ROS1 – Antibody Lexicon

Synonyms: Repressor of Silencing 1

by Jan Klos

Background

ROS1 protein (encoded by the gene ROS1 on 16q22.1) is a receptor of tyrosine kinase. It was first identified in 1986. It shows structural similarity to the anaplastic lymphoma kinase (ALK). It is an integral membrane protein with tyrosine kinase activity and may function as a growth and differentiation factor receptor. Presence of the rearrangements of ROS1 gene predicts response to targeted therapy in cancer.

Positive staining in tumors

Since the initial description in glioblastoma, ROS1 fusions have been detected in a range of malignancies including inflammatory myofibroblastic tumor, cholangiocarcinoma, ovarian cancer, gastric cancer, colorectal cancer, angiosarcoma, spitzoid melanoma and non-small cell lung carcinoma NSCLC (2007). ROS1 protein expression in lung cancer cells detected by IHC (moderate and strong cytoplasmic reactivity) shows 100% sensitivity and 92% specificity for ROS1 rearrangements detected by FISH.
Positive immunohistochemical staining may be also seen in reactive epithelial proliferations, including areas of type II pneumocyte hyperplasia and bronchiolar metaplasia. The expression is occasionally seen in non-neoplastic alveolar epithelium at the periphery of the tumor or in subpleural pneumocytes. Expression of ROS1 protein is also detected in multinucleated giant cells of osteoclast-type.
Lung carcinomas with ROS1 rearrangements commonly show solid and/or papillary growth pattern, mucinous and/or signet ring cell features, and frequent psammomatous calcifications.
Lung adenocarcinomas show a number of oncogenic alterations and some of them will predict response to targeted therapies.

EGFR kinase domain mutations occur in 10–20% and predict response to EGFR tyrosine kinase inhibitors (TKIs).
Rearrangements involving the anaplastic lymphoma kinase (ALK) gene occur in about 2-5% and predict response to the multitargeted kinase inhibitor crizotinib.
Rearrangements involving the ROS1 oncogene are found in 1-2% of non-small cell lung carcinomas and are also associated with response to crizotinib.
KRAS mutations occur in approximately 25% cases, and typically are mutually exclusive with EGFR and ALK alterations, and are associated with a lack of response to EGFR TKIs.
NTRK………………………..

Staining pattern is variable depending on translocation partner.

In some cases the staining may also be positive due to aneuploidy leading to aberrant expression.

Control tissue:

Application

The D4D6 antibody can detect full length ROS1 protein. It is also reactive in some macrophages/giant cells and in non-neoplastic pneumocyte proliferations. In particular, care must be taken not to over-interpret the significance of ROS1 expression restricted to reactive pneumocytes at the periphery of the tumor since it could be mistaken for lepidic tumor growth. Although typically weak, in rare cases ROS1 protein expression can be strong in this reactive compartment. In metastatic bone lesions, the osteoclast-type giant cells express ROS1 may cause some confusion.

Faint (1+) staining in rare tumor cells is not uncommon, and is most often not associated with a ROS1 translocation. In cases with moderate (2+) and strong (3+) positive cytoplasmic staining. ROS1 immunohistochemistry is 100% sensitive and 92% specific for its translocations.
ROS1 staining patterns may vary due to different intracellular localizations depending on different partners of ROS1 fusions.
Immunohistochemistry for ROS1 protein is an excellent screening tool for rearrangements of the ROS1 gene in tumors and should be used in panel for diagnostic work up of all cases of non-squamous /non-small cell lung carcinoma, which are negative for mutations in EGFR/ALK/KRAS/BRAF.
All positive immunohistochemical staining for ROS1 should trigger confirmatory FISH or NGS testing.

by Jan Klos

Background

Positive staining in tumors

Application

Selected references