Differential expression of multiple isoforms of the ELKS mRNAs involved in a papillary thyroid carcinoma

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Abstract
A novel gene, ELKS, whose 5′ portion was fused to the RET gene, was found in a papillary thyroid carcinoma. A cDNA of this gene obtained from a human‐brain cDNA library revealed that it encoded a peptide of 948 amino acids, termed ELKSα. We identified four other isoforms, which encoded ELKSβ, ELKSγ, ELKSδ, and ELKSϵ proteins consisting, respectively, of 992, 720, 1088, and 1116 amino acid residues. Analysis of the gene structure revealed that the isoforms were generated by alternative splicing. Isoforms β, γ, δ, and ϵ all contain an optional exon (exon14a), but ELKSγ, ‐δ, and ‐ϵ lack exon 1b. ELKSγ lacks exons 3 to 6. ELKSδ and ‐ϵ lack exons 12 and 17; ELKSϵ contains an optional exon (exon 6a). Analysis by RT‐PCR suggested that ELKSα and ELKSβ mRNAs are abundant in the brain, ELKSδ and ELKSϵ mRNAs predominate in testis and thyroid, and ELKSϵ mRNA predominates in other tissues. To prove whether the fusion of different ELKS isoforms to RET (between ELKS coiled‐coil domains and the RET kinase domain) could produce chimeric proteins that could be autophosphorylated, we synthesized ELKSγ‐RET, ELKSδ‐RET, and ELKSϵ‐RET fusion proteins in vitro. Immunoblotting with anti‐ELKS, anti‐RET, and anti‐phosphotyrosine antibodies demonstrated that the chimeric proteins were constitutively phosphorylated at tyrosine residues, whereas native RET protein was not. These results indicate that the ELKS gene is alternatively spliced, and that every type of ELKS‐RET chimeric protein having oligomerization domains can activate RET's cytoplasmic tyrosine kinase.