Relationships between CBP and p300 in Epigenetics and

Rubinstein-Taybin Syndrome

Yu Wang

Malvern College Qingdao, Qingdao, China, 266000

Keywords: CREBBP, Acetyltransferase, Tumors, Rubinstein-Taybi Syndrome, HDAC Inhibitors.

Abstract: Rubinstein-Taybi Syndrome (RSTS) is a rare genetic disorder and accounts for one case in every 125,000 to

100,000 cases. It involves poor development of facial features, distal limb abnormalities similar to broaden

thumbs and first toes and also mental retardations. RSTS could also extend to other malformations, including

neurological and renal malformations. There are many epigenetic mechanisms underlying RSTS. For

example, histone acetyltransferases (HATs), histone deacetylases (HDACs) and histone deacetylase inhibitors

(HDACI), transcription co-activators. CBP and p300 are enzymes that are encoded by CREB-binding protein

(CREBBP) gene and EP300 gene and with these two genes germline mutations, the amount of CBP and p300

could be abnormal, which leads to difficulty in transcription. CBP and p300 are histone acetyltransferases,

they are responsible for histone acetylation and increasing gene expression. However, with histone

deacetylase, they alter the shape of the chromosome and silence a specific region of the DNA to repress the

gene expression. This problem could be solved by histone deacetylase inhibitor. HDACI inhibits HDACs to

increase histone acetylation, which then increases gene expression. Nowadays, RSTS mutations could cause

tumor growths. It is considered that whether if CBP and p300 as histone acetylases could function to repress

tumor development. This paper discusses the underlying epigenetic mechanisms within RSTS and the

suitability of using it as a tumor repressor. It is found that CBP could interact with Sam68 and process to

prevent tumor formation, which Sam68 is a strong transcription repressor.

1 INTRODUCTION

Rubinstein-Taybi Syndrome (RSTS) is a congenital

developmental abnormality characterised by

psychomotor delay, facial abnormalities and also

relating to cardiac cancers, digestive and skin

malformations, especially tumor formations

(VanGils et al. 2021, Edward 2017, Boot et al. 2018).

This syndrome involves de novo heterozygous

mutations within genes and are analyzed mostly from

epigenetic aspects.

RSTS is caused by mutations of cAMP response

element-binding protein (CREBBP) and E1A-

associated protein p300 (EP300) gene. These two

genes are responsible for encoding two transcription

co-activators and they also act as histone

acetyltransferases (HATs), CREB-binding protein

(CBP) and p300. CBP and p300 are highly

homologous, which they are in the same HATs group

(Edward 2017).

Histone acetylation by CBP and p300 could

activate gene transcription. However, histone

deacetylase (HDACTs) are responsible in silencing

genes. Without gene expression, disease like RSTS

could occur. Scientists have found a therapeutic

method that histone deacetylase inhibitors (HDACI)

could inhibit the function of HDACs, which it is

responsible for reactivating genes (Edward 2017).

Tumors are associated with RSTS. Nevertheless,

the relationship between the phenotypes and

genotypes have not be clear (Boot et al. 2018). It is

suggested that CBP and p300 as histone

acetyltransferase could have the possibility to repress

tumor growth by preventing specific gene

uncontrolled transcription, which this paper will

discuss about. The mechanisms of different elements

considering Rubinstein-Taybi Syndrome have been

compared. In addition, these mechanisms are tested

in theory of repressing tumor development, which in

turns could bring another use of CBP and p300.

152

Wang, Y.

Relationships between CBP and p300 in Epigenetics and Rubinstein-Taybin Syndrome.

