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Fanca deficiency reduces A/T transitions in somatic hypermutation and alters class switch recombination junctions in mouse B cells.

Nguyen TV, Riou L, Aoufouchi S, Rosselli F - J. Exp. Med. (2014)

Bottom Line: Whereas SHM involves an error-prone repair process that introduces novel point mutations into the Ig gene, the mismatches generated during CSR are processed to create double-stranded breaks (DSBs) in DNA, which are then repaired by the NHEJ pathway.As several lines of evidence suggest a possible role for the FANC pathway in SHM and CSR, we analyzed both processes in B cells derived from Fanca(-/-) mice.Here we show that Fanca is required for the induction of transition mutations at A/T residues during SHM and that despite globally normal CSR function in splenic B cells, Fanca is required during CSR to stabilize duplexes between pairs of short microhomology regions, thereby impeding short-range recombination downstream of DSB formation.

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Affiliation: Centre National de la Recherche Scientifique UMR 8200, Institut Gustave Roussy, 94805 Villejuif, France Université Paris Sud, 91400 Orsay, France Programme Equipe Labellisées, Ligue Contre le Cancer, 75013 Paris, France.

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Reduced A/T transitions during SHM in Fanca−/− mice. (A) Distribution of mutations in the JH4 intronic region (506 bp) that was amplified from Peyer’s patch PNAhigh B cells isolated from WT and Fanca−/− mice. (B) Proportion of sequences with numbers of mutations per clone (the central circle shows the total number of analyzed sequences) from WT (n = 5) and Fanca−/− mice (n = 5). (C) The spectrum of base substitutions is expressed as a percentage of the total number of mutations (left), and the frequency of mutation (right) was corrected for base composition. Gray boxes denote a significant decrease in mutation frequency compared with WT (the χ2 test was applied according to the SHMTool algorithm; *, P < 0.05; **, P < 10−3; ***, P < 10−4). Data for the SHM are from five independent experiments. (D) Differential expression of Polη in FANCA- and FANCG-deficient cells. Whole cell extracts were prepared from the indicated human lymphoblast cell lines and analyzed by immunoblotting for the expression of Polη, MSH2, FANCA, FANCG, and Vinculin (asterisks indicate nonspecific bands). Representative data from two independent experiments are shown.
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fig1: Reduced A/T transitions during SHM in Fanca−/− mice. (A) Distribution of mutations in the JH4 intronic region (506 bp) that was amplified from Peyer’s patch PNAhigh B cells isolated from WT and Fanca−/− mice. (B) Proportion of sequences with numbers of mutations per clone (the central circle shows the total number of analyzed sequences) from WT (n = 5) and Fanca−/− mice (n = 5). (C) The spectrum of base substitutions is expressed as a percentage of the total number of mutations (left), and the frequency of mutation (right) was corrected for base composition. Gray boxes denote a significant decrease in mutation frequency compared with WT (the χ2 test was applied according to the SHMTool algorithm; *, P < 0.05; **, P < 10−3; ***, P < 10−4). Data for the SHM are from five independent experiments. (D) Differential expression of Polη in FANCA- and FANCG-deficient cells. Whole cell extracts were prepared from the indicated human lymphoblast cell lines and analyzed by immunoblotting for the expression of Polη, MSH2, FANCA, FANCG, and Vinculin (asterisks indicate nonspecific bands). Representative data from two independent experiments are shown.

Mentions: To determine the impact of Fanca loss-of-function on SHM, we compared the levels and patterns of somatic mutation in the nonselected intronic region flanking the rearranged VHDJH4 genes (JH4 intronic region) in Peyer’s patch peanut agglutininhigh (PNAhigh) B cells isolated from WT and Fanca−/− mice. To calculate mutation frequency, the accumulated number of unique mutations was divided by the theoretical maximum number of the corresponding type of mutation to correct for base composition (MacCarthy et al., 2009). We observed that the distributions of mutations along the analyzed region were similar in Fanca−/− and WT mice (Fig. 1 A) but that the overall mutation frequency was significantly lower (P < 10−3) in B cells from Fanca−/− mice (Table 1). However, Fanca−/− mice showed a consistent but nonsignificant decrease in the proportion of highly mutated sequences (>10 mutations) and an increase in the proportion of sequences with <5 mutations (Fig. 1 B). Consequently, the reduced mutation frequency we observed could be accounted for solely by the reduction in the number of highly mutated sequences. Alternatively, the number of DNA lesions generated by AID or the number of mutations introduced during the course of SHM could prevent Fanca−/− cells from proliferating normally within the GC, leading to an apparent reduction in mutation frequency; i.e., the accumulation, not the generation, of mutations may be compromised in Fanca−/− mice.


Fanca deficiency reduces A/T transitions in somatic hypermutation and alters class switch recombination junctions in mouse B cells.

