Phenotypic Mutation 'xander' (pdf version)
Allelexander
Mutation Type splice acceptor site (4 bp from exon)
Chromosome19
Coordinate
Base Change
Gene Nfkb2
Gene Name nuclear factor of kappa light polypeptide gene enhancer in B cells 2, p49/p100
Synonym(s) NF kappaB2, p52
Chromosomal Location 46,304,737-46,312,090 bp (+)
MGI Phenotype Homozygotes for targeted null mutations exhibit gastric hyperplasia, enlarged lymph nodes, enhanced cytokine production by activated T cells, absence of Peyer's patches, increased susceptibility to Leishmania major, and early postnatal mortality.
Accession Number

NCBI RefSeq: NM_019408; MGI: 1099800

Mapped Yes 
Amino Acid Change
Institutional SourceAustralian Phenomics Network
Ref Sequences
Ensembl: ENSMUSP00000107512 (fasta)
Gene Model not available
SMART Domains

DomainStartEndE-ValueType
Pfam:RHD 40 220 2.1e-62 PFAM
IPT 227 326 3.48e-27 SMART
low complexity region 351 382 N/A INTRINSIC
low complexity region 391 409 N/A INTRINSIC
ANK 487 522 5.58e1 SMART
ANK 526 555 9.78e-4 SMART
ANK 559 591 3.74e0 SMART
ANK 599 628 3.36e-2 SMART
ANK 633 663 1.3e1 SMART
ANK 667 696 4.26e-4 SMART
low complexity region 707 721 N/A INTRINSIC
ANK 729 758 2.35e3 SMART
DEATH 764 851 5.52e-16 SMART
low complexity region 879 894 N/A INTRINSIC
Phenotypic Category Autosomal Recessive
Penetrance 100% 
Alleles Listed at MGI

All alleles(7) : Targeted, knock-out(5) Chemically induced(2)

Lab Alleles
AlleleSourceChrCoordTypePredicted EffectPPH Score
IGL01466:Nfkb2 APN 19 46308016 missense probably damaging 0.96
IGL01791:Nfkb2 APN 19 46309839 unclassified 0.00
IGL01966:Nfkb2 APN 19 46309690 missense probably benign 0.04
IGL03296:Nfkb2 APN 19 46309928 missense probably damaging 1.00
pale_fire UTSW 19 46311626 missense possibly damaging 0.96
R0270:Nfkb2 UTSW 19 46311626 missense possibly damaging 0.96
R0561:Nfkb2 UTSW 19 46309862 missense possibly damaging 0.93
R1944:Nfkb2 UTSW 19 46308052 missense probably damaging 1.00
R2217:Nfkb2 UTSW 19 46307724 splice site probably null
R2878:Nfkb2 UTSW 19 46307441 missense possibly damaging 0.86
R4493:Nfkb2 UTSW 19 46308439 missense probably damaging 0.99
R4494:Nfkb2 UTSW 19 46308439 missense probably damaging 0.99
R4495:Nfkb2 UTSW 19 46308439 missense probably damaging 0.99
R4731:Nfkb2 UTSW 19 46308964 missense possibly damaging 0.74
R4752:Nfkb2 UTSW 19 46307567 missense probably benign 0.02
R4753:Nfkb2 UTSW 19 46307567 missense probably benign 0.02
R4777:Nfkb2 UTSW 19 46307567 missense probably benign 0.02
R4780:Nfkb2 UTSW 19 46309922 missense probably damaging 1.00
R4820:Nfkb2 UTSW 19 46308054 missense probably damaging 0.99
R4837:Nfkb2 UTSW 19 46307567 missense probably benign 0.02
R4839:Nfkb2 UTSW 19 46307567 missense probably benign 0.02
R5514:Nfkb2 UTSW 19 46311408 missense probably damaging 1.00
R5519:Nfkb2 UTSW 19 46307567 missense probably benign 0.02
R5549:Nfkb2 UTSW 19 46307567 missense probably benign 0.02
R5615:Nfkb2 UTSW 19 46307567 missense probably benign 0.02
R5616:Nfkb2 UTSW 19 46307567 missense probably benign 0.02
R5709:Nfkb2 UTSW 19 46310521 missense probably damaging 1.00
R6053:Nfkb2 UTSW 19 46311812 missense probably damaging 1.00
S24628:Nfkb2 UTSW 19 46307567 missense probably benign 0.02
Mode of Inheritance Autosomal Recessive
Local Stock None
Repository

