SDHA Mutations Research Articles

Yeast model for evaluating the pathogenic significance of SDHB, SDHC and SDHD mutations in PHEO-PGL syndrome

SDH genes, encoding succinate dehydrogenase, act as tumour suppressor genes, linking mitochondrial dysfunction with tumourigenesis. Heterozygous germline mutations in SDHA, SDHB, SDHC, SDHD and in the assembly factor encoding gene SDHAF2 have all been shown to predispose to heritable endocrine neoplasias such as pheochromocytomas (PHEO) and paragangliomas (PGLs) called 'PHEO-PGL syndrome'. SDH genes mutations, in addition to deletions or truncations which are most likely pathogenic, often include missense substitutions which can be of uncertain significance. Unclassified missense substitutions may be difficult to interpret unless the cause-effect link between mutation and the disease is established by functional and insilico studies or by the familial segregation with the phenotype. Using the yeast model, here, we report functional investigations on several missense SDH mutations found in patients affected by pheochromocytomas or paragangliomas. The aim of this study was to evaluate whether and to which extent the yeast model may be useful for establishing the pathological significance of missense SDH mutations in humans. The results of our study demonstrate that the yeast is a good functional model to validate the pathogenic significance of SDHB missense mutations while, for missense mutations in SDHC and SDHD genes, the model can be informative only when the variation involves a conserved residue in a conserved domain.

SDHA loss of function mutations in a subset of young adult wild-type gastrointestinal stromal tumors

BackgroundA subset of KIT/PDGFRA wild-type gastrointestinal stromal tumors (WT GIST) have been associated with alteration of the succinate dehydrogenase (SDH) complex II function. A recent report identified four non-syndromic, KIT/PDGFRA WT GIST harboring compound heterozygous or homozygous mutations in SDHA encoding the main subunit of the SDH complex II.MethodsNext generation sequencing was applied on five pediatric and one young adult WT GIST, by whole exome capture and SOLiD 3-plus system sequencing. The putative mutations were first confirmed by Sanger sequencing and then screened on a larger panel of 11 pediatric and young adult WT GIST, including 5 in the context of Carney triad.ResultsA germline p.Arg31X nonsense SDHA mutation was identified in one of the six cases tested by SOLiD platform. An additional p.D38V missense mutation in SDHA exon 2 was identified by Sanger sequencing in the extended KIT/PDGFRA WT GIST patients cohort. Western blotting showed loss of SDHA expression in the two cases harboring SDHA mutations, while expression being retained in the other WT GIST tumors. Results were further confirmed by immunohistochemistry for both SDHA and SDHB, which showed a concurrent loss of expression of both proteins in SDHA-mutant lesions, while the remaining WT tumors showed only loss of SDHB expression.ConclusionsGermline and/or somatic aberrations of SDHA occur in a small subset of KIT/PDGFRA WT GISTs, outside the Carney’s triad and are associated with loss of both SDHA and SDHB protein expression. Mutations of the SDH complex II are more particularly associated with KIT/PDGFRA WT GIST occurring in young adults. Although pediatric GIST consistently display alterations of SDHB protein expression, further molecular studies are needed to identify the crucial genes involved in their tumorigenesis.

Recessive germline SDHA and SDHB mutations causing leukodystrophy and isolated mitochondrial complex II deficiency

BackgroundIsolated complex II deficiency is a rare form of mitochondrial disease, accounting for approximately 2% of all respiratory chain deficiency diagnoses. The succinate dehydrogenase (SDH) genes (SDHA, SDHB, SDHC and...

