Colorectal Adenocarcinomas Harboring ALK Fusion Genes : a Clinicopathologic and Molecular Genetic Study of 12 Cases and Review of the Literature

This study determined the frequency, clinicopathologic, and genetic features of colorectal carcinomas driven by oncogenic fusions of the anaplastic lymphoma kinase gene (ALK). Out of 8150 screened tumors, 12 (0.15%) were immunohistochemically ALK-positive with D5F3 antibody. These cancers harbored CAD-ALK (n=1), DIAPH2-ALK (n=2), EML4-ALK (n=2), LOC101929227-ALK (n=1), SLMAP-ALK (n=1), SPTBN1-ALK (n=4) and STRN-ALK (n=1) fusions as detected by an RNA-based NGS assay. ALK fusion carcinomas were diagnosed mostly in older patients with a 9:3 female predominance (median age, 72 years). All tumors, except a rectal one, occurred in the right colon. Most tumors were stage T3 (n=7) or T4 (n=3). Local lymph node and distant metastases were seen at presentation in 9 and 2 patients. These tumors showed moderate (n=6) or poor (n=3) glandular differentiation, solid medullary growth pattern (n=2), and pure mucinous morphology (n=1). DNA mismatch repair deficient phenotype was identified in 10 cases. Tumor-infiltrating lymphocytes were prominent in 9 carcinomas. In 4 carcinomas, tumor cells showed strong, focal (n=3) or diffuse PD-L1 immunoreactivity. CDX2 expression and loss of CK20 and MUC2 expression were frequent. CK7 was expressed in 5 tumors. Four patients died of disease within 3 years and 7 were alive with follow-up ranging from 1–8 years. No mutations in BRAF, RAS and genes encoding components of PI3K-AKT/MTOR pathway were identified. However, one tumor had a loss-of-function PTEN mutation. Aberration of p53 signaling, TP53 mutations and/or nuclear accumulation of p53 protein was seen in 9 cases. ALK fusion colorectal carcinomas are a distinct and rare subtype of colorectal cancers displaying some features of mismatch repair-deficient tumors