Australian PhenomeBank: 93

Last Updated 2017-03-28 2:17 PM by Katherine Timer
Record Created 2010-02-01 4:00 PM by Nora G. Smart
Record Posted 2010-02-02
Phenotypic Description
The xander mutation was discovered in a screen of ethylnitrosourea (ENU)-mutagenized G3 C57BL/6 mice, which had their blood lymphocyte subsets screened by flow cytometry. Homozygous xander (xdr/xdr)mice have reduced numbers of circulating B cells in the blood as assessed by the B220 cell-surface marker, which is the B cell form of CD45 (see the record for belittle). T lymphocytes are present in normal numbers in these animals (1) (Figure 1A).
 
After B cell formation occurs in the bone marrow (BM), B cell maturation in the spleen proceeds from transitional type I (T1) cells to transitional type 2 (T2) cells, which then mature into recirculating (follicular) B2 cells and marginal zone (MZ) B cells (2). These B cell subsets are distinguished by cell surface expression of IgM, IgD, and CD21. Analysis of B cell development in xdr/xdr mice demonstrated normal development in the BM, but reduced numbers of T2, mature, and MZ B cells in the spleen, blood and lymph nodes compared with wild type mice (Figure 1). The number of B2 cells in the peritoneal cavity of xdr/xdr animals is normal, while the number of B1 cells is increased sevenfold. B1 cells differ from B2 cells in terms of anatomical localization, restricted immunoglobin (Ig) repertoire, cell surface markers and ability to self-renew in the periphery (3). BM chimeras using mixtures of normal and mutant BM into irradiated wild type mice, demonstrated that the B cell defect in xdr/xdr animals is cell autonomous. Although xdr versus wild type chimeras were equally reconstituted with the two donor marrows and formed similar numbers of mutant and wild type pre-B cells and immature B cell in their BM, the numbers of more mature B cells of xdr/xdr origin showed a dramatic and selective deficiency compared with donor wild type cells.  
 
Xdr/xdr mice also exhibit abnormalities in secondary lymphoid organs (SLOs) with abnormally small lymph nodes and disturbed splenic architecture. The white pulp of the spleen is subdivided into T and B cell zones, and surrounded by marginal metallophilic macrophages (MMM). Follicular dendritic cells (FDCs) form prominent networks within the B cell follicles and provide critical interactions for B and T cells during antibody responses. While T cell zones in xdr/xdr mice are normal, B cell follicles and FDC networks are reduced. MMMs are also reduced in number (Figure 2).
Nature of Mutation
The xander mutation was mapped to Chromosome 19, and corresponds to a T to A transversion in the acceptor splice site of intron 6 (GTAAG -> GAAAG) of the Nfkb2 gene on Chromosome 19 (position 3163 in the Genbank genomic region NC_000085 for linear genomic DNA sequence of Nfkb2). Nfkb2 contains 23 exons. RT-PCR amplification of exons 5-8 in C57BL/6 mice yielded two products; one reads into intron 5 and contains a premature stop codon. Amplification of the same exons in xdr/xdr mice results in the production of five aberrant transcripts. Two of the aberrant transcripts retained an unspliced intron between exon 6 and 7. In the other three transcripts, exon 6 was spliced to cryptic splice acceptor sites within exon 7 resulting in the absence of the first 34 or 37 bases of exon 7. All of the transcripts contain stop codons that prematurely truncate NF-κB2 in the middle of the DNA-binding domain (Figure 3). 
Protein Prediction
The Nfkb2 gene encodes a roughly 100 kD 899 amino acid protein (p100), which is cleaved at amino acids 454-455 to produce an active 52-kD transcription factor (p52) that is a member of the NF-κB family of transcription factors [reviewed by (4;5)]. Members of this family include NF-κB1 (p50 and its precursor p105), RelA (also known as p65), c-Rel, and RelB, all of which are characterized by the presence of an N-terminal Rel homology domain (RHD) that is responsible for homo- and heterodimerization as well as for sequence-specific DNA binding (Figure 4). Unlike RelA, c-Rel, and RelB, p52 and p50 do not contain a C-terminal transcription activation domain (TAD). Thus, p52 and p50 rely on interactions with other transcription factors or NF-κB proteins that contain this domain in order to positively regulate transcription. p52 is able to heterodimerize with most other NF-κB proteins, but preferentially binds to RelB, as does the p100 precursor (6-8). p52 homodimers likely repress transcription by competing with other NF-κB dimers for binding sites in the promoters of NF-κB activated genes. However, association with the transcription factor BCL3 converts p52 homodimers to a transcriptionally active trimer (9).
 
