ROS-1 rearrangements with at least 12 different partner proteins have been identified in a small subset of lung non-small cell carcinomas (1–2%), which shows susceptibility to tyrosine kinase inhibitors (TKIs) similar to ALK rearranged tumors.  ROS-1 is considered a oncogene found on chromosome 6.  The exact mechanism of activation of this gene protein product with the various gene rearrangement partners is not understood.  The protein function is similar to that of the ALK family, which is why this mutation was studied for possible response to ALK inhibitors (crizotinib).

 

Recently, ROS-1 mutated tumors have been approved for TKI therapy with identification of a rearrangement by FISH analysis.  Like ALK, ROS-1 FISH utilizes a break apart probe to identify the presence of a gene rearrangement.  Other successful modalities for identification of ROS-1 rearrangements include ‘next generation’ sequencing (NGS) and immunohistochemistry.

 

Immunohistochemistry (IHC) has been studied as an alternative to FISH as a screening modality.  Based on multiple studies, the sensitivity of IHC appears to be near 100% with the specificity of at least 92%.  These studies were performed using the D4D6 rabbit monoclonal antibody clone (Cell Signaling Technology, Danvers, Massachusetts).

Unlike ALK, there is no known normal tissue counterpart which can be used as a control.  Therefore, known ROS-1 positive tumors or cell lines  (HCC78 cell line with the SLC34A2-ROS1 rearrangement) are generally used.  ROS-1 expression is cytoplasmic with described expression ranging from finely granular to globular cytoplasmic staining and membranous staining.  No consensuses has been established as to the minimal threshold of positivity.

 

Possible interpretation pitfalls include weak staining of type II pneumocytes and alveolar macrophages along with osteoclast-type giant cells in bone biopsies.  Like any immunostain, contextual evaluation is critical.