2. I PROCESSI DI METILAZIONE GIOCANO UN RUOLO
NELLA PATOGENESI UMANA
3. The mechanism(s) by which the native MeCP2 protein operates in the
cell are not well understood.
Historically, MeCP2 has been characterized as a proximal gene
silencer with 2 functional domains:
1. a methyl DNA binding domain and
2.a transcription repression domain.
However, several lines of new data indicate that MeCP2 structure and
function relationships are more complex: an analysis of cell-based
experiments suggesting MeCP2 is a regulator, rather than a strict
silencer, of transcription. The new data establish MeCP2 as a
multifunctional nuclear protein, with potentially important roles in
chromatin architecture, regulation of RNA splicing, and active
transcription..
methyl DNA binding protein 2 (MeCP2)
4. Andreas Rett, che aveva osservato un comportamento insolito, tragico e affascinante insieme,
in alcune bambine sedute nella sala d'aspetto del suo studio. Correva l'anno 1966.
Nel 1999, il team di Huda Zoghbi del Baylor College of Medicine di Houston scoprì che la
sindrome di Rett è associata a una mutazione del gene MeCP2, localizzato sul cromosoma X. Il
gene MeCP2 è un repressore della trascrizione genica.
La proteina MeCP2 è espressa in modo predominante nei neuroni maturi. MeCP2 modula le
connessioni sinaptiche e quindi la comunicazione cellulare attraverso la regolazione di alcuni
geni, tra cui il brain-derived neurotrophic factor (BDNF).
SINDROME DI RETT
Guy J, Gan J, Selfridge J, Cobb S, Bird A. Reversal of neurological defects in
a mouse model of Rett syndrome. Science 2007; 315:1143-1147
Guy et al., con un sistema murino molto sofisticato hanno silenziato la sintesi
della proteina MeCP2 causando la patologia, poi ne hanno reinstaurato la
sintesi con una remissione della sintomatologia
Più recentemente, si è visto che le MeCP2 interagisce con la proteina YB-1
che è coinvolta nella matutazione degli RNA e in particolare della subunità
NR1 dei recettori per il glutammato NMDA
6. The alterations of DNA methylation level and
patterns are a common feature of human cancer
cells. A global DNA hypomethylation has been
observed in many cancers, without obvious
sequence specificity
Recently, an extensive study of about 1200 CpG islands has
indicated that hypermethylated CpG islands are not randomly
distributed and the patterns of the hypermethylation might
be specific of subclasses of cancers.
The methylation status of tumor suppressor genes has been
extensively investigated and such alterations have been
reported in many human tumors (Robertson and Jones,
2000).
METILAZIONE DEL DNA E CANCRO
7. Several reports link genome hypomethylation to
genome instability. In particular, it was shown
recently that strongly reduced DNMT activity in a
transgenic mouse model caused demethylation of
centromeric satellite and other repeat sequences,
which resulted in a variety of chromosome defects
and concomitant tumorigenesis
METILAZIONE DEL DNA E CANCRO
8. Figure 1. Epigenetic cancer therapy. DNA methyltransferases (DNMTs) can
cause ectopic methylation and gene silencing. These events are called
epimutations and promote tumorigenesis if directed to tumor suppressor
genes. Importantly, the maintenance of epimutations requires the continuous
activity of DNMTs. This accounts for the principal reversibility of
epimutations by DNMT inhibitors.
9. Il mantenimento della metilazione del
DNA (da parte di DNMT1) è
indispensabile nella duplicazione del DNA
per mantenere metilati specifiche porzioni
del DNA.
Nel topo la mutazione di DNMT1 è letale
10. Human DNA methyltransferases (DNMTs) and their functionally important
domains.
All known DNMTs share a highly conserved C-terminal catalytic domain.