NF-κB dimers bind to DNA sequences known as κB sites. Most κB sites appear to be 10 base pairs (bp) in length with the consensus sequence 5‘-GGRNNWYYCC-3' (where R= purine, N= any base, W= adenine or thymine and Y = pyrimidine) [reviewed by (10)]. p52/RelB heterodimers have been reported to bind to κB sites that diverge significantly from classical κB sites with the sequence 5’-RGGAGAYTTR-3’. However, other data suggest that p52/RelB is unable to bind these sites in vitro (11). The p50/p52 subunits have been shown to bind to the 5’-GGGRN-3’ half-site, while RelA, c-Rel, and RelB preferentially bind to the 5’-YYCC-3’ half-site. p50 and p52 homodimers prefer an 11 bp site comprising two 5 bp half-sites separated by a central A/T bp (10)
The RHD domain of p52 has been co-crystallized with NF-κB DNA-binding sites, either as a homodimer (12) or as a heterodimer with RelB (13) (Figure 5). The crystal structure of RHD domains from various NF-κB transcription factors reveals that they are composed of two immunoglobulin (Ig)-like folds linked by a short polypeptide (12;14;15). The N-terminal nine-stranded β-barrel Ig-like domain of p52 is involved in DNA recognition, while the seven-stranded C-terminal β-barrel is responsible for dimer formation. The DNA is contacted by five loops per monomer, two of them in the N-terminal domain, two in the C-terminal domain and one being the five-amino acid interdomain linker. The DNA recognition loop in the N-terminal domain contacts four consecutive G:C base pairs in the major goove through side chain interactions of His62, Arg54, Arg52 and Lys221, respectively (5’ to 3’). The p52/DNA complex contains many water molecules, which mediate the interactions in the protein-DNA interface (12). In most NF-κB dimers, each monomer contributes symmetrical β-strand elements that pack against each other to form a β-sheet dimer interface. Roughly, 12 side chains from each subunit mediate symmetrical (for homodimers) or pseudo symmetrical (for heterodimers) inter-subunit contacts (10;16). The amino acid residues at the dimer interface are highly conserved across NF-κB proteins, and include Leu249, Ala286 and Val288. In p52, Tyr285 controls the specificity of dimerization. This residue is located at the lower part of the dimer interface, and along with Leu249 contributes most to the hydrophobic interface (12).
Although NF-κB dimer structures are relatively similar, major differences occur within the insert regions of these proteins. The insert region is located within an extension between the last two β-strands of the N-terminal domain (Figure 5), and shows very little sequence homology amongst different NF-κB family members. The insert region in p52 forms an autonomous module of two helices connected by a short loop. Helix αA points with its N-terminal end towards the phosphate backbone from the minor groove side. In p52, the insert region is rotated away from the core domain surface, opening up a deep, mainly polar cleft. The insert region presents potential interaction surfaces to bind to other proteins (12).
 
In addition to being members of the NF-κB family, both the p100 and p105 proteins contain multiple C-terminal ankyrin repeats (seven in p100), a common feature shared by IκB family members (Figure 4). These are proteins that inhibit the NF-κB pathway by sequestering NF-κB proteins in the cytoplasm (4;5). The folding of the ankyrin repeats allows p100 to mask its nuclear localization signal (NLS) at amino acids 337-341. Between the RHD and the ankyrin repeats is a glycine-rich region (GRR) that is essential for determining the site of p100 cleavage into the p52 subunit. This region also prevents full degradation of p100 through the proteasome (17;18). The p100 RHD domain is phosphorylated at residues 99, 108, 115, and 123 by the IKK-1 kinase (also called IKK-α; see Background). p100 is also phosphorylated at Ser871 by IKK-1, and at Ser865 and Ser869 by the NF-κB inducing kinase (NIK; see the record for lucky). Phosphorylation at all of these sites is required for efficient processing of p100 to p52 (7;19;20), and results in the creation of a binding site for the recognition subunit of the E3 ligase SCFβ-TrCP , homologous to those found in IκB proteins. This triggers K48-linked ubiquitination of p100 on Lys856 and subsequent degradation to p52 (21;22). p100 also contains a C-terminal death domain (DD) at amino acids 764-851 that associates with the S9 subunit of the 19 S protease (22) (Figure 6). Death domains are conserved regions that are usually involved in homotypic protein interactions, and are composed of a bundle of six α-helices.

