Human periplakin: genomic organization in a clonally unstable region of chromosome 16p with an abundance of repetitive sequence elements

Periplakin, a member of the plakin family of proteins, has been recently characterized by cDNA cloning, and the corresponding gene, PPL, has been mapped to human chromosome 16p13.3 (Aho et al., 1998, Genomics 48: 242-247). Periplakin has also been shown to serve as an autoantigen in a malignancy-associated autoimmune blistering disease, paraneoplastic pemphigus (Mahoney et al., 1998, J. Invest. Dermatol. 111: 308-313). In this study, we have elucidated the intron-exon organization of human PPL and characterized its promoter region. The flanking 5' sequences were rich in G and C ( approximately 80%) and included multiple AP2 sites and a SP1 site, while no canonical TATA or CCAAT sequences were found. The functionality of the upstream sequences (-709 to +135) as a promoter in cultured epidermal keratinocytes was detected by a CAT reporter gene, and a limited region (-382 to +135) showed activity in cultured dermal fibroblasts, attesting to cell-type specificity of the promoter. The genomic organization, including the intron-exon borders, was determined by direct nucleotide sequencing of human genomic P1 clones. Comparative analysis of cDNA and genomic sequences revealed that PPL consists of 22 exons, with the distribution of exons in PPL being consistent with that of other plakin genes: 21 small exons, separated by large introns, encode the amino-terminal globular domain, and 1 large exon encodes the entire rod and the tail domains. Characterization of four P1 clones spanning the PPL locus revealed multiple Alu repeats, 20 of them within 33 kb of the entirely sequenced segments (0.60/kb), in addition to numerous MIR and L1 elements. These repetitive elements could lead to the clonal instability detected throughout the genomic P1 clones and may give rise to the genomic rearrangements possibly underlying the paraneoplastic pemphigus.