The N-terminal domains differ strongly between DNMT1 and DNMT3 enzymes
and contain several motifs for regulatory functions: a PCNA binding domain
(PBD), a replication foci targeting domain (RFTD), a CXXC domain implicated in
DNA binding, a PWWP domain linked to protein targeting and an ATRX domain
implicated in histone deacetylase interactions.
The DNMT3A and DNMT3B proteins are similar and are probably the products
of a recent gene duplication event.
11. 5-Azacytidine (Vidazae) has been approved
for the treatment of myelodysplastic
syndrome.
This has been a major milestone in the
field of cancer epigenetics and provides an
important validation for the concept of an
epigenetic cancer therapy
12.
13. Deacetilasi istoniche: una nuova classe di farmaci
nella terapia antitumorale
Generalmente agiscono causando apoptosi di
cellule tumorali,
ma non di cellule non tumorali; le HDAC sono
generalmente associate a fattori di trascrizione
oncogeni.
14. The effects of HDAC inhibitors on gene expression are highly selective,
leading to transcriptional activation of certain genes such as the cyclin-
dependent kinase inhibitor p21WAF1/CIP1 but repression of others.
HDAC inhibition not only results in acetylation of histones but also
transcription factors such as p53, GATA-1 and estrogen receptor-alpha.
The functional significance of acetylation of non-histone proteins and the
precise mechanisms whereby HDAC inhibitors induce tumor cell growth
arrest, differentiation and/or apoptosis are currently the focus of
intensive research.
Several HDAC inhibitors have shown impressive antitumor activity in vivo
with remarkably little toxicity in preclinical studies and are currently in
phase I clinical trial.
15. HDACs are classified in four groups based on their homology to
yeast histone deacetylases:
Class I which includes HDAC1, -2, -3 and -8 are related to yeast
RPD3 gene;
Class II which includes HDAC4, -5, -6, -7, -9 and -10 are related
to yeast Hda1 gene;
Class III, also known as the sirtuins are related to the Sir2 gene
and include SIRT1-7, and
Class IV which contains only HDAC11 has features of both Class I
and II.
Classificazione delle deacetilasi istoniche
16. The “classical” HDIs act exclusively on Class I and Class II HDACs by
binding to the zinc containing catalytic domain of the HDACs. These
classical HDIs fall into several groupings, in order of decreasing
potency:
(i) hyroxamic acids, such as Tricostatine A,
(ii) cyclic tetrapeptides (such as trapoxin B), and the depsipeptides,
(iii) benzamides,
(iv) electrophilic ketones, and
(v) the alophatic acid compounds such as phenylbutyrate and valproic
acid.
"Second generation" HDIs include SAHA/Vorinostat,
Belinostat/PXD101, MS275, LAQ824/LBH589, CI994, and
MGCD0103.
The sirtuin Class III HDACs are NAD+ dependent and are therefore
inhibited by nicotinamides, as well derivatives of NAD,
dihydrocoumarin, naphthopyranone, and 2-hydroxynaphaldehydes.[4]
Classificazione degli inibitori delle istone deacetilasi
17. VORINOSTAT (Zolinza) è il primo inibitore delle acetilasi
istoniche approvato per il trattamento di neoplasie
Nel 2006 il VORINOSTAT è stato approvato per il
trattamento del linfoma a cellule T cutaneo
Esistono studi preclinici che indicano una attività
antinfiammatoria del vorinostat
Nel 2007 ricerche presso la Mayo Clinics hanno dimostrato
che il Vorinostat è efficace nel glioblastoma ricorrente
N
H
O
OHO
H
N
N-hydroxy-N'-phenyl-octanediamide
18. HDAC inhibitors can activate both the death-
receptor and intrinsic apoptotic pathways
19. Histone deacetylase inhibitors as therapeutics for polyglutamine
disorders Rachel Butler and Gillian P. Bates
Nature Reviews Neuroscience 7, 784-796 (October 2006)
20. P53 puo’ agire a diversi livelli:
Puo’ attivare proteine per il riparo
del DNA
Puo’ regolar e la duplicazione
cellulare a livello G1/S
Puo’ iniziare processi apoptotici
P53
23. Il ruolo degli INSULATORS nella regolazione della
espressione genica
Es. i geni globinici e LCR HS4,
scs di Drosophila che isolano hsp70)
In genere sono sequenze ricche di isole CpG non metilate
Ci sono proteine che legano gli insulators
24. Transcription factors are associated with the nuclear matrix. It
has been proposed that the nuclear matrix recruits
transcription factors, facilitating their interaction with
regulatory DNA elements.