Figure 6. Domain structure of the NF-κB2 p100 protein. The location of the xander mutation is shown. The image is interactive; click to view additional mutations in Nfkb2.

 
 
Both p100 and p105 have been shown to bind to and form high molecular-weight complexes with NF-κB proteins through interactions with their RHD, ankryin repeats and their processing regions (23-27). The processing region of p100 occurs at amino acids 453-480, and can form homotypic α-helical dimers (27). However, the p100 C-terminus appears to bind preferentially to RelB (6-8;24). This preference is strengthened by the strong interaction of the p100 RHD domain with RelB. Furthermore, the TAD of RelB, but not RelA, directly interacts with the processing region of p100 (26). p52/RelB dimers are generated from the proteolytic cleavage of a unique pool of p100/RelB (28). Other p52-containing dimers like p52/RelA or p52/c-Rel are generated from p100/p65 and p100/c-Rel pools (29)
 
The xander mutation results in abnormal splicing of the Nfkb2 gene with premature truncation of the NF-κB2 protein in the middle of the DNA-binding domain (Figure 6). Western blot analysis demonstrates the absence of p100 in xdr/xdr mice (1).
Expression/Localization
Nfkb2 mRNA is ubiquitously expressed in all organs, with highest levels in the lymph nodes and thymus, and moderate levels in the ileum and ovary (30). Strong expression has been reported in the surface epithelial layer of mouse stomach (31). During mouse embryogenesis, Nfkb2 expression is generally absent (32;33), although p52 is present in embryonic day (E) 18 muscle (34). Nfkb2 mRNA and protein are present in oocytes and trophoblast giant cells (35).
 
Nfkb2 expression levels are detectable in all tested cell types (36). In unstimulated cells, NF-κB2 proteins are cytoplasmic, but p52-containing dimers are translocated into the nucleus in response to various stimuli (37). In human tissues, high levels of nuclear p52 have been demonstrated in accessory cells of the immune system in the spleen, including FDCs in the germinal centers (GC), and dendritic cells (DCs) and macrophages in the T cell zone (38).
Background
The NF-κB signaling pathway functions in essentially all mammalian cell types and is activated in response to injury, infection, inflammation and other stressful conditions requiring rapid reprogramming of gene expression. Typically, the rapid and transient activation of NF-κB complexes in response to a wide range of stimuli such as proinflammatory cytokines tumor necrosis factor (TNF)-α, interleukin (IL)-1β, IL-6, and CD40L (see the record for walla), DNA damaging agents, Toll-like receptor (TLR) agonists or viruses is regulated by the canonical NF-κB pathway. In the resting cell, NF-κB dimers are kept inactive in the cytoplasm through their association with the IκB inhibitory molecules, which can include p105 and p100 (23). In response to stimulation, IκBs are phosphorylated by the IκB kinase (IKK) complex, composed of IKK-1 (or IKK-α), IKK-2 (or IKK-β) and IKK-γ, at conserved serine residues (please see the record for panr2).  This modification induces the K48-linked polyubiquitination of IκB molecules and subsequent recognition by the 26S proteasome as substrates for proteolysis. Degradation of IκBs allows the NF-κB dimers to translocate into the nucleus, where they are able to activate the transcription of target genes, including genes encoding various cytokines [for review see (4;5). The genes encoding the IκB proteins have been reported to be NF-κB target genes. Therefore, NF-κB activation generates feedback inhibition within the pathway, whereby re-synthesized IκBs sequester nuclear NF-κB dimers and mediate nuclear export of the inhibited NF-κB dimers to ensure appropriate termination of the canonical NF-κB response. Genetic studies have demonstrated that the IKK-2 and NEMO subunits of the IKK complex are required for canonical NF-κB activation (39), while IKK-1 is required for the non-canonical or alternative NF-κB pathway (37).
 
