Molekulargenetische Charakterisierung der Bruchpunktregion 19q13.4 bei gutartigen follikulären Schilddrüsentumoren und Charakterisierung des Kandidaten-Gens ZNF331

Abstract

The significance in the high incidence of thyroid nodules is that some of these nodules may harbour thyroid tumors. Still, the molecular mechanisms underlying the tumor formation are not fully understood. In this study, the positional cloning approach was used to characterize the chromosomal breakpoints specific for the two main cytogenetic subgroups in benign follicular thyroid tumors which habor chromosomal alterations in 19q13 and 2p21, respectively. In the cytogenetic subgroup with 19q13 aberrations, a candidate gene ZNF331 was identified at a 25 kbp distance proximal to the breakpoint region. ZNF331 consists of 15 exons and encodes for a 463 aa protein with a 42 aa KRAB-A domain and a 332 aa zinc finger region. The normal expression pattern of ZNF331 shows 3 transcripts of 2.1, 4.0 und 4.8 kbp. Exclusively in cell lines established from thyroid adenomas with 19q13 aberration, the expression of a 3.4 kbp transcript was detected. Hidden translocations involving chromosome 19 may mask a higher rate of clonal aberrations in benign thyroid tumors. FISH analyses did not confirm the existence of such translocations in a subset of 38 thyroid adenomas with apparently normal karyotype. The characterization of the canine orthologue of ZNF331 showed a high percentage of sequence identity with its human counterpart at the nucleic acid level as well as at the amino acid level. By FISH, canine ZNF331 was mapped to the terminal region of canine chromosome 1, i.e. CFA1q33.In the cytogenetic subgroup with 2p21 aberrations, a candidate gene, THADA, covering the 450-kbp breakpoint region was identified and first characterized. In two cell lines with 2p21 aberrations, the chromosomal breakpoint was narrowed down to intron 28 of THADA. Analyses of fusion transcripts resulting from the aberrations pointed to truncation of THADA rather than fusion to particular coding sequences as the critical effect of the chromosomal breaks.