Terminal Deoxynucleotidyl Transferase (TdT)

Abstract

Terminal deoxynucleotidyl transferase (TdT) is a DNA polymerase with the unique capacity to add nucleotides to a DNA strand in an untemplated fashion. This capacity is specifically utilised in developing B‐cells and T‐cells in vertebrates to generate extra diversity in the antigen receptor repertoire. During the process of V(D)J recombination developing B‐ and T‐cell rearrange the DNA encoding the immunoglobulin (Ig) and T‐cell receptor (TCR) genes, respectively. Following the introduction of double‐strand DNA breaks and prior to the ligation by members of the nonhomologous end joining (NHEJ) pathway, TdT can add nontemplated (N‐)nucleotides to the open DNA ends. Through the capacity to generate longer junctions with new nucleotide compositions, TdT is critical in driving the generation of the enormous antigen receptor repertoire with a potential 1015 unique sequences that facilitate recognition of the diverse range of pathogens vertebrates are exposed to.

Key Concepts

  • TdT is a DNA polymerase with nontemplated activity.
  • TdT adds random nucleotides in the junctional region during V(D)J recombination.
  • TdT expression is tightly regulated in early B‐ and T‐cell progenitors.
  • Detection of nuclear TdT expression using recombinant antibodies is an important biomarker for diagnostics and minimal residual disease monitoring of acute leukaemias.
  • TdT expression is regulated by IL‐7 receptor signalling.
  • Foetal B‐ and T‐cell progenitors have reduced TdT expression and function.

Keywords: terminal deoxynucleotidyl transferase; V(D)J recombination; immunoglobulin; T‐cell receptor; adaptive immunity; B‐cell; nontemplated nucleotide; immunoglobulin; polymerase

Figure 1. IGH gene rearrangement and junctional region composition. VDJ recombination in IGH locus initiates through Dh to Jh rearrangement and proceeds with coupling of Vh gene to the DJh joint. Subsequently, the VDJh exon is transcribed and spliced to the IGHM exon followed by translation to Igμ protein. Similar recombination events within IGL loci lead to the formation of functional IgL protein. Finally, IgH and IgL chains are paired into complete Ig (BCR) which is expressed on the B‐cell membrane with CD79a and CD79b proteins. VDJ junctional region is processed through exonuclease activity and random nucleotide additions by TdT to form a unique sequence encoding complementary determining region (CDR)3, which encodes the key loops responsible for antigen binding.
Figure 2. Schematic representation of V(D)J recombination. The RAG proteins initiate DNA break formation in two gene segments at the border of the RSSs. This results in two hairpinned coding ends and two blunt signal ends. The blunt signal ends are directly joined by a complex of Ku70, Ku80 and DNA‐PKcs. The coding ends are held together by NBN, MRE1 and RAD50, while Ku70, Ku80 and DNA‐PKcs recruit Artemis that cleaves the hairpins. A complex of DNA ligase IV, XRCC4, XLF and TdT is recruited, which regulates N‐nucleotide additions and ligation of the ends into a coding joint. Adapted from Berkowska et al. ().
Figure 3. Timeline of discoveries related to TdT function.
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van Zelm, Menno C(May 2020) Terminal Deoxynucleotidyl Transferase (TdT). In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0028615]