chromosome that tends to form a gap or constriction and may tend to break when the cell is exposed to partial replication stress.

Question 22

FMR1 alleles size range

Answer 22

• Normal= 6-50 rpts
• Intermediate= 46-58 (under 50= stable, 50-58 may show instability)
• Premutation= 55-200 (unmethylated)
• Full mutation= 200-1000 (with methylation)

Question 23

Interpretation of intermediate FMR1 alleles.

Answer 23

• Often transmitted stably, more unstable towards the higher end of range
• Stability can vary within a family.
• Stability correlates with presence of AGG motifs within CGG tract.
• Not known if alleles in this range are involved in POI, FXTAS or dev delay

Question 24

Interpretation of premutation FMR1 alleles.

Answer 24

• Smaller alleles can be detected by PCR. Larger alleles need S.blot.
• Probability of conversion to a full mutation on maternal transmission in a single generation is low for alleles between 59-70, but 90% for alleles >90rpts
• extreme skewing in favour of mutant allele makes small premutations hard to detect in females.

Question 25

Interpretation of full mutation FMR1 alleles.

Answer 25

• 20% full mutation patients show mosaicism for a premutation, which may be the same size (methylation mosaic). Usually smaller.
• Usually extensive somatic variation of repeat number in peripheral blood of full mutation patients.