Biomedical subjects

JoAnn Sekiguchi

Publications and source records attributed to JoAnn Sekiguchi.

6 recordsLinked to original sources

PubMed2005

Artemis-independent functions of DNA-dependent protein kinase in Ig heavy chain class switch recombination and development.

Assembly of Ig genes in B lineage cells involves two distinct DNA rearrangements. In early B cell development, site-specific double strand breaks (DSBs) at germ-line V, D, and J gene segments are joined via nonhomologous end-joining (NHEJ) to form variable region exons. Activated mature B cells can change expressed Ig heavy chain constant region exons by class switch recombination (CSR), which also involves DSB intermediates. Absence of any known NHEJ factor severely impairs joining of cleaved V, D, and J segments. In NHEJ, DNA-dependent protein kinase (DNA-PK), which is comprised of the Ku70/80 end-binding heterodimer and the catalytic subunit (DNA-PKcs), activates Artemis to generate a nuclease that processes DSBs before ligation. Because inactivation of DNA-PKcs components also severely affects CSR, we tested whether DNA-PK also functions in CSR via activation of Artemis. To obviate the requirement for V(D)J recombination, we generated DNA-PKcs- and Artemis-deficient B cells that harbored preassembled Ig heavy chain and kappa-light chain "knock-in" (HL) alleles. We found that Artemis-deficient HL B cells undergo robust CSR, indicating that DNA-PKcs functions in CSR via an Artemis-independent mechanism. To further elucidate potential Artemis-independent functions of DNA-PKcs, we asked whether the embryonic lethality associated with double-deficiency for DNA-PKcs and the related ataxia-telangiectasia-mutated (ATM) kinase was also observed in mice doubly deficient for ATM and Artemis. We found that ATM/Artemis double-deficient mice were viable and born in normal Mendelian numbers. Therefore, we conclude that DNA-PKcs has Artemis-independent functions in CSR and normal development.

Alleles↗

PubMed2004

Artemis and p53 cooperate to suppress oncogenic N-myc amplification in progenitor B cells.

The nonhomologous DNA end-joining (NHEJ) pathway contains six known components, including Artemis, a nuclease mutated in a subset of human severe combined immunodeficient patients. Mice doubly deficient for the five previously analyzed NHEJ factors and p53 inevitably develop progenitor B lymphomas harboring der(12)t(12;15) translocations and immunoglobin heavy chain (IgH)/c-myc coamplification mediated by a breakage-fusion-bridge mechanism. In this report, we show that Artemis/p53-deficient mice also succumb reproducibly to progenitor B cell tumors, demonstrating that Artemis is a tumor suppressor in mice. However, the majority of Artemis/p53-deficient tumors lacked der(12)t(12;15) translocations and c-myc amplification and instead coamplified IgH and N-myc through an intra- or interchromosome 12 breakage-fusion-bridge mechanism. We discuss this finding in the context of potential implications for mechanisms that may target IgH locus translocations to particular oncogenes.

Animals↗

PubMed2003

Impaired V(D)J recombination and lymphocyte development in core RAG1-expressing mice.

RAG1 and RAG2 are the lymphocyte-specific components of the V(D)J recombinase. In vitro analyses of RAG function have relied on soluble, highly truncated "core" RAG proteins. To identify potential functions for noncore regions and assess functionality of core RAG1 in vivo, we generated core RAG1 knockin (RAG1(c/c)) mice. Significant B and T cell numbers are generated in RAG1(c/c) mice, showing that core RAG1, despite missing approximately 40% of the RAG1 sequence, retains significant in vivo function. However, lymphocyte development and the overall level of V(D)J recombination are impaired at the progenitor stage in RAG1(c/c) mice. Correspondingly, there are reduced numbers of peripheral RAG1(c/c) B and T lymphocytes. Whereas normal B lymphocytes undergo rearrangement of both JH loci, substantial levels of germline JH loci persist in mature B cells of RAG1(c/c) mice, demonstrating that DJH rearrangement on both IgH alleles is not required for developmental progression to the stage of VH to DJH recombination. Whereas VH to DJH rearrangements occur, albeit at reduced levels, on the nonselected alleles of RAG1(c/c) B cells that have undergone D to JH rearrangements, we do not detect VH to DH rearrangements in RAG1(c/c) B cells that retain germline JH alleles. We discuss the potential implications of these findings for noncore RAG1 functions and for the ordered assembly of VH, DH, and JH segments.

