SDHB

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Succinate dehydrogenase complex, subunit B, iron sulfur (Ip)
PDB 2h89 EBI.png
PDB rendering based on 2h89.
Available structures
PDB Ortholog search: PDBe, RCSB
Identifiers
Symbols SDHB ; CWS2; IP; PGL4; SDH; SDH1; SDH2; SDHIP
External IDs OMIM185470 MGI1914930 HomoloGene2255 GeneCards: SDHB Gene
EC number 1.3.5.1
Orthologs
Species Human Mouse
Entrez 6390 67680
Ensembl ENSG00000117118 ENSMUSG00000009863
UniProt P21912 Q9CQA3
RefSeq (mRNA) NM_003000 NM_023374
RefSeq (protein) NP_002991 NP_075863
Location (UCSC) Chr 1:
17.35 – 17.38 Mb
Chr 4:
140.96 – 140.98 Mb
PubMed search [1] [2]

Succinate dehydrogenase [ubiquinone] iron-sulfur subunit, mitochondrial (SDHB) also known as iron-sulfur subunit of complex II (Ip) is a protein that in humans is encoded by the SDHB gene.[1][2][3]

The succinate dehydrogenase (also called SDH or Complex II) protein complex catalyzes the oxidation of succinate (succinate + ubiquinone => fumarate + ubiquinol). SDHB is one of four protein subunits forming succinate dehydrogenase, the other three being SDHA, SDHC and SDHD. The SDHB subunit is connected to the SDHA subunit on the hydrophilic, catalytic end of the SDH complex. It is also connected to the SDHC/SDHD subunits on the hydrophobic end of the complex anchored in the mitochondrial membrane. The subunit is an iron-sulfur protein with three iron-sulfur clusters. It weighs 30 kDa.

Function[edit]

Figure 1: Function of the SDHB protein. Electrons are transferred from the Citric Acid Cycle to the Respiratory Chain. Electron path is shown by red arrows.

The SDH complex is located on the inner membrane of the mitochondria and participates in both the Citric Acid Cycle and Respiratory chain.

SDHB acts as an intermediate in the basic SDH enzyme action shown in Figure 1:

  1. SDHA converts succinate to fumarate as part of the Citric Acid Cycle. This reaction also converts FAD to FADH2.
  2. Electrons from the FADH2 are transferred to the SDHB subunit iron clusters [2Fe-2S],[4Fe-4S],[3Fe-4S].
  3. Finally the electrons are transferred to the Ubiquinone (Q) pool via the SDHC/SDHD subunits.This function is part of the Respiratory chain.

Gene[edit]

The gene that codes for the SDHB protein is nuclear, not mitchondrial DNA. However, the expressed protein is located in the inner membrane of the mitochondria. The location of the gene in humans is on the first chromosome at locus p36.1-p35. The gene is coded in 1162 base pairs, partitioned in 8 exons. The expressed protein has 280 amino acids.

RNA editing[edit]

The mRNA transcripts of the SDHB gene in human are edited through an unknown mechanism at ORF nucleotide position 136 causing the conversion of C to U and thus generating a stop codon resulting in the translation of the edited transcripts to a truncated SDHB protein with an R46X amino acid change. This editing has been shown in monocytes and some human lymphoid cell-lines,[4] and is enhanced by hypoxia[disambiguation needed].[5]

Clinical significance[edit]

Germline mutations in the gene can cause familial paraganglioma (in old nomenclature, Paraganglioma Type PGL4). The same condition is often called familial pheochromocytoma. Less frequently, renal cell carcinoma can be caused by this mutation.

Paragangliomas related to SDHB mutations have a high rate of malignancy. When malignant, treatment is currently the same as for any malignant paraganglioma/pheochromocytoma.

Cancer[edit]

Paragangliomas caused by SDHB mutations have several distinguishing characteristics:

  1. Malignancy is common, ranging from 38%-83%[6][7]in carriers with disease. In contrast, tumors caused by SDHD mutations are almost always benign. Sporadic paragangliomas are malignant in less than 10% of cases.
  2. Malignant paragangliomas caused by SDHB are usually (perhaps 92%[7]) extra-adrenal. Sporadic pheochromocytomas/paragangliomas are extra-adrenal in less than 10% of cases.
  3. The penetrance of the gene is often reported as 77% by age 50[6] (i.e. 77% of carriers will have at least one tumour by the age of 50). This is likely an overestimate. Currently (2011), families with silent SDHB mutations are being screened[8] to determine the frequency of silent carriers.
  4. The average age of onset is approximately the same for SDHB vs non-SDHB related disease (approximately 36 years).

