The role of KRAS splice variants in cancer biology

Abstract

The three mammalian RAS genes (HRAS, NRAS and KRAS) encode four proteins that play central roles in cancer biology. Among them, KRAS is mutated more frequently in human cancer than any other oncogene. The pre-mRNA of KRAS is alternatively spliced to give rise to two products, KRAS4A and KRAS4B, which differ in the membrane targeting sequences at their respective C-termini. Notably, both KRAS4A and KRAS4B are oncogenic when KRAS is constitutively activated by mutation in exon 2 or 3. Whereas KRAS4B is the most studied oncoprotein, KRAS4A is understudied and until recently considered relatively unimportant. Emerging work has confirmed expression of KRAS4A in cancer and found non-overlapping functions of the splice variants. The most clearly demonstrated of these is direct regulation of hexokinase 1 by KRAS4A, suggesting that the metabolic vulnerabilities of KRAS-mutant tumors may be determined in part by the relative expression of the splice variants. The aim of this review is to address the most relevant characteristics and differential functions of the KRAS splice variants as they relate to cancer onset and progression.

Keywords: KRAS; KRAS4A; KRAS4B; alternative splicing; glycolysis; oncogene; oncoprotein.

FIGURE 1

Three RAS genes encode four RAS proteins. (A) The four RAS isoforms have nearly identical GTP/GDP binding (G) domains (grey) (aa1–165) but distinct hypervariable (HVR) regions (aa166–188/189, not to scale), which direct membrane trafficking. Color coding of the HVR distinguishes each RAS isoform (HRAS in orange, NRAS in yellow, KRAS4A in blue, and KRAS4B in green). Sequence and posttranslational modifications of the HVR of KRAS4A and KRAS4B are indicated by colors highlighting the residues that are palmitoylated (red), phosphorylated (green) or farnesylated (blue). Basic residues in the HVRs of KRAS4A and KRAS4B are indicated in bold. (B) The two KRAS isoforms are splice variants utilizing alternative 4th exons. Oncogenic mutations occur in exons 1 or 2 (*) such that when KRAS is mutant, both splice variants give rise to oncogenic proteins.

FIGURE 2

KRAS4A regulates hexokinase 1 catalytic activity. Hexokinase 1 (HK1) is resident on the outer mitochondrial membrane (OMM) owing to an N-terminal targeting sequence. KRAS4A undergoes a cycle of palmitoylation/depalmitoylation on cysteine 180 that drives association with the plasma membrane or OMM, respectively. KRAS4A interacts with HK1 on the OMM in a GTP-dependent fashion and stimulates HK1 activity by limiting the allosteric feedback inhibition physiologically mediated by glucose-6-phosphate.