PTPRH mutations in NSCLC regulate EGFR phosphorylation

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The dysregulation of receptor tyrosine kinases (RTK) has garnered plenty of interest within the cancer field, and attention has begun to turn to phosphatases regulating RTK behavior. Under normal cellular conditions, protein tyrosine phosphatases remove phosphate groups from tyrosine residues, thus maintaining signaling homeostasis. In whole genome sequencing primary mouse mammary tumors from the polyoma virus middle T antigen (PyMT) mouse model, we found a mutation in the protein tyrosine phosphatase receptor type H (Ptprh) gene. Targeted resequencing of 45 mouse tumors showed a conserved heterozygous or homozygous mutation present in 80% of tumors. This C>T mutation results in a valine to methionine shift within one of the fibronectin domains of PTPRH. Previous literature has shown interactions between PTPRH and epidermal growth factor receptor (EGFR). To determine the relevancy of PTPRH mutations in human cancer, data from The Cancer Genome Atlas (TCGA) was analyzed and revealed PTPRH mutations in five percent of non‐small cell lung cancer (NSCLC) patients. Moreover, patients with a mutation in PTPRH were mutually exclusive from those with mutation or amplification of EGFR. We hypothesize a mutation in PTPRH results in a failure of PTPRH to dephosphorylate EGFR, resulting in inappropriate maintenance of downstream signaling pathways important for proliferation and evading apoptosis. Since NCSLC patients with EGFR mutations are successfully treated with tyrosine kinase inhibitors (TKI), we also hypothesize tumors with a mutation in PTPRH will be sensitive to TKIs. In support of this, we demonstrated mouse tumors with a mutation in Ptprh had increased phosphorylated EGFR (pEGFR). Furthermore, CRISPR mediated knockout of PTPRH in H23 NSCLC cells leads to increased pEGFR. Pathway signature analysis applied to microarray gene expression data from the Breast TCGA dataset (due to low sample size in the NSCLC dataset), and single sample gene set enrichment analysis applied to RNA sequencing data from the NSCLC TCGA dataset both predicted an increase in PI3K and AKT activity. This suggested the EGFR residue targeted by PTPRH was tyrosine 1197. Western blots on Ptprh mutant mouse tumors confirmed increased levels of pAKT. Additionally, immunohistochemistry for pEGFR 1197 revealed increased staining in mouse tumors with a mutation in Ptprh, with sub‐cellular location in the nucleus rather than the membrane. To determine whether TKIs may be an effective treatment for NSCLC patients who harbor a PTPRH mutation, H1155 and H2228 NSCLC cell lines with PTPRH mutations in the fibronectin and phosphatase domains respectively, were subjected to a dose response curve with the TKI osimertinib. These lines show significant growth differences as compared to the negative control cell line A427. While more work is needed to elucidate the role of mutant PTPRH in NSCLC, preliminary data suggests mutant PTPRH fails to dephosphorylate EGFR, and patients with a mutation in PTPRH may benefit from TKI therapy.