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Phosphorylation of pRb: mechanism for RB pathway inactivation in MYCN ‐amplified retinoblastoma

2017; Wiley; Volume: 6; Issue: 3 Linguagem: Inglês

10.1002/cam4.1010

2045-7634

Kathryn G. Ewens, Tricia R. Bhatti, Kimberly Moran, Jennifer Richards‐Yutz, Carol L. Shields, Ralph C. Eagle, Arupa Ganguly,

Head and Neck Surgical Oncology

Abstract A small, but unique subgroup of retinoblastoma has been identified with no detectable mutation in the retinoblastoma gene ( RB 1) and with high levels of MYCN gene amplification. This manuscript investigated alternate pathways of inactivating pR b, the encoded protein in these tumors. We analyzed the mutation status of the RB 1 gene and MYCN copy number in a series of 245 unilateral retinoblastomas, and the phosphorylation status of pR b in a subset of five tumors using immunohistochemistry. There were 203 tumors with two mutations in RB 1 ( RB 1 −/− , 83%), 29 with one ( RB 1 +/− , 12%) and 13 with no detectable mutations ( RB 1 +/+ , 5%). Eighteen tumors carried MYCN amplification between 29 and 110 copies: 12 had two ( RB 1 −/− ) or one RB 1 ( RB 1 +/− ) mutations, while six had no mutations ( RB 1 +/+ ). Immunohistochemical staining of tumor sections with antibodies against pR b and phosphorylated Rb (ppRb) displayed high levels of pR b and ppRb in both RB 1 +/+ and RB 1 +/− tumors with MYCN amplification compared to no expression of these proteins in a classic RB 1 −/− , MYCN ‐low tumor. These results establish that high MYCN amplification can be present in retinoblastoma with or without coding sequence mutations in the RB 1 gene. The functional state of pR b is inferred to be inactive due to phosphorylation of pR b in the MYCN ‐ amplified retinoblastoma without coding sequence mutations. This makes inactivation of RB 1 by gene mutation or its protein product, pR b, by protein phosphorylation, a necessary condition for initiating retinoblastoma tumorigenesis, independent of MYCN amplification.