Animals↗

PubMed2003

Defective DNA repair and increased genomic instability in Artemis-deficient murine cells.

In developing lymphocytes, the recombination activating gene endonuclease cleaves DNA between V, D, or J coding and recombination signal (RS) sequences to form hairpin coding and blunt RS ends, which are fused to form coding and RS joins. Nonhomologous end joining (NHEJ) factors repair DNA double strand breaks including those induced during VDJ recombination. Human radiosensitive severe combined immunodeficiency results from lack of Artemis function, an NHEJ factor with in vitro endonuclease/exonuclease activities. We inactivated Artemis in murine embryonic stem (ES) cells by targeted mutation. Artemis deficiency results in impaired VDJ coding, but not RS, end joining. In addition, Artemis-deficient ES cells are sensitive to a radiomimetic drug, but less sensitive to ionizing radiation. VDJ coding joins from Artemis-deficient ES cells, which surprisingly are distinct from the highly deleted joins consistently obtained from DNA-dependent protein kinase catalytic subunit-deficient ES cells, frequently lack deletions and often display large junctional palindromes, consistent with a hairpin coding end opening defect. Strikingly, Artemis-deficient ES cells have increased chromosomal instability including telomeric fusions. Thus, Artemis appears to be required for a subset of NHEJ reactions that require end processing. Moreover, Artemis functions as a genomic caretaker, most notably in prevention of translocations and telomeric fusions. As Artemis deficiency is compatible with human life, Artemis may also suppress genomic instability in humans.

Animals↗

PubMed2002

Increased ionizing radiation sensitivity and genomic instability in the absence of histone H2AX.

In mammalian cells, DNA double-strand breaks (DSBs) cause rapid phosphorylation of the H2AX core histone variant (to form gamma-H2AX) in megabase chromatin domains flanking sites of DNA damage. To investigate the role of H2AX in mammalian cells, we generated H2AX-deficient (H2AX(Delta)/Delta) mouse embryonic stem (ES) cells. H2AX(Delta)/Delta ES cells are viable. However, they are highly sensitive to ionizing radiation (IR) and exhibit elevated levels of spontaneous and IR-induced genomic instability. Notably, H2AX is not required for NHEJ per se because H2AX(Delta)/Delta ES cells support normal levels and fidelity of V(D)J recombination in transient assays and also support lymphocyte development in vivo. However, H2AX(Delta)/Delta ES cells exhibit altered IR-induced BRCA1 focus formation. Our findings indicate that H2AX function is essential for mammalian DNA repair and genomic stability.

Amino Acid Sequence↗

PubMed2002

Leaky Scid phenotype associated with defective V(D)J coding end processing in Artemis-deficient mice.

Radiosensitive severe combined immune deficiency in humans results from mutations in Artemis, a protein which, when coupled with DNA-dependent protein kinase catalytic subunit (DNA-PKcs), possesses DNA hairpin-opening activity in vitro. Here, we report that Artemis-deficient mice have an overall phenotype similar to that of DNA-PKcs-deficient mice-including severe combined immunodeficiency associated with defects in opening and joining V(D)J coding hairpin ends and increased cellular ionizing radiation sensitivity. While these findings strongly support the notion that Artemis functions with DNA-PKcs in a subset of NHEJ functions, differences between Artemis- and DNA-PKcs-deficient phenotypes, most notably decreased fidelity of V(D)J signal sequence joining in DNA-PKcs-deficient but not Artemis-deficient fibroblasts, suggest additional functions for DNA-PKcs. Finally, Artemis deficiency leads to chromosomal instability in fibroblasts, demonstrating that Artemis functions as a genomic caretaker.

Animals↗