25. ESPRESSIONE GENICA IN EUCARIOTE: SEQUENZA DI EVENTI
3’ TRASCRIZIONE
MODIFICAZ.
TP
TRADUZIONE
RNA, trascritto primario
Capping e poliadenilazione
Splicing o maturazione
Fuoriuscita dal nucleo
riconoscimento da parte dei ribosomi
traduzione
Modificazioni post-traduzionali
Editor's Notes
a | MECP2 (methyl-CpG-binding protein 2) gene structure. The positions and frequencies of the most common mutations that are associated with Rett syndrome in females are indicated. b | The two main protein isoforms, MeCP2A (486 amino acids) and MeCP2B (498 amino acids), are produced by alternative splicing of the MECP2 transcript and differ in their N-terminal regions, which are encoded by exon 2 of the gene in the case of MeCP2A and exon 1 in MeCP2B. MBD, methyl-CpG binding domain; NLS, nuclear localization signal; poly(A), polyadenylation; TRD, transcriptional repression domain; X, stop codon.
HDAC inhibitors can activate both the death-receptor and intrinsic apoptotic pathways. The death-receptor pathway is triggered by the ligation of death receptors, which results in binding of adaptor proteins (FADD) and the recruitment and activation of membrane-proximal activator caspases (caspase-8). These caspases in turn activate downstream effector caspases (caspase-3). The intrinsic apoptotic pathway is activated by internal stresses that induce mitochondrial membrane disruption mediated by pro-apoptotic BCL2 proteins (BAX/BAK). HDAC inhibitors can induce the activation of the intrinsic apoptotic pathway, and anti-apoptotic BCL2 proteins inhibit mitochondrial membrane damage. HDAC inhibitors might still induce cell death in the absence of caspase activation after the production of reactive oxygen species (ROS). Asterix's denote components of the death pathways that are affected by HDAC inhibitors. Those components that inhibit cell death are shown in yellow, and those that promote death are in purple. BAK, BCL2 antagonist/killer protein; BAX, B-cell-associated X protein; BCL2, B-cell lymphoma 2; Cyto c , cytochrome c ; FADD, FAS-associated via death domain; HTRA2, high temperature requirement 2.
The function of non-histone proteins is also affected by acetylation (Ac)/deacetylation at specific lysine residues (K). Therefore, histone deacetylase (HDAC) inhibitors could have far reaching effects on multiple cellular processes. Two examples of non-histone acetylated proteins that might be affected by HDAC inhibition and involved in Huntington's disease and other polyglutamine repeat diseases are p53 and heat shock protein 90 (HSP90). a | p53 is acetylated under conditions of stress or HDAC inhibition by its cofactor CREB binding protein (CBP). Acetylation increases p53 activity by enhancing its stability and DNA binding activity. The end result is upregulated expression of p53-controlled genes, such as HTT. b | HSP90 is a chaperone that complexes with other chaperones, such as p23, to maintain correct conformational folding of is client proteins. HDAC6 deacetylates HSP90; inhibition of HDAC6 would result in hyperacetylated HSP90, which would be unable to interact with its co-chaperones and properly fold its client proteins. Inhibition of HDACs in this pathway would therefore lead to misfolded client proteins being targeted for degradation via the ubiquitin–proteasome system. HAT, histone acetyltransferase.
