Loss of heterozygosity with acquisition of homozygous KIT-activating mutation promotes gastrointestinal stromal tumor progression

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Background: Gastrointestinal stromal tumors (GISTs) originate from interstitial cells of Cajal and represent the most common mesenchymal tumor of GI tract. The activating mutations of KIT and platelet-derived growth factor receptor α (PDGFRA) have been demonstrated in 88% and 6% of GISTs respectively. Multiple genetic events are involved in tumor initiation and progression. Activation of an oncogene can result from a single hit of gain-of-function mutation, but two hits leading to loss of heterozygosity (LOH) are necessary for inactivation of a tumor suppressor gene for the initiation of neoplasia. Little is known about the significance and mechanism of LOH of oncogenes in tumor progression. The LOH of activating-oncogenes is not infrequent, i.e. in more than 8% of GIST. These GIST clones with LOH prevail and dominate, strongly suggestive of proliferative and metastatic advantages. However, the significance and mechanisms of LOH acquiring homozygous activating-oncogene mutation remain unknown. Methods: Genomic DNA and cDNA sequencing analysis of KIT, single-nucleotide polymorphisms (SNPs), immunohistochemistry of GISTs clones, and 3D structural analysis of the mutated KIT. Results: Real-time genetic studies in GIST show initial development of overexpression of KIT without mutation, to coexistence of various heterozygous-activating mutations; eventually, one clone dominates. Comparisons within the same patient show that clonal evolution from heterozygous to LOH acquiring homozygous (diploid) KIT-activating mutation results in dominance of the latter, with augmented KIT signaling and doubling of mitotic figures. Using SNPs for allelotyping, we found that mitotic nondisjunction, rather than mitotic recombination, represents an important mechanism of the second hit. Conclusion: GIST clones with LOH acquiring homozygous KIT-activating mutation gain a selective advantage over the heterozygous counterpart. Mitotic nondisjunction, in addition to the commonly recognized mitotic recombination, is an important and perhaps the primary mechanism of LOH acquiring homozygous KIT-activating mutation during GIST progression.

No significant financial relationships to disclose.

Carbonic anhydrase II gene expression in cell lines from human pancreatic adenocarcinoma

Current evidence suggests that carbonic anhydrase II (CA II) is produced by pancreatic duct cells but not by pancreatic acinar or islet cells. The aim of this study was to determine whether CA II homologous RNA and CA II immunoreactive protein are produced by cell lines established from human pancreatic adenocarcinomas. A 1.7-Kb CA II homologous RNA was detected in poly(A+) RNA isolated from normal human pancreas, normal human liver, and to varying degrees in the cell lines examined. The CA II immunoreactivity corresponding to approximately 30 kD (consistent with the established molecular mass of CA II) was also detected by immunoblotting in normal human pancreas, normal human liver, and some of the cell lines. We also found that the levels of CA II homologous RNA increase in the pancreatic adenocarcinoma cell lines following treatment with the differentiating agent, retinoic acid.