Zaika A I, Kovalev S, Marchenko N D, Moll U M
Department of Pathology, State University of New York at Stony Brook, 11794, USA.
Cancer Res. 1999 Jul 1;59(13):3257-63.
The p73 gene is a structural and, in overexpression systems, functional p53 homologue. Ectopic p73 expression can activate a broad subset of p53-responsive genes, induce apoptosis, and act as a growth suppressor. Yet, viral oncoproteins that antagonize p53 (adenovirus E1B 55K, SV40 large T, and human papillomavirus E6) do not antagonize p73. This could suggest that inactivation of p73, in contrast to p53, is not required for tumorigenesis. Also, p73 is not activated by DNA damage. Because intragenic p73 mutations in tumors have not been reported and imprinting is idiosyncratic, tumor-specific changes in wild-type p73 expression levels become the most reliable guide toward identifying the normal function of p73 and its role in tumorigenesis. We analyzed 77 invasive breast cancers and 7 breast cancer cell lines for p73 mRNA expression levels, allelic origin, intragenic mutations, and COOH-terminal splice variants. A range of normal tissues, including breast, showed very low p73 expression, with little variation from tissue to tissue. In contrast, 38% (29 cases) of breast cancers had elevated p73 mRNA ranging from 5-25-fold above normal, with the remaining tumors (64%) falling within the normal range. Moreover, five of seven cell lines (71%) also exhibited p73 overexpression (13-73-fold). Yet, no correlation with p21 mRNA and protein levels was present, although four of the five lines were mutant for p53. Mutation analysis of the eight highest expressers showed wild type status. Eight of 14 informative samples were biallelic, whereas the remaining 6 samples showed monoallelic expression. Tumors and cell lines with p73 overexpression tended to exhibit a complex profile of up to six different COOH-terminal splice variants, whereas normal and transformed tissues with low p73 mRNA predominantly expressed p73 alpha. We confirm the previously described variants p73 gamma and delta in breast tissue and describe two novel isoforms, p73 epsilon and phi, thereby further enlarging combinatorial possibilities. Together, our in vivo data show that p73 does not have a role as a classic Knudson-type tumor suppressor in breast cancer.
p73基因是一种结构上与p53同源且在过表达系统中功能上也与p53同源的基因。异位表达p73可激活p53反应基因的一个广泛子集，诱导细胞凋亡，并发挥生长抑制作用。然而，拮抗p53的病毒癌蛋白（腺病毒E1B 55K、SV40大T抗原和人乳头瘤病毒E6）并不拮抗p73。这可能表明，与p53不同，肿瘤发生并不需要p73失活。此外，p73不会被DNA损伤激活。由于尚未报道肿瘤中的p73基因内突变，且印记具有特异性，野生型p73表达水平的肿瘤特异性变化成为确定p73正常功能及其在肿瘤发生中作用的最可靠指导。我们分析了77例浸润性乳腺癌和7个乳腺癌细胞系中的p73 mRNA表达水平、等位基因起源、基因内突变以及COOH末端剪接变体。包括乳腺在内的一系列正常组织显示p73表达非常低，组织间差异很小。相比之下，38%（29例）的乳腺癌p73 mRNA升高，范围是正常水平的5至25倍以上，其余肿瘤（64%）在正常范围内。此外，7个细胞系中的5个（71%）也表现出p73过表达（13至73倍）。然而，尽管5个细胞系中有4个p53发生了突变，但p73与p21 mRNA和蛋白水平之间没有相关性。对8个最高表达者进行的突变分析显示为野生型状态。14个信息丰富样本中的8个是双等位基因，而其余6个样本显示单等位基因表达。p73过表达的肿瘤和细胞系倾向于表现出多达六种不同COOH末端剪接变体的复杂图谱，而p73 mRNA水平低的正常组织和转化组织主要表达p73α。我们在乳腺组织中证实了先前描述过的变体p73γ和δ，并描述了两种新的异构体p73ε和φ，从而进一步扩大了组合可能性。总之，我们的体内数据表明p73在乳腺癌中并不作为经典的Knudson型肿瘤抑制因子发挥作用。
