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