Mutations causing disease have been seen in exons 1 through 7, but not 8. As with the SDHC and SDHD genes, SDHB is a tumor suppressor gene. Note the SDHA gene is not a tumor suppressor gene.

Tumor formation generally follows the Knudson "two hit" hypothesis. The first copy of the gene is mutated in all cells, however the second copy functions normally. When the second copy mutates in a certain cell due to a random event, Loss of Heterozygosity (LOH) occurs and the SDHB protein is no longer produced. Tumor formation then becomes possible.

Given the fundamental nature of the SDH protein in all cellular function, it is not currently understood why only paraganglionic cells are affected. However, the sensitivity of these cells to oxygen levels may play a role.

Disease pathways[edit]

The precise pathway leading from SDHB mutation to tumorigenesis is not determined; there are several proposed mechanisms.[9]

Generation of reactive oxygen species[edit]

Figure 2: Disease Pathways for SDHB mutations. Electron path during normal function is shown by solid red arrows. Red dashed arrow shows superoxide generation (Pathway 1). Purple dashed arrow shows diffusion of succinate to block PHD (Pathway 2). Black crosses indicate the non-mutated process is blocked.

When succinate-ubiquinone activity is inhibited, electrons that would normally transfer through the SDHB subunit to the Ubiquinone pool are instead transferred to O2 to create Reactive Oxygen Species (ROS) such as superoxide. The dashed red arrow in Figure 2 shows this. ROS accumulate and stabilize the production of HIF1-α. HIF1-α combines with HIF1-β to form the stable HIF heterodimeric complex, in turn leading to the induction of antiapoptotic genes in the cell nucleus.

Succinate accumulation in the cytosol[edit]

SDH inactivation can block the oxidation of succinate, starting a cascade of reactions:

  1. The succinate accumulated in the mitochondrial matrix diffuses through the inner and outer mitochondrial membranes to the cytosol (purple dashed arrows in Figure 2).
  2. Under normal cellular function, HIF1-α in the cytosol is quickly hydroxylated by prolyl hydroxylase (PHD), shown with the light blue arrow. This process is blocked by the accumulated succinate.
  3. HIF1-α stabilizes and passes to the cell nucleus (orange arrow) where it combines with HIF1-β to form an active HIF complex that induces the expression of tumor causing genes.[10]

This pathway raises the possibility of a therapeutic treatment. The build-up of succinate inhibits PHD activity. PHD action normally requires oxygen and alpha-ketoglutarate as cosubstrates and ferrous iron and ascorbate as cofactors. Succinate competes with α-ketoglutarate in binding to the PHD enzyme. Therefore, increasing α-ketoglutarate levels can offset the effect of succinate accumulation.

Normal α-ketoglutarate does not permeate cell walls efficiently, and it is necessary to create a cell permeating derivative (e.g. α-ketoglutarate esters). In-vitro trials show this supplementation approach can reduce HIF1-α levels, and may result in a therapeutic approach to tumours resulting from SDH deficiency.[11]

Impaired developmental apoptosis[edit]

Paraganglionic tissue is derived from the neural crest cells present in an embryo. Abdominal extra-adrenal paraganglionic cells secrete catecholamines that play an important role in fetal development. After birth these cells usually die, a process that is triggered by a decline in nerve growth factor (NGF)which initiates apoptosis (cell death).

This cell death process is mediated by an enzyme called prolyl hydroxylase EglN3. Succinate accumulation caused by SDH inactivation inhibits the prolyl hydroxylase EglN3.[12] The net result is that paranglionic tissue that would normally die after birth remains, and this tissue may be able to trigger paraganglioma/pheochromocytoma later.

Glycolysis upregulation[edit]

Inhibition of the Citric Acid Cycle forces the cell to create ATP glycolytically in order to generate its required energy. The induced glycolytic enzymes could potentially block cell apoptosis.

Interactive pathway map[edit]

Click on genes, proteins and metabolites below to link to respective articles. [§ 1]

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TCACycle_WP78 go to article go to article go to article go to article go to HMDB go to article go to article go to article Go to article go to article go to article go to article go to article go to article Go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to HMDB go to article go to article go to HMDB go to article go to article go to HMDB go to article go to article go to HMDB go to article go to article go to article go to article go to article go to article go to article go to article go to article go to article go to WikiPathways go to article go to article go to article go to article
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  1. ^ The interactive pathway map can be edited at WikiPathways: "TCACycle_WP78". 

