Analysis of tumor supressor genes p16 and Rb in childhood acute lymphoblastic leukemia

细胞周期的进展是由一系列细胞周期蛋白和细胞周期蛋白依赖性激酶（CDK）复合物共同调控的，这些复合物引发视网膜母细胞瘤蛋白（Rb）的磷酸化，从而释放出一些与Rb相关的转录因子，如G1期到S期进展所需的E2F家族。p16/INK4a作为CDK抑制剂之一负调控这一过程，并被认为是一种肿瘤抑制因子，因为它在多种肿瘤中经常失活。然而，出乎意料的是，最近的报道揭示了p16表达增强与包括儿童ALL在内的几种恶性肿瘤的不利结果的关联。在我们研究儿童ALL中p16的过程中，我们发现了一个具有非随机易位的b前体ALL细胞系（KOPN-79），其p16表达水平极高。为了阐明该细胞系细胞周期进程的机制，我们检测了p16/Rb通路中的关键调控因子，并鉴定了Rb基因的结构异常。得到的Rb产物在口袋区域内的位置被重排和截断，对E2F-1的结合能力较差。我们的发现可以解释，至少部分地，儿童ALL过表达p16的不利结果的主要原因。

Progression of the cell cycle is cooperatively regulated by a series of cyclin and cyclin-dependent kinase (CDK) complexes which elicit phosphorylation of retinoblastoma protein (Rb) resulting in release of several RB-associated transcription factors such as the E2F family required for G1 to S phase progression. p16/INK4a negatively regulates this process as one of CDK inhibitors, and is regarded as a tumor suppressor because of its frequent inactivation in a wide variety of tumors. Unexpectedly, however, recent reports have revealed the association of an enhanced p16 expression with an unfavorable outcome in several malignancies including childhood ALL. Along with the process of our study characterizing p16 in childhood ALL, we found a B-precursor ALL cell line (KOPN-79) with nonrandom translocations expressing p16 at extremely high levels. To clarify mechanism(s) allowing this cell line to cell cycle progression, we examined key regulators in the p16/Rb pathway, and identify the structural abnormality of the Rb gene. The resultant Rb product was rearranged and truncated at the site within the pocket region, and showed the poor binding capacity to the E2F-1. Our findings might explain, at least in part, the primary cause for an unfavorable outcome of childhood ALL over-expressing p16.