Attenuation of G2 checkpoint function precedes human cell immortalization

Area of Science:

• Cell biology
• Genetics
• Cancer research

Background:

• The G2 checkpoint prevents mitosis in cells with DNA damage, protecting against genetic instability.
• Immortal human cell lines often exhibit genetic instability, but the underlying mechanisms are not fully understood.

Purpose of the Study:

• To investigate if a weakened G2 checkpoint contributes to the genetic instability observed in immortal human cell lines.
• To determine if attenuation of the G2 checkpoint is a factor in the conversion of normal cells to an immortal state.

Main Methods:

• Comparing the G2 checkpoint response to gamma-radiation-induced DNA damage in normal human fibroblasts (IMR-90) versus SV40-transformed and immortal derivative cells.
• Quantifying the inhibition of mitosis and cyclin B1/p34CDC2 kinase activity post-irradiation.
• Measuring the expression levels of cyclin B1 and p34CDC2 in different cell types.

Main Results:

• Normal IMR-90 fibroblasts exhibited a stringent G2 checkpoint response to gamma radiation.
• SV40-transformed and immortal cells showed significantly attenuated G2 delay and reduced inhibition of cyclin B1/p34CDC2 kinase activity.
• Elevated expression of cyclin B1 (8-fold) and p34CDC2 (2.5-fold) was observed in SV40-transformed IMR-90 cells.

Conclusions:

• Attenuation of the G2 checkpoint is a characteristic of immortal human cell lines.
• This weakened checkpoint allows cells with damaged chromatids to enter mitosis, potentially promoting genetic alterations necessary for immortality.
• The findings suggest a mechanism by which normal cells may acquire immortality through genetic instability driven by checkpoint defects.