ABSTRACT
PTC-1, the predominant form of PTC oncogene in human papillary thyroid carcinoma, encodes a fusion protein containing the N-terminus of H4 (D10S170) fused 5' to the ret tyrosine kinase domain. Accordingly, the PTC-1 expression is driven by the H4 gene promoter. Our study showed that H4 is expressed in various human tissues, including thyroid. Furthermore, we have localized the transcriptional start sites of H4 to a region 100 to 190 bp upstream of the translation initiation site (ATG) by primer extension assay, and the H4 promoter to a region within 259 bp upstream of the ATG site by luciferase assay. Interestingly, protein sequence analysis indicated a potential coiled-coil domain in the N-terminal region of H4. Indeed, oligomerization was demonstrated by an in vitro assay with recombinant proteins containing this region. As dimerization is considered to be a crucial step for receptor tyrosine kinase activation, we hypothesize that both unscheduled expression of ret tyrosine kinase and constitutive oligomerization of PTC-1 proteins are responsible for PTC-1 transforming activity in thyroid.
Subject(s)
Drosophila Proteins , Oncogene Proteins/genetics , Promoter Regions, Genetic , Proteins/genetics , Proto-Oncogene Proteins/genetics , Proto-Oncogenes , Receptor Protein-Tyrosine Kinases/genetics , Base Sequence , Carcinoma, Papillary/genetics , Cytoskeletal Proteins , DNA Primers/chemistry , Gene Expression , Humans , Macromolecular Substances , Molecular Sequence Data , Oncogene Proteins, Fusion , Protein Binding , Protein-Tyrosine Kinases , Proto-Oncogene Mas , Proto-Oncogene Proteins c-ret , RNA, Messenger/genetics , RNA, Neoplasm/genetics , Recombinant Fusion Proteins , Thyroid Gland/metabolism , Thyroid Neoplasms/geneticsABSTRACT
Using non-quantitative reverse-transcription polymerase chain reaction (RT-PCR), we found that thyroid peroxidase (TPO) is expressed in all differentiated thyroid carcinomas examined, although the ratio of the shorter to longer transcript is decreased in tumors that had lost the iodide concentrating capacity. TPO expression is lost in several thyroid carcinoma cell lines (TPC-1, 8305C, 8505C) and altered in another (TC-80). Nucleotide sequencing of the PCR products revealed missense polymorphisms in the TPO gene. Four out of five samples tested are heterozygous for TPO alleles in exon 15, showing both C and T at nucleotide 2612 (GTG coding for Val, GCG for Ala). One tumor is homozygous for T at this position. In exon 8, three samples show T at nucleotide 1189 (TCG, Ser) and C at 1265 (ACC, Thr), while most published sequences report G at both positions (GCG coding for Ala at 1189 and AGC coding for Ser at 1265).
