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Inhibition of DNA repair by deoxyadenosine in resting human lymphocytes
Insights
Adenosine deaminase (ADA) deficiency leads to deoxyadenosine (dAdo) accumulation, impairing DNA repair in lymphocytes. This dAdo-induced DNA repair inhibition may increase toxicity of DNA-damaging agents.
Area of Science:
- Biochemistry
- Immunology
- Molecular Biology
Background:
- Adenosine deaminase (ADA) deficiency causes profound lymphopenia in immunodeficient children.
- Deoxyadenosine (dAdo) accumulation inhibits T lymphoblast proliferation.
- dAdo also induces DNA strand breaks in mature, non-dividing lymphocytes.
Purpose of the Study:
- To investigate the effects of dAdo on DNA repair in resting human lymphocytes.
- To determine the mechanism by which dAdo inhibits DNA repair.
- To assess the inhibitory potential of dAdo and related compounds on DNA repair.
Main Methods:
- Gamma-radiation of resting normal lymphocyte cultures.
- DNA strand break rejoining assay using DNA unwinding.
- Measurement of [3H]thymidine uptake for unscheduled DNA synthesis.
- Use of deoxycoformycin, an ADA inhibitor.
Main Results:
- dAdo significantly retarded DNA rejoining in a dose- and time-dependent manner.
- Inhibition of DNA repair by dAdo required its phosphorylation.
- 2-chlorodeoxyadenosine was a potent inhibitor of DNA repair and unscheduled DNA synthesis.
Conclusions:
- Quiescent lymphocytes exhibit a slow, balanced DNA break and repair rate.
- Accumulation of dATP inhibits DNA repair, leading to DNA strand breaks.
- dAdo and related compounds may potentiate DNA damaging agent toxicity by inhibiting DNA repair.
Abstract:
Profound lymphopenia is characteristic of immunodeficient children who lack adenosine deaminase (ADA). When ADA is inactive, deoxyadenosine (dAdo) is phosphorylated by immature T lymphoblasts and inhibits cell division. However, dAdo also causes the slow accumulation of DNA strand breaks in nondividing, mature human peripheral blood lymphocytes. To explore the basis for this phenomenon, we have assessed the effects of dAdo and other deoxynucleosides on the repair of gamma-radiation induced DNA strand breaks in resting normal lymphocyte cultures. As measured by a sensitive DNA unwinding assay, most DNA strand breaks were rejoined within 2 hr after exposure of lymphocytes to 500 rad. In medium supplemented with deoxycoformycin, a tight binding ADA inhibitor, dAdo retarded DNA rejoining in a dose and time dependent manner. The inhibition required dAdo phosphorylation. Over an 8-hr period, 10 microM dAdo gradually rendered peripheral blood lymphocytes incompetent for DNA repair. Among several other compounds tested, 2-chlorodeoxyadenosine, an ADA resistant dAdo congener with anti-leukemic and immunosuppressive activity, was the most powerful inhibitor of DNA repair, exerting significant activity at concentrations as low as 100 nM. Both dAdo and 2-chlorodeoxyadenosine blocked unscheduled DNA synthesis in irradiated resting lymphocytes, as measured by [3H]thymidine uptake. On the basis of this and other data, we suggest that quiescent peripheral blood lymphocytes break and rejoin DNA at a slow and balanced rate. The accumulation of dATP progressively retards the DNA repair process and thereby fosters the time-dependent accretion of DNA strand breaks. By inhibiting DNA repair, dAdo, 2-chlorodeoxyadenosine and related compounds may substantially potentiate the toxicity of DNA damaging agents to normal and malignant lymphocytes.