Activation-induced cytidine deaminase (AID)-dependent somatic hypermutation requires a splice isoform of the serine/arginine-rich (SR) protein SRSF1.
Somatic hypermutation (SHM) of Ig variable region (IgV) genes requires both IgV transcription and the enzyme activation-induced cytidine deaminase (AID). Identification of a cofactor responsible for the fact that IgV genes are much more sensitive to AID-induced mutagenesis than other genes is a key question in immunology. Here, we describe an essential role for a splice isoform of the prototypical serine/arginine-rich (SR) protein SRSF1, termed SRSF1-3, in AID-induced SHM in a DT40 chicken B-cell line. Unexpectedly, we found that SHM does not occur in a DT40 line lacking SRSF1-3 (DT40-ASF), although it is readily detectable in parental DT40 cells. Strikingly, overexpression of AID in DT40-ASF cells led to a large increase in nonspecific (off-target) mutations. In contrast, introduction of SRSF1-3, but not SRSF1, into these cells specifically restored SHM without increasing off-target mutations. Furthermore, we found that SRSF1-3 binds preferentially to the IgV gene and inhibits processing of the Ig transcript, providing a mechanism by which SRSF1-3 makes the IgV gene available for AID-dependent SHM. SRSF1 not only acts as an essential splicing factor but also regulates diverse aspects of mRNA metabolism and maintains genome stability. Our findings, thus, define an unexpected and important role for SRSF1, particularly for its splice variant, in enabling AID to function specifically on its natural substrate during SHM.
Department of Bioscience and Biotechnology,  Okayama University Graduate School of Natural Science and Technology, Tsushima-Naka, Kita-Ku, Okayama 700-8530,  Japan.
Yuichi
Y
Kanehiro
Kagefumi
K
Todo
Misaki
M
Negishi
Junji
J
Fukuoka
Wenjian
W
Gan
Takuya
T
Hikasa
Yoshiaki
Y
Kaga
Masayuki
M
Takemoto
Masaki
M
Magari
Xialu
X
Li
James L
JL
Manley
Hitoshi
H
Ohmori
Naoki
N
Kanayama
D000818:Animals; D001402:B-Lymphocytes; D015153:Blotting, Western; D002645:Chickens; D047369:Chromatin Immunoprecipitation; D003564:Cytidine Deaminase / Q000378:metabolism*; D017931:DNA Primers / Q000235:genetics; D018076:DNA, Complementary / Q000096:biosynthesis; D051379:Mice; D041681:NIH 3T3 Cells; D009687:Nuclear Proteins / Q000235:genetics / Q000378:metabolism*; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D012313:RNA / Q000302:isolation & purification; D016601:RNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D000068103:Serine-Arginine Splicing Factors; D027041:Somatic Hypermutation, Immunoglobulin / Q000276:immunology*; D000098830:AICDA (Activation-Induced Cytidine Deaminase)
D017931:DNA Primers; D018076:DNA, Complementary; D009687:Nuclear Proteins; D020033:Protein Isoforms; D016601:RNA-Binding Proteins; D000068103:Serine-Arginine Splicing Factors; D012313:RNA; D000098830:AICDA (Activation-Induced Cytidine Deaminase); D003564:Cytidine Deaminase