DOI: 10.5220/0011210700003444

In Proceedings of the 2nd Conference on Artiﬁcial Intelligence and Healthcare (CAIH 2021), pages 152-156

ISBN: 978-989-758-594-4

2 OVERVIEWS OF RUBINSTEIN-

TAYBI SYNDROME

2.1 Symptoms and Phenotypes

Considering Rubinstein-Taybi

Syndrome (RSTS)

Rubinstein-Taybi Syndrome is a rare congenital

genetic disorder characterized by abnormal

development of physical features such as short

stature, facial abnormalities, broaden thumbs and first

toes, however, facial dysmorphism and distal limb

abnormalities are reported the most common (Figure

1) (Boot et al. 2018). Facial dysmorphism only occurs

to be a feature late in the childhood, owing to the fact

that phenotype is evolutionary, and the adult’s

phenotype could be distinct from the appearance of

them as a new born. Broaden thumbs could be a type

of distal limb abnormality, although it is not always

constant, it is found to be within 69% to 97% of the

total RSTS cases (VanGils et al. 2021). In addition,

disabilities like psychomotor delays, mental

retardations have also been expressed by RSTS

patients. RSTS is reported to occur in one case per

100,000 and 125,000 births (Hennekam 2006), and

children carrying a pathogenic varient in p300 could

cause a pre-eclampsia and hypertension during

pregnancy (VanGils et al. 2021). Several

malformations are discovered, including cardiac,

digestive and skin malformations, also an increasing

number of patients have developed benign and

malignant tumors, with meningiomas and

pilomatricomas the most common (Boot et al. 2018).

Figure 1: A patient with mild RSTS.

Phenotype like facial dysmorphism with long

eyelashes, prominent beaked nose, posteriorly rotated

ears and also strabismum are expressed. Figure 1 (A

and B) including broadened thumbs and first toes, and

broadened terminal phalanges in Figure 1(C and D)

(VanGils et al. 2021, Bartsch et al. 2002).

2.2 Genotypes and Causes Associated

with Rubinstein-Taybi Syndrome

Rubinstein-Taybi Syndrome is an autosomal

dominant trait and caused by de novo heterozygous

mutations within the genes that encode the CREB-

binding protein (CBP) and EP300 (VanGils et al.

2021, Edward 2017). CREB-binding protein

(CREBBP) gene encodes CBP and it is located on

chromosome 16p13.3, with this gene germline

mutation, it leads to Rubinstein-Taybi Syndrome type

I. RSTS I accounts for 50% to 60% of the total RSTS

cases and CREBBP gene germline mutation is

associated with 106 point mutations and 21 deletions.

On the other hand, for Rubinstein-Taybi Syndrome

type II, which is caused by E1A-binding protein

(EP300 or p300) gene mutation on chromosome

22q13.2 (Boot et al. 2018). RSTS II accounts for

approximately 10% of the total RSTS cases. EP300

gene encodes the protein EP300/p300, this gene

germline mutation includes 27 point mutations, 6

exonic deletions and 1 whole-gene deletion (Edward

2017). Moreover, severe RSTS could be caused by

chromosome 16p13.3 deletion syndrome, and the

deleted DNA length expands approximately from 40

kb to >3 Mb (Boot et al. 2018; Bartsch et al. 2006).

3 EPIGENETICS WITHIN

RUBINSTEIN-TAYBI

SYNDROME

3.1 CBP and p300: Transcription

Co-activators

CBP and p300 are highly homologous and are the

only two members within the KAT3 family. So most

of the missense mutations are sited on the Lysine

Acetyltransferase domain (KAT domain) for both

CREBBP gene and EP300 gene (VanGils et al. 2021).

As CREB-binding protein (CBP) and p300 are

transcription co-activators, they interact with KAT

domain to activate transcription. Transcription co-

activators are proteins that bind with transcription

factors to start the transcription process. For example,

cAMP response element-binding protein (CREB) as

a transcription factor will need to recruit CBP in order

to start transcription. This involves interactions

Relationships between CBP and p300 in Epigenetics and Rubinstein-Taybin Syndrome

153

between activation domain of transcription factors

and CBP’s multiple protein-protein interaction

domains, including the transcriptional adaptor zinc-

binding (TAZ) domain. The CH1 and CH3 (cysteine-

/histidine-rich region) where zinc-binding domains

are localized regions regulate most of the CBP’s

protein-protein interactions (Edward 2017).