References[edit]

  1. ^ "Entrez Gene: succinate dehydrogenase complex". 
  2. ^ Kita K, Oya H, Gennis RB, Ackrell BA, Kasahara M (January 1990). "Human complex II (succinate-ubiquinone oxidoreductase): cDNA cloning of iron sulfur (Ip) subunit of liver mitochondria". Biochem. Biophys. Res. Commun. 166 (1): 101–8. doi:10.1016/0006-291X(90)91916-G. PMID 2302193. 
  3. ^ Au HC, Ream-Robinson D, Bellew LA, Broomfield PL, Saghbini M, Scheffler IE (July 1995). "Structural organization of the gene encoding the human iron-sulfur subunit of succinate dehydrogenase". Gene 159 (2): 249–53. doi:10.1016/0378-1119(95)00162-Y. PMID 7622059. 
  4. ^ Baysal BE (2007). "A recurrent stop-codon mutation in succinate dehydrogenase subunit B gene in normal peripheral blood and childhood T-cell acute leukemia". PLoS ONE 2 (5): e436. doi:10.1371/journal.pone.0000436. PMC 1855983. PMID 17487275.  open access publication - free to read
  5. ^ Baysal BE, De Jong K, Liu B, Wang J, Patnaik SK, Wallace PK, Taggart RT (2013). "Hypoxia-inducible C-to-U coding RNA editing downregulates SDHB in monocytes". PeerJ 1: e152. doi:10.7717/peerj.152. PMC 3775634. PMID 24058882. 
  6. ^ a b Neumann HP, Pawlu C, Peczkowska M, Bausch B, McWhinney SR, Muresan M, Buchta M, Franke G, Klisch J, Bley TA, Hoegerle S, Boedeker CC, Opocher G, Schipper J, Januszewicz A, Eng C (August 2004). "Distinct clinical features of paraganglioma syndromes associated with SDHB and SDHD gene mutations". JAMA 292 (8): 943–51. doi:10.1001/jama.292.8.943. PMID 15328326. 
  7. ^ a b Brouwers FM, Eisenhofer G, Tao JJ, Kant JA, Adams KT, Linehan WM, Pacak K (November 2006). "High frequency of SDHB germline mutations in patients with malignant catecholamine-producing paragangliomas: implications for genetic testing". J. Clin. Endocrinol. Metab. 91 (11): 4505–9. doi:10.1210/jc.2006-0423. PMID 16912137. 
  8. ^ Conference: National Institute of Health (U.S.A.), "SDHB-related Pheochromocytoma: Recent Discoveries & Current Diagnostic and Therapeutic Approaches", September 29, 2006
  9. ^ Gottlieb E, Tomlinson IP (November 2005). "Mitochondrial tumour suppressors: a genetic and biochemical update". Nat. Rev. Cancer 5 (11): 857–66. doi:10.1038/nrc1737. PMID 16327764. 
  10. ^ Selak MA, Armour SM, MacKenzie ED, Boulahbel H, Watson DG, Mansfield KD, Pan Y, Simon MC, Thompson CB, Gottlieb E (January 2005). "Succinate links TCA cycle dysfunction to oncogenesis by inhibiting HIF-alpha prolyl hydroxylase". Cancer Cell 7 (1): 77–85. doi:10.1016/j.ccr.2004.11.022. PMID 15652751. 
  11. ^ MacKenzie ED, Selak MA, Tennant DA, Payne LJ, Crosby S, Frederiksen CM, Watson DG, Gottlieb E (May 2007). "Cell-permeating alpha-ketoglutarate derivatives alleviate pseudohypoxia in succinate dehydrogenase-deficient cells". Mol. Cell. Biol. 27 (9): 3282–9. doi:10.1128/MCB.01927-06. PMC 1899954. PMID 17325041. 
  12. ^ Lee S, Nakamura E, Yang H, Wei W, Linggi MS, Sajan MP, Farese RV, Freeman RS, Carter BD, Kaelin WG, Schlisio S (August 2005). "Neuronal apoptosis linked to EglN3 prolyl hydroxylase and familial pheochromocytoma genes: developmental culling and cancer". Cancer Cell 8 (2): 155–67. doi:10.1016/j.ccr.2005.06.015. PMID 16098468. 

Further reading[edit]

External links[edit]