Title: Rett Syndrome is caused by mutations in Xlinked MECP2, encoding methylCpGbinding protein 2
1Rett Syndrome is caused by mutations in X-linked
MECP2, encoding methyl-CpG-binding protein 2
- Presented By
- Vanessa Bowers
Amir, R., I.B. Van den Veyver, M. Wan, C.Q. Tran,
U. Francke, and H.Y. Zoghbi. 1999. Rett Syndrome
is caused by mutations in X-linked MECP2,
encoding methyl-CpG-binding protein 2. Nature
Genetics. 23185-188.
2What is Rett Syndrome
- A progressive neurodevelopmental disorder that
begins at early infancy - Caused by a mutation in the methyl-cysteine
binding protein 2 gene (MeCP2) - Most common cause of mental retardation in
females - Shows high lethality in hemizygous males (causing
miscarriage, still birth, or early death)
3Symptoms Stages of Rett Syndrome(RTT)
- Patients with RTT appear to be born healthy and
develop normally until about 6-18 months of age - Several stages then follow
- After initial regression, the condition
stabilizes and female patients usually survive
into adulthood
4What is the MeCP2 gene?
- MeCP2 gene is located on the X-chromosome (mapped
to the region Xq28) - It is a transcriptional repressor that prevents
unscheduled transcription throughout the body - The MeCP2 gene encodes the MeCP2 protein which
binds methylated cytosine residue of DNA
dinucleotides and mediates transcriptional
silencing through the interaction with histone
deacetylase and the corepressor mSIN3A - These cytosine residues are subject to
transcriptional silencing after DNA methylation
5Why is transcriptional silencing important in the
body?
- It is a mechanism of transcriptional control
where DNA in no longer accessible for future
transcription - This is important because in different stages of
development, certain genes need to be silenced or
turned off and others need to be turned on in
order for proper development - In RTT, the mutation of the MeCP2 gene doesnt
allow for normal transcriptional silencing,
therefore causing the disease
6What does the MeCP2 protein look like?
- MeCP2 protein is a chain of 486 amino acids
- Two functional domains
- - an 85 amino acid methyl-CpG binding domain
(MBD) - - a 104 amino acid transcription repression
domain (TRD) - The MBD is essential for the binding of the
protein to methyl cytosine at the beginning of
genes in the body - The TRD recruits other repressor proteins
(mSIN3a, histone deascetylase) to inhibit
transcription
7Where does RTT originate?
- RTT is caused by mutations in the MeCP2 gene,
which causes changes in the corresponding MeCP2
protein - Mutations occur in either of the parents
reproductive cells sperm or egg - Mutations are usually sporadic and not familial
8There are two scenarios for the origin of
mutations in Rett Syndrome 1) The
father has a mutation on his X chromosome or
2) The mother has a mutation on one of her
x chromosomes
Scenario 1
Scenario 2
9X InactivationX-inactivation is where one
x-chromosome is randomly inactivated in every
cell. Therefore when the mother passes her mutant
x chromosome onto her daughter, she my or may not
express RTT due to x-inactivation of her mutant X
chromosome, and the expression of the fathers
normal X chromosome.
Father YXNF
Mother XNMXMM
10Rett Syndrome is Neurodevelopmental Disorder
- MeCP2 is very abundant and highly expressed in
the brain. In the developing cerebral cortex, the
appearance of MeCP2 correlates with neuronal
maturation. - With this high abundance of MeCP2 within the
brain, these mutations of the MeCP2 protein
affect the development of neurons and therefore
cause many of the neurological symptoms that are
related to Rett Syndrome
11The Experiment
- 21 sporadic and 8 familial RTT patients genomic
DNA were screened by conformation-sensitive gel
electrophoresis - Sporadic patients no known family members
expressed the disorder - Familial patients were
- 5 pairs of full sisters
- 2 pairs of half sisters
- 1 pair of second half cousins
12Results
- Among sporadic patients 3 missense,
- 1 nonsense, and 1 frameshift mutations were
identified among patients 6, 22, 24, 29, and 39 - In two half sisters, the same missense mutation
was found in each, but was not present in the
mother
13Quick Definitions
- Missense mutation - A mutation that changes a
codon so that it codes for a different amino acid
- Nonsense mutation - A mutation in which one of
the three terminator codons in the RNA appears in
the middle of a genetic message, causing
premature termination of transcription, and
releases incomplete, nonfunctional polypeptides
from the ribosome. - Frameshift mutation - A mutation resulting from
an addition or subtraction that is not an exact
multiple of 3 base pairs in a coding sequence,
causing the subsequent codons to be read out of
phase and a completely different set of amino
acids to be made into protein.
14Patient 39 a missense mutation occurred at amino
acid 133, Arginine was replaced by Glutamine
Patient 24 a missense mutation occurred at
amino acid 155, Phenylalanine was replaced by
Serine Patient 6 a missense mutation occurred
at amino acid 158, Threonine was replaced by
Methionine
The electropherograms above depict the nucleotide
sequences (data from DNA sequencing) performed on
the sporadic patients
15- Patient 22 a nonsense mutation occurred with a
substitution of cytosine to thymine, which
converted a CGA codon to a TGA codon
16- Patient 29 A frameshift mutation occurred from
an insertion at codon 208 which shifted the
reading frame and introduced a stop codon after
27 amino acids
17Familial Patients Results
- In two half sisters with the same mother, the
same missense mutation of cytosine replaced by
thymine was identified - No mutation was found in the mother (she is an
obligate carrier)
Sister
Mother
Sister
18Discussion
- These missense mutations disrupt the structure of
the methyl binding region (MBR) of the MeCP2
protein - The nonsense and frameshift mutations disrupt the
transcription repression domain (TRD) - These disruptions of the MBD and the TRD of the
MeCP2 protein interferes with their functions of
transcriptional silencing and cause Rett syndrome
19Conclusion
- Rett syndrome patients display some type of
mutation (missense, nonsense, or frameshift).
These mutations affect the MBD or TRD domains of
the MeCP2 protein, which cause defects that dont
allow for the proper functioning of the MeCP2
gene as a transcriptional silencer. Without
proper functioning of this gene, patients will
develop Rett Syndrome.
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