Nguyen TV, Riou L, Aoufouchi S, Rosselli F - J. Exp. Med. (2014)

Reduced A/T transitions during SHM in Fanca−/− mice. (A) Distribution of mutations in the JH4 intronic region (506 bp) that was amplified from Peyer’s patch PNAhigh B cells isolated from WT and Fanca−/− mice. (B) Proportion of sequences with numbers of mutations per clone (the central circle shows the total number of analyzed sequences) from WT (n = 5) and Fanca−/− mice (n = 5). (C) The spectrum of base substitutions is expressed as a percentage of the total number of mutations (left), and the frequency of mutation (right) was corrected for base composition. Gray boxes denote a significant decrease in mutation frequency compared with WT (the χ2 test was applied according to the SHMTool algorithm; *, P < 0.05; **, P < 10−3; ***, P < 10−4). Data for the SHM are from five independent experiments. (D) Differential expression of Polη in FANCA- and FANCG-deficient cells. Whole cell extracts were prepared from the indicated human lymphoblast cell lines and analyzed by immunoblotting for the expression of Polη, MSH2, FANCA, FANCG, and Vinculin (asterisks indicate nonspecific bands). Representative data from two independent experiments are shown.
© Copyright Policy - openaccess
Related In: Results  -  Collection

License 1 - License 2
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getmorefigures.php?uid=PMC4042646&req=5

fig1: Reduced A/T transitions during SHM in Fanca−/− mice. (A) Distribution of mutations in the JH4 intronic region (506 bp) that was amplified from Peyer’s patch PNAhigh B cells isolated from WT and Fanca−/− mice. (B) Proportion of sequences with numbers of mutations per clone (the central circle shows the total number of analyzed sequences) from WT (n = 5) and Fanca−/− mice (n = 5). (C) The spectrum of base substitutions is expressed as a percentage of the total number of mutations (left), and the frequency of mutation (right) was corrected for base composition. Gray boxes denote a significant decrease in mutation frequency compared with WT (the χ2 test was applied according to the SHMTool algorithm; *, P < 0.05; **, P < 10−3; ***, P < 10−4). Data for the SHM are from five independent experiments. (D) Differential expression of Polη in FANCA- and FANCG-deficient cells. Whole cell extracts were prepared from the indicated human lymphoblast cell lines and analyzed by immunoblotting for the expression of Polη, MSH2, FANCA, FANCG, and Vinculin (asterisks indicate nonspecific bands). Representative data from two independent experiments are shown.
Mentions: To determine the impact of Fanca loss-of-function on SHM, we compared the levels and patterns of somatic mutation in the nonselected intronic region flanking the rearranged VHDJH4 genes (JH4 intronic region) in Peyer’s patch peanut agglutininhigh (PNAhigh) B cells isolated from WT and Fanca−/− mice. To calculate mutation frequency, the accumulated number of unique mutations was divided by the theoretical maximum number of the corresponding type of mutation to correct for base composition (MacCarthy et al., 2009). We observed that the distributions of mutations along the analyzed region were similar in Fanca−/− and WT mice (Fig. 1 A) but that the overall mutation frequency was significantly lower (P < 10−3) in B cells from Fanca−/− mice (Table 1). However, Fanca−/− mice showed a consistent but nonsignificant decrease in the proportion of highly mutated sequences (>10 mutations) and an increase in the proportion of sequences with <5 mutations (Fig. 1 B). Consequently, the reduced mutation frequency we observed could be accounted for solely by the reduction in the number of highly mutated sequences. Alternatively, the number of DNA lesions generated by AID or the number of mutations introduced during the course of SHM could prevent Fanca−/− cells from proliferating normally within the GC, leading to an apparent reduction in mutation frequency; i.e., the accumulation, not the generation, of mutations may be compromised in Fanca−/− mice.

Bottom Line: Whereas SHM involves an error-prone repair process that introduces novel point mutations into the Ig gene, the mismatches generated during CSR are processed to create double-stranded breaks (DSBs) in DNA, which are then repaired by the NHEJ pathway.As several lines of evidence suggest a possible role for the FANC pathway in SHM and CSR, we analyzed both processes in B cells derived from Fanca(-/-) mice.Here we show that Fanca is required for the induction of transition mutations at A/T residues during SHM and that despite globally normal CSR function in splenic B cells, Fanca is required during CSR to stabilize duplexes between pairs of short microhomology regions, thereby impeding short-range recombination downstream of DSB formation.

View Article: PubMed Central - HTML - PubMed

Affiliation: Centre National de la Recherche Scientifique UMR 8200, Institut Gustave Roussy, 94805 Villejuif, France Université Paris Sud, 91400 Orsay, France Programme Equipe Labellisées, Ligue Contre le Cancer, 75013 Paris, France.

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Related in: MedlinePlus