Genomics-based characterization of tumor suppressor genes in the cardiovascular system, a role for adenomatosis polyposis coli gene in human cardiovascular development and disease

Apoptosis (programrned cell death) is an important process, which, in conjunction with cell proliferation, maintains cell number homeostasis. Although apoptosis has been more extensively investigated in other tissues [1,2], only recently has this process been suspected as a significant contributor to both disease and nomal development of the cardiovascular system [3,4,5,6]. Grasping a comprehension of the underlying genetic mechanisms of apoptosis is especially crucial considering that cardiac myocytes irreversibly exit the cell cycle and thus fail to proliferate during pathological conditions. Despite great strides in understanding the molecular pathways of apoptosis, there still remain numerous questions to be answered. identifying key genes that are involved in the regulatory process of apoptosis in the cardiovascular system will serve as a basis for creating more effective therapeutic treatments in cardiovascular disease and provide an understanding of how cardiac development is rnodulated. This review provides a brief summary of recent data implicating genes that may be involved in apoptosis in the cardiovascular system. It also outlines the continued usefulness of large-scale generation of expressed sequence tags (ESTs) to establish expression profites from the cardiovascular system and as a means of identifying potentially significant apoptotic regulators previously characterized in other tissues but not as yet in the cardiovascuiar system. INTRODUCTiON Cell death can occur either by necrosis or apoptosis. Apoptotic and necrotic cells are generally different in morphology and in the sequence of events surrounding their demise. Morphologically, necrosis is characterized by cell lysis and organelle destruction whereas apoptosis results in cell shrinkage, blebbing, chromatin condensation and DNA fragmentation [q. The major difference between necrosis and apoptosis is the inflarnmatory response: cellular debris is accumulated in necrotic cells whereas in apoptotic cells, there exists an efficient degradation and disposal process through phagocytosis by neighbouring cells. Apoptosis is essential for normal development through its involvement in controlling cell number homeostasis in conjunction with cell proliferation. However, this balance can be petturbed leading to abnonal, often fatal conditions (such as embryonic lethality) [8,9]; recent studies have sought to identify modulators of apoptosis in an attempt to decipher how the process can be positively or negatively regulated. Early work in the nematode C. elegans [10,11; for reviews see 12.1 31 has provided significant preliminary data in understanding the more complex regulation in higher organisms, such as in mammalian systems. Apoptosis can be initiated or controlled by developmental and environmental factors such as DNA damage. viral infection. cellular damage and loss of celltell or cell-substrate contact; it is also under the regulation of a number of genes. These regdatory genes can be classified into 3 categories: 1. Effectors of apoptosis (e.g., interleukin-1 -beta converting enzyme (CE) family) which are implicated in the onset of apoptosis. 2. Suppressors of apoptosis (e.g., Bcl-2) which are very important for the regulation of apoptosis and are invoived in pathogenesis of many diseases such as lymphomas and leukemia [14,15]. 3. Intermediate regulators of apoptosis (e.g., Fas/Fas iigand, p53 and c-myc) which can interact with receptor complexes and other apoptotic regulators to induce or suppress apoptosis. To obtain a profile of genes expressed in the cardiovascular system from a developmental (fetal versus adult heart) and disease (normal adult versus hypertrophic heart) perspective, large-scale sequencing of expressed sequence tags (ESTs) from human cardiovascular cDNA libraries has been investigated. To date, our laboratory has sequenced over 51,000 ESTs from these libraries, and a catalogue of over 5,000 genes from the cardiovascular system and a profile of their differential expression has been published [16]. From this, genes that are known to be involved in apoptotic pathways either as effectors, suppressors or intermediate regulators -can be identified in the cardiovascular system and further characterized. GENES INVOLVED IN APOPTOSIS A CARDIOVASCULAR PERSPECTIVE The control of apoptosis has been linked to a vanety of genes and gene families. Differential expression of these genes between tissues, developmental and disease states and, indeed, between organisrns, has been studied to establish pathway(s) of apoptotic regulation. From our data, we have extracted and modified two specific functional subsets of genes identified in the cardiovascular systern [16], namely "apoptosis" (Table 1 a) and 'DNA synthesis/replication" (Table 1 b). In Table la, the genes were further characterized based on their mode of apoptotic regulation as described previously in the literature, namely 'effectors", "suppressors" or "intermediate regulators" of apoptosis. The two subsets were selected since both apoptosis and DNA synthesis/replication are involved in controlling cell number homeostasis. TABLE 1: In Silico Northern analysis of known genes expressed in the human cardiovascular systern: a) genes involved in apoptosis, and b) genes involved in cell division (DNA synthesis/replication). Modified from Hwang and Dempsey et al., Circulation 1997;96:4146-4203 URL: http:l/americanheart.org/Scientifidpu bdscipu b/genome.html Accession numbers listed correspond to the putatively identified gene. In the event that an EST matched to more than one entry, the entry with the greatest number of EST hits was chosen. ESTs matching significantly to GenBank database entries ( P d 0 ' ' O ) but exhibiting ~ 9 5 % sequence identity were designated as homotogs of the most significant match. ESTs matching significantiy only to non-human GenBank database entries were designated as isologs of those genes. Intensities reflect the frequency of a given EST in a particular Iibrary, as a percentage of total ESTs in that library. lntensities are defined as follows: