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Diseases That Result from Expansion of Trinucleotide Repeats

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Title: Diseases That Result from Expansion of Trinucleotide Repeats


1
Diseases That Result from Expansion of
Trinucleotide Repeats
  • Type? trinucleotide repeat diseases
  • xuyan

2
Type? diseases differ from the Type?disease
Type?disease Type?diseases
Repeating trinucleotide unit CAG A variety of trinucleotides,
Encode amino acids Yes (Glu) No
Quantity of trinucleotide repeats Exceed 35 Glu Massive repeats(gt200)
Affected part Brain Numerous of body
3
  • The best studied Type?diseases is fragile X
    syndrome(X????????) ,so named because the mutant
    X chromosome is especially susceptible(?????) to
    damage.

4
  • The fragile X syndrome is the most common form of
    inherited mental retardation(????) and accounts
    for approximately 40 of cases with X-linked
    mental retardation. 

5
  • Other characteristics of the fragile X syndrome
    include a wide range of cognitive(???),
    behavioral(???), and physical features such as
    variable IQ (profound to mild mental
    retardation), autistic-like(??????) features,
    hyperactivity(????), increased testicular
    (??)volume, macrocephaly(???), and large ears.  .

6
  • The fragile X mental retardation-1 (FMR1) gene
    codes for the mRNA-binding fragile X mental
    retardation protein (FMRP).
  •  
  • FMRP is normally made in many tissues, especially
    in the brain and testes(??).

7
Where is the FMR1 gene located?
  • The FMR1 gene is located on the long (q) arm of
    the X chromosome at position 27.3.
  • Cytogenetic Location Xq27.3

8
  • Almost all cases of fragile X syndrome are caused
    by expansion of the CGG trinucleotide repeat in
    the FMR1 gene. In these cases, CGG is abnormally
    repeated from 200 to more than 1,000 times, which
    makes this region of the gene unstable.

9
  • A normal allele of FMR1 gene contains anywhere
    that is repeated in a part of the gene that
    corresponds to the 5 noncoding portion of the
    messenger RNA (Figure 1).

10
Figure 1
11
  • The CGG repeat in FMR1 that can be categorized
    into three classes based on the size of the
    repeat normal (5-55 repeats), premutation
    (60-200 repeats), and full mutation (200-2000
    repeats).  The full mutation is the
    disorder-causing form of the repeat, and the
    premutation is the carrier form of the repeat. 

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  • Full mutations result in hypermethylation of the
    DNA in and around the CGG tract, curtailed gene
    expression and no FMRP being produced.

14
  • Without adequate FMRP, severe learning problems
    or mental retardation and the other features of
    fragile X syndrome can developed.

15
  • Smaller expansions of the CGG repeat, or
    premutations, do not cause Fragile X syndrome
    but may show expansion into full mutations over
    one or more generations.

16
  • Unlike an abnormal HD allele, FMR1 allele causes
    disease as the result of a gain of function , an
    abnormal FMR1 allele causes disease as the result
    of a loss of function .

17
Conclusion
  • The fragile X syndrome is the best studied
    Type?diseases ,which is a X-linked disease .The
    most characteristic of the fragile X syndrome is
    mental retardation. Almost all cases of fragile X
    syndrome are caused by expansion of the CGG
    repeat in the FMR1 gene. In these cases, CGG is
    abnormally repeated from 200 to more than 1,000
    times

18
  • Although there is no effective treatment for
    any of the disease caused by trinucleotide
    expansion, the risk of transmitting or possessing
    a mutant allele can be assessed through genetic
    screening.

19
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