It is proposed that the activation of CREB towards

its targeted promoters is based on the phosphorylation

of CREB by Protein Kinase A (PKA) at Serine-133

in response to advance levels of cAMP. Activation by

protein kinase A results in CREB binding with the

cAMP response element, because PKA caused CREB

phosphorylation, which this action stimulates

interactions with many transcription factors and

allows it to recruit transcription co-activators, such as

CBP. Then CBP can further recruit RNA polymerase

II and promot transcription of targeted genes (Everett

et al. 2009).

To sum up, CBP as a transcription coactivator is

responsible for CREB-dependent transcriptional

activation, the interactions between CBP and CREB

depends on CREB phosphorylation at Serine 133

located within the kinase-inducible domain (KIK)

(Edward 2017). As shown in Figure 2.

Figure 2: Function of CREB and CBP.

Figure 2 shows protein kinase A causes

phosphorylation of the CREB transcription factor and

recruit transcription co-activator CREB-binding

protein (CBP), which then leads to CBP further

recruit RNAPII to activate transcription process

(Everett et al. 2009).

3.2 CBP and p300: Histone Acetylation

CREB-binding protein and EP300 are considered

histone acetyltransferase (HATs). HATs are enzymes

responsible for adding an acetyl group acetyl CoA to

the lysine residues on the histone tails and form ε-N-

acetyl lysine (Annabelle et al. 2012). Bromodomain

could recognize acetylated lysine residues, the KIT11

domain is a lysine acetyltransferase able to transfer

acetyl groups on to histone N-terminals (Edward

2017). This process activates transcription by altering

the shape of the chromatin via acetylation directly to

the histone N-terminal tails of histones H2A, H2B,

H3, and H4. This includes H3K14, H3K18, H3K27,

and H3K56 shown by Figure 4A and B (Edward

2017). Acetylation by HATs reduces the attractions

between the positive histone and the negative DNA,

owing to the fact that acetylation decreases the

positive charge on the histone and they then

developed repulsion. Therefore, the chromatin would

be loosen/relaxed. Therefore, RNA polymerase could

function to copy information and transcribe.

Figure 3: Histone Acetylation.

Figure 3 shows histone acetylation on the lysine

residue K14 (H3K14 acetylation) on histone tail.

Figure 4: Function of Histone Acetylation.

Acetylation involved in relaxing the structure of

the chromosome in order to successfully start the

transcription by the RNA polymerase (Annabelle et

al. 2012).

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4 THERAPEUTIC METHODS

FOR RUBINSTEIN-TAYBI

SYNDROME

4.1 Histone Deacetylase (HDACs) and

Histone Deacetylase Inhibitors

(HDI)

Epigenetic mechenisms are reversible, so epigenetic

markers can be regulated by Histone acetylation and

histone deacetylation by keeping a balance between

them. Histone deacetylase (HDACs) are enzymes

responsible for removing the acetyl group on the

histone tail, in contadiction to the HATs, it increases

the positive charge on the histone, causing the

negative DNA strand and histone binding more

strongly together. This repressed the gene expression

by preventing RNA polymerase to assist the forming

of mRNA strand. The gene is silenced, which then

stops the transcription process.

The statement mentioned above states that

mutations within CREB-binding protein gene and

EP300 gene germline mutation caused Rubinstein-

Taybi Syndrome. To be more specific, deletion of one

of the copies of the gene could lead to trancribing an

abnormal amount of the normal histone acetylase

enzymes. This is thought to be correlated with

HDACs causing the gene to be silenced. Shown by

Figure 4, the arrow pointing from the right to the left

represents HDACs causing the gene the be repressed.