Some of the inducers of the canonical NF-κB pathway are also able to trigger the non-canonical NF-κB pathway. The non-canonical NF-κB pathway drives the post-translational processing of p100 to mature p52 through IKK-1 and NIK, and results in the activation of p52/RelB heterodimers (4;7;8;40), and appears to be mostly restricted to a subset of tumor necrosis factor (TNF) receptors including lymphotoxin-β receptor (LTβR), B cell activating receptor (BAFFR), CD40, receptor activator of NF-κB (RANK) and TNF-related weak inducer of apoptosis (TWEAK) (19;41-47). These receptors are involved in secondary lymphoid organogenesis (SLO), B cell differentiation, survival and homeostasis, osteoclastogenesis, and angiogenesis (37), and bind to TNF receptor associated factors (TRAFs) to regulate NIK activity. TRAF2 and TRAF5 positively regulate NIK activity under certain conditions (48;49), but in other contexts, TRAF2 and TRAF3 form a complex with NIK to mediate NIK degradation (50-53). After stimulation with BAFFR/CD40, the complex is destabilized by TRAF2/3 degradation, permitting the release of NIK from the complex (51-53). After NIK is activated, it is able to bind to and phosphorylate several substrates including IKK-1 and p100, and serves as a docking molecule between IKK-1 and p100 (19;20;54). Phosphorylation of p100 by IKK-1 results in polyubiquitination and processing to p52 (20;22). The non-canonical NF-κB pathway is generally slower than the activation of the canonical pathway, leading to delayed, but sustained activation of nuclear p52-containing complexes. 
 
Although p100 processing defines the non-canonical NF-κB pathway, the interaction of both p100 and p52 with other NF-κB subunits suggests an important role for NF-κB2 proteins in the canonical pathway as well. NIK stabilization also induces canonical NF-κB signaling by activating IKK-2 (52), and p100 processing has been shown to regulate the nuclear localization of RelA in addition to RelB (24). Other studies suggest that the steady-state expression of RelB is regulated by the canonical pathway, which in turn impacts non-canonical NF-κB dimer activation (55).
 
Figure 7. Canonical and non-canonical NF-κB signaling pathways. In the canonical pathway, several membrane receptors, including TNFR (tumor-necrosis factor receptor), IL-1R (interleukin-1 receptor) and TLRs (toll-like receptors), signal through kinases and adaptors (TRAFs), resulting in IKK activation. This activation occurs after the K63 ubiquitination of TRAFs and RIP. TAK1 and its adaptor proteins TAB1 and TAB2 bind ubiquitin chains to TRAF and NEMO (IKKγ) resulting in the activation of the IKK complex (NEMO, IKKα and IKKβ). Ubiquitination can be inhibited by deubiquitinating enzymes (DUB). Stimulation of the T-cell receptor (TCR) and B-cell receptor (BCR) results in the recruitment of Src and Syk family kinases. These kinases activate a phosphorylation cascade which leads to the activation of protein kinase C (PKC). The phosphorylation of CARMA1 (CARD (caspase-recruitment domain)-MAGUK (membrane-associated guanylate kinase) protein 1) recruits BCL 10 (B-cell lymphoma 10) and MALT1 (mucosa-associated lymphoid tissue lymphoma translocation gene 1), forming the CBM complex and activating the IKK complex. The IKK complex phosphorylates both IκB, p105 and TPL2 (or MAP3K8), resulting in IκB and p105 ubiquitination and degradation (small pink circles) by 26S proteasome. Degradation of IκB releases activated NF-κB dimers for translocation to the nucleus. A subset of TNFRs such as the lymphotoxin-β receptor (LT-βR),CD40, B-cell-activating factor receptor (BAFFR) and receptor activator of Nf-κB (RANK) can activate the canonical or non-canonical NF-κB signaling pathways. In the non-canonical pathway, the receptors bind to TRAFs to regulate NIK activity. TRAF3 and TRAF2 are recruited to the receptor along with cIAP1/2. TRAF2 undergoes K63 self-ubiquitination and is responsible for the K63 ubiquitination of cIAP1/2. TRAF3 is degraded by K48 ubiquitination, enhanced by the K63 ubiquitination of TRAF2 and cIAP1/2. (Gray arrows represent ubiquitination dependence.) As TRAF levels decrease, NIK is released and phosphorylates IKKα which phosphorylates p100. Phosphorylation and ubiquitination of p100 leads to the 26S proteasomal degradation of p100 and the processing of p52. P52 and RelB are released for translocation to the nucleus. This image is interactive. Click on the image to view mutations in the pathway (red) and the genes affected by these mutations (black). Click on the mutations for more specific information.
Mice homozygous for targeted null mutations of Nfkb2 exhibit absence of Peyer's patches (PPs) and reduced lymph nodes, increased susceptibility to certain pathogens, impaired B cell maturation, aberrant T cell function, and early postnatal mortality [reviewed by (39)]. They also exhibit disruption of splenic architecture that includes an absence of the splenic follicular marginal zone and marked depletion of B cell follicular areas or germinal centers (56-58). The defects in lymph node, PPs, and splenic architecture suggest a general defect in SLO. Similar defects are also present in mice lacking LTβ (59), LTβR (60), NIK (61) or RelB (8;62). LTBR signaling through the non-canonical NF-κB pathway is required in stromal cells (58) to induce p52/RelB-dependent expression of stromal cell-derived factor (SDF), Epstein-Barr virus-induced molecule 1 (EB-1) and B lymphocyte chemoattractrant (BLC), chemokines that are crucial for lymphoid organogenesis. The promoters for all of these factors contain the divergent κB DNA binding sites that have been reported to be bound by the p52/RelB heterodimer (63). However, recent data suggests that the p50/RelA dimers can also activate expression of these factors (55). Both RelA knockouts that have been rescued from embryonic lethality and mice lacking both Nfkb1 and Nfkb2 exhibit a much more severe defect in SLO (64;65), suggesting that both canonical and non-canonical NF-κB pathways may be important during this process.
 
