HER2 Overexpression in Retinoblastoma: A Potential Therapeutic Target?

Emerging therapeutic targets for retinoblastoma

INTRODUCTION

Retinoblastoma (Rb) is an early childhood intraocular tumor of the retina and is managed by multimodal therapeutic approaches. Recent advanced targeted delivery of chemotherapeutic drugs to the eye has improved the possibility of globe salvage. However, enucleation is inevitable for advanced and recurrent Rb. The cumulative knowledge of identification of newer molecular biology tools, exosomal cargo, role of cancer stem cells (CSCs), and its microenvironment in the progression of the diseases warrants a relook at the traditional treatment protocol and explore the feasibility of targeted therapies.

AREAS COVERED

This review covers Rb pathobiology, novel molecular-targeted therapeutics, and strategies targeting Rb CSCs and provides an update on potential therapeutic targets such as second messengers and exosomal cargo.

EXPERT OPINION

The emergence of early diagnosis and multimodality treatment protocols have significantly improved the clinical outcome of children with advanced Rb; however, the problem of tumor recurrence has not yet been overcome. Improved understanding of the molecular pathways, identification, and characterization of CSCs opens up new targeted therapy approaches. The contemporary evidence from other fields shows promising evidence that combining conservative treatment modalities with targeting therapies specific for CSCs in clinical practice is essential for achieving high globe salvage rate in Rb patients.

In situ analysis of Her2 DNA and RNA in retinoblastoma and adjacent retina

Retinoblastoma (RB) is an ocular tumor of early childhood. Current treatments attempt to preserve visual function, but may spare chemoresistant tumor cells. One potential therapeutic target for RB is HER2, (ERBB2), expressed in RB in truncated form. In this study, we tested the hypothesis that Her2 DNA and RNA are expressed in RB tumors and adjacent retina. We examined 24 human RB tumors as well as normal-appearing adjacent retinal tissues for Her2 DNA and RNA expression by in situ hybridization. We also examined 28 RB tumors for HER2 protein immunoreactivity. 21/22 RB tumors expressed Her2 DNA and 14/19 tumors expressed Her2 RNA. In 17 paired cases, there were three cases in which Her2 DNA was detected, but not RNA. We also saw Her2 RNA signal in six instances of "normal" adjacent retinal tissue. Heterogeneous HER2 protein expression in specific tumor regions also was confirmed by quantitative HER2 immunohistochemistry. In summary, Her2 DNA and RNA are expressed in many RB tumors, and in some adjacent ocular tissues, with hetereogenous protein expression throughout. These results may provide important insights regarding RB tumor progression, and drug targeting approaches designed to spare the eye, preserve vision and improve quality of life for RB patients.

Targeting tyrosine kinases for treatment of ocular tumors

Uveal melanoma is the most common intraocular primary malignant tumor in adults, and retinoblastoma is the one in children. Current mainstay treatment options include chemotherapy using conventional drugs and enucleation, the total removal of the eyeball. Targeted therapies based on profound understanding of molecular mechanisms of ocular tumors may increase the possibility of preserving the eyeball and the vision. Tyrosine kinases, which modulate signaling pathways regarding various cellular functions including proliferation, differentiation, and attachment, are one of the attractive targets for targeted therapies against uveal melanoma and retinoblastoma. In this review, the roles of both types of tyrosine kinases, receptor tyrosine kinases and non-receptor tyrosine kinases, were summarized in relation with ocular tumors. Although the conventional treatment options for uveal melanoma and retinoblastoma are radiotherapy and chemotherapy, respectively, specific tyrosine kinase inhibitors will enhance our armamentarium against them by controlling cancer-associated signaling pathways related to tyrosine kinases. This review can be a stepping stone for widening treatment options and realizing targeted therapies against uveal melanoma and retinoblastoma.