Stem Cell Expression Profiling

The human genome has approximately 20,000 to 25,000 genes that are used by a cell to perform various functions. The first step in gene expression is the conversion of DNA to messenger RNA (mRNA) through a process called transcription. The mRNAs are then converted into proteins, the functional units of the cell by a process known as translation. Events during transcription such as alternative splicing, as well as the activity of proteins whose function is to regulate transcription, can alter the nature and number of distinct mRNAs expressed from a single gene, thus [Page 1071]increasing biological complexity. Further complexity is achieved by the fact that apart from the genes that code for proteins, there are subsets of RNA that do not code for protein but regulate the expression of genes. These subsets include microRNA (miRNA) and other noncoding RNA (ncRNA). Given all the internal and external conditions, the characteristic set of RNA expressed by a cell at any particular point in time is known as its expression profile.

While most cells in the body contain the entire copy of the genome, at any given time cells express only a subset of these genes as mRNA. The specific type and quantity of mRNAs expressed depend upon multiple factors that take into account the type of cell (skin cell, liver cell, stem cell, and so on) and the external conditions that are present around the cell. External stimuli can drastically alter the mRNAs expressed in the cell at a particular time. In addition to different sets of genes being expressed, the amount of mRNA expressed from a particular gene can also change based on internal and external stimuli. A classic example is when a stem cell that is pluripotent or multipotent is forced to differentiate into a particular type of somatic cell because of an external stimulus such as a growth hormone. In such a case, a distinct change can be observed in the nature and amount of mRNAs expressed by the stem cell.

The expression profile of a cell can thus be thought of as its genetic signature and is an extremely useful tool for research and clinical treatment. Databases such as the Gene Expression Omnibus (GEO) from the National Center for Biotechnology Information (NCBI) contain many such expression profiles that have been determined through experimentation and that are freely available and searchable. Gene expression profiles can be used to identify cell types, to understand how cells respond to stimuli, and to identify certain diseases.