In addition to SLO, LTBR signaling through the NF-κB non-canonical pathway is critical for thymic organogenesis and self-tolerance. This is evidenced by the development of autoimmune phenotypes in mice with mutations in Nfkb2, Ltβr, Map3k14 (encoding NIK), and Ikkα (66-69)Nfkb2-/- animals display an increased infiltration of activated T cells in organs and higher titers of autoreactive antibodies. As for SLOs, the defect occurs in the stromal compartment with impaired development and maturation of the thymic epithelial cells that express the autoimmune regulator (AIRE), a transcription factor necessary to develop T cell self-tolerance. Mice with mutations in Aire also develop autoimmune disease (70)
 
Nfkb2-/- animals show multiple immune cell abnormalities. The observed reduction in mature B cells results from a survival defect caused by lack of BAFFR activation of the non-canonical NF-κB pathway, and impaired expression of the pro-survival factor, Bcl-2 (43;57;71). Although Nfkb2-/- animals display abnormal humoral responses, this defect is B cell extrinsic and surviving B cells appear to function normally (56;57). Mice deficient in BCL3, which can form active transcriptional complexes with NF-κB proteins, also exhibit impaired humoral immune responses and lack splenic germinal centers (72;73). The role of NF-κB2 in T cell function is complex. Nfkb2-/- T cells appear normal in vitro (57), but CD4+ T helper (Th)1 responses mounted by Nfkb2-/- mice are abnormal, causing high susceptibility to the Leishmania major pathogen. Normal interferon (IFN)-γ responses do not develop due to impaired CD40-dependent production of the interleukin (IL)-12 cytokine by Nfkb2-/- macrophages (74). Defective IFN-γ production also causes susceptibility to the intracellular parasite Toxoplasma gondii, but macrophage function and IL-12 production are normal in this context. Instead, mice display a reduction in IFN-γ producing T cells (75). Mice deficient in RelB are similarly susceptible to T. gondii (76). In DCs, p100 functions as a negative regulator of RelB (77), and Nfkb2-/- DCs display increased RelB activity and enhanced ability to induce CD4+ T cell responses. 
 
