Requirement for deoxycytidine kinase in T and B lymphocyte development

Abstract

Deoxycytidine kinase (dCK) is a rate-limiting enzyme in deoxyribonucleoside salvage, a metabolic pathway that recycles products of DNA degradation. dCK phosphorylates and therefore activates nucleoside analog prodrugs frequently used in cancer, autoimmunity, and viral infections. In contrast to its well established therapeutic relevance, the biological function of dCK remains enigmatic. Highest levels of dCK expression are found in thymus and bone marrow, indicating a possible role in lymphopoiesis. To test this hypothesis we generated and analyzed dCK knockout (KO) mice. dCK inactivation selectively and profoundly affected T and B cell development. A 90-fold decrease in thymic cellularity was observed in the dCK KO mice relative to wild-type littermates. Lymphocyte numbers in the dCK KO mice were 5-to 13-fold below normal values. The severe impact of dCK inactivation on lymphopoiesis was unexpected given that nucleoside salvage has been thought to play a limited, "fine-tuning" role in regulating deoxyribonucleotide triphosphate pools produced by the de novo pathway. The dCK KO phenotype challenges this view and indicates that, in contrast to the great majority of other somatic cells, normal lymphocyte development critically requires the deoxyribonucleoside salvage pathway.

Erk5 contributes to maintaining the balance of cellular nucleotide levels and erythropoiesis

“…Deoxynucleoside salvaging during B cell, T cell and erythroid development is necessary to fine tune dNTP levels and avoid replication stress in their highly proliferative progenitors. 18,19 The results presented here show that erythroid progenitors without Erk5 have higher dTTP levels, associated with enhanced mutagenesis and inefficient erythropoiesis in bone marrow, but not in the spleen. On average, bone marrow contains twice the amount of thymidine than spleen.…”

“…19 Thymidine salvaging in bone marrow could generate replication stress in highly proliferative haematopoietic precursors of Erk5 ¡/¡ VavCre mice if dNTP metabolism is not properly geared, and these are subsequently forced to immigrate to more permissive niches such as the spleen. 18 In addition to erythroid progenitors, B cell development also showed signs of stress, as the mature B220 high B cell population was underrepresented in Erk5 ¡/¡ VavCre marrow. Of note, while this manuscript was being submitted the alterations of B-cell development reported here have been confirmed in another paper.…”

“…Fuelled by extracellular deoxynucleosides imported into the cell, 17 the salvage pathway is necessary for proper haematopoietic development. 18,19 Haematopoietic tissues contain high thymidine levels, which increases the cellular dTTP levels produced by the salvage pathway. 19 A high dTTP concentration prevents UTP incorporation into DNA and replication stress in erythroid and lymphoid lineages.…”

“…19 A high dTTP concentration prevents UTP incorporation into DNA and replication stress in erythroid and lymphoid lineages. 18,19 Alterations in the activity of enzymes metabolising nucleotides can lead to mutagenesis and tumourigenesis in vitro and in vivo [20][21][22] and produce developmental defects in lymphoid and erythroid lineages, 18,23 underscoring the importance of nucleotide metabolism for proper haematopoietic development.…”

Human Biodistribution and Radiation Dosimetry of 18F-Clofarabine, a PET Probe Targeting the Deoxyribonucleoside Salvage Pathway

“…T-and B-cell development is strongly associated with dCK expression (19). In dCK knockout mouse studies, lymphocytes were reduced 5-to 13-fold below reference values (19), suggesting that lymphopoiesis is highly dCK-dependent.…”