Beta-2 microglobulin

β2 microglobulin also known as B2M is a component of MHC class I molecules, MHC class I molecules have α1, α2, and α3 proteins which are present on all nucleated cells (excludes red blood cells).[5][6] In humans, the β2 microglobulin protein[7] is encoded by the B2M gene.[6][8]

B2M
Available structures
PDBOrtholog search: PDBe RCSB
Identifiers
AliasesB2M, entrez:567, IMD43, beta-2-microglobulin, Β2 microglobulin
External IDsOMIM: 109700 MGI: 88127 HomoloGene: 2987 GeneCards: B2M
Gene location (Human)
Chr.Chromosome 15 (human)[1]
Band15q21.1Start44,711,487 bp[1]
End44,718,877 bp[1]
RNA expression pattern


More reference expression data
Orthologs
SpeciesHumanMouse
Entrez

567

12010

Ensembl

ENSG00000166710
ENSG00000273686

ENSMUSG00000060802

UniProt

P61769

P01887

RefSeq (mRNA)

NM_004048

NM_009735

RefSeq (protein)

NP_004039

NP_033865

Location (UCSC)Chr 15: 44.71 – 44.72 MbChr 2: 122.15 – 122.15 Mb
PubMed search[3][4]
Wikidata
View/Edit HumanView/Edit Mouse

Structure and function
Schematic representation of MHC class I

β2 microglobulin lies beside the α3 chain on the cell surface. Unlike α3, β2 has no transmembrane region. Directly above β2 (that is, further away from the cell) lies the α1 chain, which itself is next to the α2.

β2 microglobulin associates not only with the alpha chain of MHC class I molecules, but also with class I-like molecules such as CD1 and Qa.

An additional function is association with the HFE protein, together regulating the expression of hepcidin in the liver which targets the iron transporter ferroportin on the cytoplasmic membrane of enterocytes and macrophages for degradation resulting in increased iron uptake from food and decreased iron release from recycled red blood cells in the MPS (mononuclear phagocyte system) respectively. Loss of this function causes iron excess and hemochromatosis.

Mice models deficient for the β2 microglobulin gene have been engineered. These mice demonstrate that β2 microglobulin is necessary for cell surface expression of MHC class I and stability of the peptide binding groove. In fact, in the absence of β2 microglobulin, very limited amounts of MHC class I (classical and non-classical) molecules can be detected on the surface. In the absence of MHC class I, CD8 T cells cannot develop. (CD8 T cells are a subset of T cells involved in the development of acquired immunity.)

Clinical significance

In patients on long-term hemodialysis, it can aggregate into amyloid fibers that deposit in joint spaces, a disease, known as dialysis-related amyloidosis.

Low levels of β2 microglobulin can indicate non-progression of HIV.

Levels of β2 microglobulin can be elevated in multiple myeloma and lymphoma, though in these cases primary amyloidosis (amyloid light chain) and secondary amyloidosis (amyloid associated protein) are more common. The normal value of β2 microglobulin is < 2 mg/L.[9] However, with respect to multiple myeloma, the levels of β2 microglobulin may also be at the other end of the spectrum. Diagnostic testing for multiple myeloma includes obtaining the β2 microglobulin level, for this level is an important prognostic indicator. As of 2011, a patient with a level < 4 mg/L is expected to have a median survival of 43 months, while one with a level > 4 mg/L has a median survival of only 12 months.[10] β2 microglobulin levels cannot, however, distinguish between monoclonal gammopathy of undetermined significance (MGUS), which has a better prognosis, and smouldering (low grade) myeloma.[11][12]

Loss-of-function mutations in this gene have been reported in cancer patients unresponsive to immunotherapies.

References
  1. ENSG00000273686 GRCh38: Ensembl release 89: ENSG00000166710, ENSG00000273686 - Ensembl, May 2017
  2. GRCm38: Ensembl release 89: ENSMUSG00000060802 - Ensembl, May 2017
  3. "Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
  4. "Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
  5. "Entrez Gene: Beta-2-microglobulin".
  6. Güssow D, Rein R, Ginjaar I, Hochstenbach F, Seemann G, Kottman A, Ploegh HL (1 November 1987). "The human beta 2-microglobulin gene. Primary structure and definition of the transcriptional unit". J. Immunol. 139 (9): 3132–8. PMID 3312414.
  7. Cunningham BA, Wang JL, Berggård I, Peterson PA (November 1973). "The complete amino acid sequence of beta 2-microglobulin". Biochemistry. 12 (24): 4811–22. doi:10.1021/bi00748a001. PMID 4586824.
  8. Suggs SV, Wallace RB, Hirose T, Kawashima EH, Itakura K (November 1981). "Use of synthetic oligonucleotides as hybridization probes: isolation of cloned cDNA sequences for human beta 2-microglobulin". Proc. Natl. Acad. Sci. U.S.A. 78 (11): 6613–7. doi:10.1073/pnas.78.11.6613. PMC 349099. PMID 6171820.
  9. Pignone M, Nicoll D; McPhee SJ (2004). Pocket guide to diagnostic tests (4th ed.). New York: McGraw-Hill. p. 191. ISBN 0-07-141184-4.
  10. Munshi NC, Longo DL, Anderson KC (2011). "Chapter 111: Plasma Cell Disorders". In Loscalzo J, Longo DL, Fauci AS, Dennis LK, Hauser SL (eds.). Harrison's Principles of Internal Medicine (18th ed.). McGraw-Hill Professional. pp. 936–44. ISBN 0-07-174889-X.
  11. Rajkumar S. V. "MGUS and Smoldering Multiple Myeloma: Update on Pathogenesis, Natural History, and Management." Hematology, American Society of Hematology Education Program. doi: 10.1182/asheducation-2005.1.340 (ASH Education Book January 1, 2005 vol. 2005 no. 1 340-345) Accessed 23 May 2014.
  12. Bataille R. and Klein B. "Serum levels of beta-2 microglobulin and interleukin-6 to differentiate monoclonal gammopathy of uncertain significance." Blood 1992 80(9) p2433 Accessed 23 May 2014.

Further reading
  • beta+2-Microglobulin at the US National Library of Medicine Medical Subject Headings (MeSH)
  • Human B2M genome location and B2M gene details page in the UCSC Genome Browser.
  • Overview of all the structural information available in the PDB for UniProt: P61769 (Beta-2 microglobulin) at the PDBe-KB.


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