including ZIP kinase (Kawai et al., 1998), NIP3 (Chen et al., 1997), FADD
(Chinnaiyan et al., 1995), PERP (Attardi et al., 2000), and CIDE-A and CIDE-B
(Inohara et al., 1998), which therefore serve as positive controls for the screen
(Albayrak et al., submitted).
The first gene isolated by this screen and further characterized was NDF (Neu
Differentiation Factor) (Grimm and Leder, 1997; Grimm et al., 1998). NDF is a
growth factor that is first synthesized as a membrane-spanning precursor protein,
from which the secreted growth factor moiety is proteolytically released. Interestingly
enough, NDF can, when overexpressed, also function as an oncogene (Krane and
Leder, 1996). This is probably achieved by activating receptors on neighboring cells,
which subsequently proliferate and eventually undergo malignant transformation.
One interpretation of NDF’s proapoptotic activity is that it is a safeguard against its
activation as an oncogene. Experiments indicated that its apoptosis-inducing domain
is indeed encoded by the cytoplasmic moiety and therefore is physically separable
from its extracellular growth factor domain, which mediates the oncogenic activity.
Hence, only NDF-overexpressing cells undergo apoptosis. This finding is—much
like the results with the DAP kinase—another example of how apoptosis contributes
to the protection against malignant cells. Nevertheless, extensive apoptosis was found
in tumors caused by NDF in transgenic mice (Grimm et al., 1998; Weinstein et al.,
1998). The activity of NDF in apoptosis induction therefore still exists in malignant
cells but must be repressed by additional mutations in order for the tumor to thrive.
ANT-1, a component of the mitochondrial ‘permeability transition’ (PT) pore,
was also isolated with this screen (Bauer et al., 1999). The PT pore is a protein complex
between the inner and outer mitochondrial membrane that has been implicated in
apoptosis induction by a wide variety of reagents (Crompton, 1999). In apoptosis
induction, it is converted into an unspecific pore by an unknown mechanism. This
leads to the collapse of the membrane potential, a block of the respiratory chain, and
to a release of apoptogenic proteins from mitochondria. Until now, ANT-1 was
known only as a transport molecule for ATP and ADP. However, recent results have
demonstrated that its transport activity is independent of its activity to induce
apoptosis. Apoptosis induced by ANT-1 is probably mediated by specific protein-
protein interactions that titrate out repressors of the PT pore. Hence, these
investigations could contribute to a mechanistic understanding of how this pore and
its components are activated for apoptosis induction (Bauer et al., 1999). ANT-1 also
constitutes an example of a gene from this screen that can be integrated into a
pathologic context: it is overexpressed in dilated cardiomyopathy (DCM), a prevalent
degenerative heart disease of unknown cause (Dorner et al., 1997) that is characterized
by an excess of apoptosis induction in the heart muscle. For this reason, it could be
postulated that this pathologic apoptosis induction is caused by overexpressed ANT-1.
A dominant activity exerted by overexpression of genes is used in this screen. The
examination of such dominant gene activities is becoming increasingly prevalent. For
example, a similar approach uses microarrays that have been spotted with plasmid
DNAs. Cells laid upon these arrays take up and express the DNA. A pilot experiment
indicated that this transfection method might be suitable to determine apoptosis-
208 GENETICS OF APOPTOSIS