P73 Shatil Amin March 27 th 2003. ..Content I.Structure and Function II.Regulation III.Is it...

P73

Shatil Amin

March 27th 2003

..Content

I. Structure and Function

II. Regulation

III. Is it involved in human cancers?

Early Findings

• Related to p53 tumor suppressor family

• Activated by DNA damage

• Mediates GI/S cell cycle arrest and apoptosis

• p73 is a transcription factor– Target genes: BAX apoptosis

WAF1(p21)(p21) Cell cycle arrest

DNA damage

p73

TARGET GENES Apoptosis Growth Arrest

P21 BAX

TP73 gene many different mRnas

• TP73 gene produces 2 classes of isoforms:– TAp73 isoform (transcriptionally active ,

apoptotic/growth inhibitory activity)

– ∆Np73 isoform

• ∆Np73 isoform lacks transcriptional activity

– Amino truncated

– Controlled by alternate promoter in the same gene

TP73 gene

Promoter 1 TAp73 isoforms (transcriptionally active…antiapoptotic and growth inhibitory activity)

Promoter 2 Np73 isoforms (NO transcriptional activity)

One gene, but two different proteins under the control of two distinct promoters

∆Np73 isoform is a dominant negative regulator of p53/TAp73

• Inhibits p53 and p73 – Competition for binding– Oligimerization

• Oncogenic properties !!

• p53 and TAp73 activate Np73 Dominant negative feedback loop

How does NAp73 inhibit

p53 and TAp73?

Tight regulation via dominant negative

feedback loop!

-Oligimerization

-Competition for binding

sites

**One gene: 2 products that are functionally antagonistic**

Involvement in Cancer?

• Ip36 locus commonly deleted in tumors– Is p73 a tumor suppressor ?

• Tp73 mutations rare in human primary tumors– Fewer than .5%

• Tp73 knockout mice don’t produce tumors

So…this evidence suggests it’s not a classical tumor suppressor

Complications in assessing the role of p73 in tumorigenesis

• Tp73 encodes two functionally opposing proteins: – An in vitro tumor suppressor (TAp73) and a putative

oncogene (∆Np73)

• Mutations may affect both TAp73 and ∆Np73 together– Deletions

• Abrogate both in vitro growth inhibitory and oncogenic activity…..(no net effect!)

• Need to discriminate between TAp73 and ∆Np73 isoforms !!

Experiments that Discriminate between TAp73 and ∆Np73

Variants of ∆Np73 (with anti-apoptotic activity) overexpressed in breast cancer cell lines, ovarian cancer, vulval cancer, and neuroblastic tumors

Np73 isoform

Ng SW et.al

Used RTPCR

• ∆Np73 expression strong adverse prognostic indicator in Neuroblastoma – No Np73 expression = 80% survival– Overexpress Np73 = none survived

• ∆Np73 function (blocking p53 and TAp73 mediated apoptosis) is key to development of tumor

• Mutation/inactivation of entire gene does not necessarily lead to cancer

• TA:∆N ratio is what may be altered in cancer !– Regulating respective promoters (mythylation)

Review of Main Points

• Tp73 gene: two functionally different proteins – TAp73: stimulates apoptosis and cell cycle arrest in

response to DNA damage– ∆Nap73: negative regulator of p53 and TAp73 with

oncogenic properties

• Enhanced expression of Np73 form associated with cancer

• Future Research: assessing TA:Np73 ratios in cancer