[1043] Rb and PTEN Deletions and p53 Mutations Are Common in Prostatic Small Cell Carcinoma

Akshay Sood, Hameed Rahimi, Wenle Wang, Nilesh Gupta, Jonathan I Epstein, Angelo M De Marzo, Tamara L Lotan. Johns Hopkins Medical Institutions, Baltimore, MD

Background: Small cell carcinoma (SCC) of the prostate is a rare subtype with an aggressive clinical course. Previously, we examined expression of retinoblastoma (Rb), PTEN, and p53 proteins by immunohistochemistry (IHC) in prostatic SCC and found Rb protein loss in 90% of prostatic SCC cases, and loss of PTEN and expression of p53 (suggestive of underlying p53 mutation) in more than half of cases. Here, we use a combination of direct sequencing and copy number variant (CNV) analysis to assess Rb, PTEN and p53 gene status in a subset of these prostatic SCC cases.
Design: Direct sequencing for exons 4, 5, 7 and 8 of p53 was performed on DNA purified from 7 prostatic SCC samples using established protocols. Sequence variants reported here were confirmed on both the forward and reverse sequencing reactions, as well as in an independent PCR. Predicted deleterious effect was assessed using the IARC database. DNA from these same 7 samples, plus one additional prostatic SCC sample was also subjected to CNV analysis using the nCounter Cancer Copy Number Assay (nanoString Technologies).
Results: CNV analysis showed Rb loss in 75% (6/8) of prostatic SCC samples, of which 83% (5/6) showed hemizygous loss and 17% (1/6) of cases showed homozygous loss at the Rb locus. Of the cases showing Rb gene loss, 100% (6/6) showed Rb protein loss by IHC. Of the cases without Rb gene loss by CNV analysis, 50% (1/2) showed Rb protein loss by IHC. For PTEN, 38% (3/8) of cases showed allelic loss, with 66% (2/3) showing hemizygous and 33% (1/3) showing homozygous loss. 100% (3/3) of cases with PTEN allelic loss showed PTEN protein loss by IHC. Similar to what we have reported previously for acinar prostatic carcinoma, 60% (3/5) of cases without PTEN allelic loss showed PTEN protein loss. Finally, 57% (4/7) of prostatic SCC cases showed p53 mutation by sequence analysis, including two cases with a well-described deleterious missense mutation in the DNA binding domain (ex 5: c.524G>A), another deleterious mutation in a splice site (in 8: c.919+1G>A) and one novel predicted deleterious mutation (ex 7 c.695,696 TC>AA). Of mutated cases, 75% (3/4) showed p53 expression by IHC compared to only 30% (1/3) of cases without detectable mutations.
Conclusions: Loss of Rb and PTEN tumor suppressors by deletion, and mutation of p53 are common events in prostatic SCC and lesions at these loci correlate with the results of validated IHC assays. Similar to previous studies of lung SCC, we found Rb gene loss in the majority of prostatic SCC cases, suggesting that this event is critical to the development of SCC in multiple organ systems.
Category: Genitourinary (including renal tumors)

Tuesday, March 5, 2013 9:30 AM

Poster Session III # 66, Tuesday Morning

 

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