Biomedical Engineering Reference
In-Depth Information
Figure 2
. The alteration of a four-element AND/OR Boolean network by cancer. The Boolean
network diagrammed consists of four binary elements. Each element receives signals from two
other elements, and sends signals to two elements. In this network, elements A and C obey the
Boolean OR function and elements B and D obey the Boolean AND function. An element can
either be "on" (1) or "off" (0), depending on the rule it obeys and the binary states of the ele-
ments from which it receives signals. For example, element A is active when either element C
or
D is active. In contrast, element B is active only when both elements A and C are active. In a
four-element Boolean network there are 16 possible initial states. Using these values, and apply-
ing Boolean logic to each element, the successor states of each element can be determined.
These successor states describe the behavior of the system. The 16 initial states, along with the
successor states for each, are shown on the truth table to the left of the diagram. Although the
number of initial states is numerous, the system eventually settles into only
three
state-cycles.
These three cycles are shaded in the truth table. Perturbation of a steady-state attractor through
mutation may upset the genetic stability and cause the cell to enter the carcinogenic cascade (a
new state,
E
). This state
E
is fundamentally unstable and results in a new set of attractors, i.e.,
cell types. These cell types are manifested as tumor cell heterogeneity.