Succinate dehydrogenase (SDH) and mitochondrial driven neoplasia

The genes for the succinate dehydrogenase (SDH) subunits SDHA, SDHB, SDHC and SDHD are encoded in the autosome. The proteins are assembled in the mitochondria to form the mitochondrial complex 2, a key respiratory enzyme which links the Krebs cycle and the electron transport chain. Thirty percent of phaeochromocytoma and paraganglioma (PHEO/PGL) are hereditary and perhaps as many as half of these familial cases are caused by germline mutations of the SDH subunits. Negative immunohistochemical staining for the SDHB subunit identifies PHEO/PGL associated with germline mutation of any of the mitochondrial complex 2 components and can be used to triage formal genetic testing of all PHEO/PGL for SDH mutations. PHEO/PGL associated with SDHA mutation also show negative staining for SDHA as well as SDHB.A unique subgroup of gastrointestinal stromal tumours (GISTs) are driven by mitochondrial complex 2 dysfunction. These SDH deficient GISTs can also be definitively identified by negative staining for SDHB and show distinct clinical and morphological features including frequent onset in childhood and young adulthood, gastric location, a tendency to multifocality, absence of KIT and PDGFRA mutations, a prognosis not predicted by size and mitotic rate and a tendency to indolent behaviour of metastases. Some of these SDH deficient GISTs are driven by classical SDH mutations, but the precise mechanisms of tumourigenesis in many (including those associated with the Carney triad) remain unknown. Germline SDHB mutation is associated with a newly recognised type of renal carcinoma which commonly but not always demonstrates distinctive morphology and can also be recognised by negative staining for SDHB.Immunohistochemistry for SDHB therefore has emerged as a useful tool to recognise these distinct neoplasias driven by mitochondrial complex 2 dysfunction and to triage formal genetic testing for the associated syndromes.

Immunohistochemical identification of SDHA-mutant gastrointestinal stromal tumors (GISTs).

10029 Background: GISTs are most commonly driven by activating mutations in KIT or PDGFRA. However, 15% of GISTs in adults and >90% in children lack such mutations. A subset of KIT/PDGFRA wild-type GISTs shows distinctive morphologic and clinical features and loss of expression of succinate dehydrogenase (SDH) B. Only a fraction of SDHB-deficient GISTs carry loss-of-function mutations in SDHB or SDHC. Due to the complexity of its locus and the presence of several pseudogenes, SDHA is rarely analyzed. Recently, mutations in SDHA were shown to lead to loss of expression of SDHA and SDHB in paraganglionomas. We sought to determine whether SDHA IHC could identify GISTs with SDHA mutations. Methods: Tumors (n=11) with features of SDH-deficient GIST (gastric origin, epithelioid morphology, multinodular/plexiform architecture) were selected from pathology archives under an IRB-approved protocol. Expression of SDHA and SDHB was determined on tumor sections by IHC. Genomic DNA was isolated from SDHA-negative tumors and amplified using primers specific to introns flanking each of the 15 SDHA exons. Amplicons were bidirectionally sequenced and compared to genomic repository data. Exons with somatic mutations were also examined in DNA from corresponding normal tissue to determine germline status. Results: All tumors (100%) were deficient for SDHB expression by IHC. Four of 11 (36%) SDHB-deficient GISTs also lacked expression of SDHA. SDHA expression was intact in the 7 remaining tumors, including 3 with known SDHB (n=2) or SDHC (n=1) mutations. Nonsense mutations in SDHA were identified in all 4 SDHA-deficient tumors, caused by a single base pair (bp) substitution (n=3) or a single bp deletion (n=1), and heterozygous mutations were also found in DNA from normal tissue of all 4 patients. Somatic loss of the 2nd allele has thus far been found in 3 of 4 tumors; 2 by loss of heterozygosity and 1 by a 13-bp deletion. Further analysis of the 4th specimen is ongoing. Conclusions: SDHA mutations are a common cause of SDH-deficient GISTs and result in combined loss of expression of both SDHA and SDHB. Loss of SDHA expression by IHC reliably predicts SDHA mutations and can be used to select cases for SDHA genetic testing.

SDHA and SDHB mutations in KIT/PDGFRA WT gastrointestinal stromal tumors.