Other NF-κB2 mutations in mice include a point mutation that results in expression of a “super-repressive” p100 protein (78), and a targeted deletion of the p100 C-terminal ankyrin repeats (designated as p100-/-) resulting in lack of NF-κB inhibition by p100 (31). Animals expressing “super-repressive” p100 display phenotypes that resemble those found in Nfkb2-/- mice, but are more severe (78). In these mice, the C-terminal phosphorylation site at amino acid 869 is substituted by a stop codon. The expressed protein cannot be processed into p52, and continues to inhibit the activation of NF-κB subunits, including RelA (77). The severe defects in SLO and impaired osteoclastogenesis found in these animals resembles Nfkb1/Nfkb2 double knockouts (65;79), and is likely due to the inhibition of both canonical and non-canonical NF-κB signaling by unprocessed p100.
 
p100-/-animals display a wide range of postnatal pathologies including gastric hyperplasia, hyperkeratosis in the heart and spleen, lymphocytic infiltrates of various tissues, enlarged lymph nodes, and enhanced cytokine production in activated T cells. Affected tissues from these mice express higher than normal levels of p52 dimers and display increased expression of NF-κB target genes (31). Unlike Nfkb2 nulls, which display a reduction in mature B cells, these animals show increased B cell survival resulting in large numbers of MZ B cells expressing autoantibodies (80). A similar phenotype is observed in BAFF transgenic mice (81)p100-/-animals also have a partial early block in B cell development in the bone marrow with B cell progenitors maintaining a myeloid differentiation potential at the expense of B lymphopoiesis (82). Mice lacking TRAF2 or TRAF3 display similar B cell abnormalities due to the constitutive activation of NIK and non-canonical NF-κB signaling (83;84)
 
In humans, C-terminal deletions and rearrangements of the NFKB2 gene are associated with the development of human cutaneous T cell lymphomas, chronic lymphocytic leukemias and multiple myelomas (85). These deletions result or rearrangements result in the removal of some or all of the sequences encoding the ANK repeat domain of p100, while sequences encoding truncated p100 remain intact. The altered NFKB2 genes and their encoded truncated p100 proteins (and sometimes processed p52) are often overexpressed in human lymphoid cell malignancies (86). The C-terminal truncations in p100 probably result in over-activation of NF-κB pathways, which often promote survival and growth. These truncations also remove the death domain, which may contribute to oncogenesis by rendering cells more resistant to apoptotic stimuli (87). The oncogenic potential of human NFKB2 mutations is supported by the phenotypes found in p100-/-mice (31).
Putative Mechanism
The xander mutation causes aberrant mRNA splicing resulting in the absence of any functional NF-κB2 protein. The phenotypes found in animals homozygous for this mutation are consistent with the reduced numbers of B cells in the spleen and the absence of FDCs and MMM described in targeted null mutations in Nfkb2 (56;57;88). Further examination of these phenotypes in xdr/xdr mice suggests that Nfkb2 function is required to establish normal numbers of mature B2 cells. The deficiency in B cell numbers first occurs after the T1 stage of B cell maturation in the spleen. 
 
Although Nfkb2 function is required for conventional B cell maturation, the analysis of xdr/xdr mice demonstrates that Nfkb2 is not required for peritoneal B cells. Instead, xdr/xdr animals display increased numbers of B1 cells, a phenotype that is similar to that seen in alymphoplasia (aly) mice with a point mutation in NIK (89). The increased numbers of B1 cells in the peritoneum is suggested to arise from a migration defect in aly/aly peritoneal B cells caused by a defect in SLO chemokine receptor signaling (90). It is possible that NIK activation of NF-κB2 in response to chemokine receptor signaling plays a role in peritoneal B cell migration. 
 
The specific pattern in B cell defects in mice deficient for NF-κB2, suggests that the non-canonical NF-κB pathway is important for B2, but not B1, survival. Both B1 and B2 cells require basal stimulation through the B cell receptor (BCR) to survive and accumulate, and BCR activation of the canonical NF-κB pathway is required for early B cell development. Defects in the BCR signaling pathway result in peripheral B cell deficiencies with more severe reductions in B1 subsets [reviewed by (2;91)]. By contrast, the defects found in xdr/xdr mice closely resemble those found in BAFFR, BAFF, or NIK-deficient animals suggesting that the non-canonical pathway is important in transducing BAFF survival signals in circulating B cells (71;92-94).
Primers Primers cannot be located by automatic search.
References
Science Writers Nora G. Smart
Illustrators Diantha La Vine
AuthorsLisa A. Miosge, Julie Blasioli, Mathieu Blery and Christopher C. Goodnow
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