Scientists reported that a whole gene deletion of

CREBBP gene and also the truncating mutation was

rescued by HDAC inhibitors (Lopez-Atalaya et al.

2012). HDAC inhibitors (HDI) are compounds that

inhibits the function of HDACs so that it allows the

lysine residues on the histone tails to be acetylased

(Figure 5). Some drugs are discovered of having the

ability to reactivate silenced genes, such as HDAC

inhibitors: SAHA-suberoylanilide, VPA-valporic

acid and TSA-trichostatin A (Edward 2017).

Figure 5: HDAC Inhibitors.

With the help of HDAC inhibitors HDAC will not

cause changes to the shape of the chromosome, which

means that the gene cannot be silenced.

4.2 Tumors Developed from RSTS

CREBBP and EP300 gene mutations have been found

in many benign and malignant tumors (Boot et al.

2018). Nevertheless, the pattern of genotype and

phenotype correlation is unclear (Hennekam 2006). It

is reported that most of the tumors occur within the

head (Miller et al. 1995). They form around parts like

large deletion or duplication group, in the groups with

nonsense or frameshift mutations, splice site

mutations and missense mutations groups (Boot et al.

2018). A total of 115 patients with RSTS, 132 tumors

are found, and most are neural crest derived tumors,

such as neuroblastoma, meningioma and

pheochromocytoma (VanGils et al. 2021).

CBP and p300 are considered cofactors for

oncoproteins or for tumor suppressor proteins

(VanGils et al. 2021). p53 as an example of tumor

suppressor protein binds to a specific region of the

DNA and stimulate the production of p21 protein,

then p21 interact with cell division-stimulating

protein and form a complex to prevent the cell to enter

the next stage of cell division (National Center for

Biotechnology Information (US), 1998). CBP as

transcription cofactor also have tumor suppressing

ability. CBP’s CH3 domain binds with several viral

oncoprotein and Sam68 (a RNA-binding protein).

CBP interacts with Sam68, because Sam68 acts as a

strong transcription repressor (Hong et al. 2002).

5 CONCLUSIONS

This paper stresses the effect of CBP and p300

responsible for the mechanism of histone acetylation

under Rubinstein-Tyabin Syndrome both from the

genotype and phenotype aspects. It is discovered that

gene germline mutations of the CREB-binding

protein (CREBBP) gene and EP300 gene would be a

prime factor of causing RSTS, which leads to the

improper number of the coded CBP and p300. Gene

expression results from histone acetylation, therefore,

histone acetyltransferase (HATs) CBP and p300 are

responsible for the normal regulating of the genes.

However, interactions between histone deacetylase

(HDACs) and histone deacetylase inhibitors

(HDACI) would regulate gene silencing involved in

RSTS. Which then suggests that tumor developments

based on gene mutations could be repressed by HATs

and HDACs. The uncontrolled cell divisions within

Relationships between CBP and p300 in Epigenetics and Rubinstein-Taybin Syndrome

155

RSTS could be possibly regulated by alternating the

epigenetic mechanisms underlying it. For instance,

CBP would interact with Sam68 to suppress DNA

transcription, therefore, over amount cell division

causing a tumor would be controlled. In addition, it is

mentioned that the CREBBP gene and EP300 gene

mutations had been found in many of the tumors, but

the underlying patterns of the genotype and

phenotype is unclear. The speculation could be CBP

mutation would cause irregular interactions with

Sam68 which lead to tumor developments, as it is

considered that changes in the structure of a specific

transcription coactivator like CBP would alter the

binding site with Sam68 to inaccurately repress tumor

developments. The ways of repressing tumors might

then encourage acknowledging many other

transcription factors and inhibitors’ functions.

ACKNOWLEDGEMENTS

I would like to gratefully thank you professor Zhibin

Wang for your helpful advises on my topic and also

the epigenetics study this summer, it is an honor to be

in your class.

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