UC San Diego

Drs. John and Ford reported in biomedicines that a variant transcript encoding receptor tyrosine kinase-like orphan receptor 1 (ROR1), namely ENST00000545203 or variant 3 (ROR1V3), was a predominant ROR1 transcript of neoplastic or normal cells in the Bioinformatic database, including GTEx and the 33 datasets from TCGA. Unlike the full-length ROR1 transcript, Drs. John and Ford deduced that ROR1V3 encoded a cytoplasmic ROR1 protein lacking an apparent signal peptide necessary for transport to the cell surface, which they presumed made it unlikely to function as a surface receptor for Wingless/Integrated (Wnt) factors. Moreover, they speculated that studies evaluating ROR1 via immunohistochemistry using any one of several anti-ROR1 mAbs actually may have detected cytoplasmic protein encoded by ROR1V3 and that anti-cancer therapies targeting surface ROR1 thus would be ineffective against “cytoplasmic ROR1-positive” cancers that express predominately ROR1V3. We generated lentivirus vectors driving the expression of full-length ROR1 or the ROR1v3 upstream of an internal ribosome entry site (IRES) of the gene encoding a red fluorescent reporter protein. Although we find that cells that express ROR1 have surface and cytoplasmic ROR1 protein, cells that express ROR1v3 neither have surface nor cytoplasmic ROR1, which is consistent with our finding that ROR1v3 lacks an in-frame initiation codon for ribosomal translation into protein. We conclude that the detection of ROR1 protein in various cancers cannot be ascribed to the expression of ROR1v3.