10087 Background: KIT/PDGFRA wild-type (WT) GISTs harbour mutations on SDHB and SDHC and, more recently, we described mutations on SDHA using massively parallel sequencing approach. We sequenced SDHA and SDHB genes in a larger series in order to validate the data. Methods: SDHA gene (1-15 exons) and SDHB gene (1-8 exons) (even not all exons in all samples) were sequenced on tumor (T) and/or peripheral blood (PB) of WT GIST patients by Sanger Sequencing method. DNA was extracted from tumor specimens by the QIAmp DNA Mini kit (Qiagen, Milan, Italy) and amplified with specific primer pairs designed to amplify exons but not SDHA pseudo-genes located on chromosomes 3 and 5. Then, PCR products were purified with the Qiaquick PCR purification kit (Qiagen, Milan, Italy) and sequenced on both strands using the Big Dye Terminator v1.1 Cycle Sequencing kit (Applied Biosystems). Sanger sequencing was performed on ABI 3730 Genetic Analyzer (Applied Biosystems). Results: SDHA gene exons were sequenced on a total of 27 WT GIST patients, in particular on T, PB and both from 12, 6 and 9 patients respectively. SDHB gene exons were sequenced on a total of 18 out of 27 patients, in particular on T, PB and both from 7, 8 and 3 patients respectively. 8 SDHA mutations were found in 5 samples (18.5%). Besides those previously identified, 5 new SDHA mutations were found in other 3 samples: one sample harboured R171C and R589Q heterozygous missense mutation in exons 5 and 13 respectively. The other one harboured G419R and E564K heterozygous missense mutations in exons 9 and 13 respectively. The third sample harboured a delCAG immediately upstream of exon 5, in heterozygosis on PB and in homozygosis on T. A SDHB heterozygous mutation (301delT) in exon 4 was found on 1 PB sample. Conclusions: the presence of SDHA mutations has been confirmed in a subgroup of WT GIST patients. All subunits of SDH complex should be sequenced on WT GIST patients in order to explore the frequency and any linkage between each other and the pathogenetic and clinical significance.

P53 Novel SDHA and SDHB mutations as a cause of isolated mitochondrial complex II deficiency

P53 Novel SDHA and SDHB mutations as a cause of isolated mitochondrial complex II deficiency

Head and neck paragangliomas: genetic spectrum and clinical variability in 79 consecutive patients

Head and neck paragangliomas (HNPGLs) are neural crest-derived tumors. In comparison with paragangliomas located in the abdomen and the chest, which are generally catecholamine secreting (sPGLs) and sympathetic in origin, HNPGLs are, in fact, parasympathetic in origin and are generally nonsecreting. Overall, 79 consecutive patients with HNPGL were examined for mutations in SDHA, SDHB, SDHC, SDHD, SDHAF2, VHL, MAX, and TMEM127 genes by PCR/sequencing. According to a detailed family history (FH) and clinical, laboratory (including metanephrines), and instrumental examinations, patients were divided into three groups: a) patients with a positive FH for HNPGL (index cases only), b) patients with a negative FH and multiple HNPGLs (synchronous or metachronous) or HNPGL associated with an sPGL, and c) patients with negative FH and single HNPGL. The ten patients in group a) proved to be SDHD mutation carriers. The 16 patients in group b) proved to be SDHD mutation carriers. Among the 53 patients in group c), ten presented with germ-line mutations (three SDHB, three SDHD, two VHL, and two SDHAF2). An sPGL was found at diagnosis or followed up in five patients (6.3%), all were SDHD mutation carriers. No SDHC, SDHA, MAX, and TMEM127 mutations were found. In SDHD mutation carriers, none of the patients affected by HNPGL associated with sPGL presented missense mutations. In conclusion, a positive FH or the presence of multiple HNPGLs is a strong predictor for germ-line mutations, which are also present in 18.8% of patients carefully classified as sporadic. The most frequently mutated gene so far is SDHD but others, including SDHB, SDHAF2, and VHL, may also be affected.

SDHA Immunohistochemistry Detects Germline SDHA Gene Mutations in Apparently Sporadic Paragangliomas and Pheochromocytomas

Pheochromocytoma-paraganglioma syndrome is caused by mutations in SDHB, SDHC, and SDHD, encoding subunits of succinate dehydrogenase (SDH), and in SDHAF2, required for flavination of SDHA. A recent report described a patient with an abdominal paraganglioma, immunohistochemically negative for SDHA, and identified a causal germline mutation in SDHA. In this study, we evaluated the significance of SDHA immunohistochemistry in the identification of new patients with SDHA mutations. This study was performed in the Erasmus Medical Center in Rotterdam (The Netherlands) and the Université Paris Descartes in Paris (France). We investigated 316 pheochromocytomas and paragangliomas for SDHA expression. Sequence analysis of SDHA was performed on all tumors that were immunohistochemically negative for SDHA and on a subset of tumors immunohistochemically positive for SDHA. Six tumors were immunohistochemically negative for SDHA. Four tumors from Dutch patients showed a germline c.91C → T SDHA gene mutation (p.Arg31X). Another tumor (from France) carried a germline SDHA missense mutation c.1753C → T (p.Arg585Trp). Loss of the wild-type SDHA allele was confirmed by loss of heterozygosity analysis. Sequence analysis of 35 SDHA immunohistochemically positive tumors did not reveal additional SDHA mutations. Our results demonstrate that SDHA immunohistochemistry on paraffin-embedded tumors can reveal the presence of SDHA germline mutations and allowed the identification of SDHA-related tumors in at least 3% of patients affected by apparently sporadic (para)sympathetic paragangliomas and pheochromocytomas.

Candidate gene analysis of gastrointestinal stromal tumors and paragangliomas in patients with carneys triad.

e12018 Background: Carneys triad (CT) describes the association of paragangliomas (PGLs) with gastrointestinal stromal tumors (GISTs) and pulmonary chondromas (PCH). CT is unique compared to other multiple neoplasia syndromes in that it is an apparently sporadic condition affecting mainly young females. CT is not always complete, with approximately ¼ of patients having all three components, and ½ having only 2 of the three (PCH and GIST). A similar syndrome (the Carney-Stratakis syndrome) is characterized by the familial association of GIST and PGL and can be inherited as an autosomal-dominant disorder due to mutations in SDHB,-C, and –D. The association of three rare tumors is strongly suggestive of a genetic disorder, yet no gene has been linked to CT. Using a novel CGH method, SOMATICS, chromosome 1p losses were determined to be frequent in both PGLs and GISTs from CT patients. Recent data implicate ARID1A as a tumor-suppressor gene frequently disrupted in ovarian clear-cell and endometrioid carcinomas. Using a candidate gene-based approach, we sequenced tumor suppressor genes in GISTs and PGLs from patients with CT. Methods: Gene candidacy was determined either by genome location (ARID1A 1p35.3), or potentially oncogenic pathway-defects in mitochondrial complex II activity (SDHAF1). Coding regions and exon-intron boundaries for ARID1A and SDHAF1 were PCR-amplified and sequenced using the Sanger method on tumor DNA from 13 patients with CT (7 paragangliomas and 11 GISTs). Results: All DNA specimens were confirmed to be negative for mutations in SDHA, SDHB, SDHC, SDHD, KIT and PDGFRA. Sequencing analysis revealed only polymorphisms, but no somatic mutations in both ARID1A and SDHAF1. Conclusions: Although the loss of chromosome 1p is a frequent feature, the ARID1A tumor suppressor gene that is located in this region, does not seem to harbor sequence alterations. These data indicate that another gene (or genes) is probably responsible for the tumor genesis in patients with CT. Further research using next generation sequencing is ongoing in our institution.

Identification of single nucleotide variants in gastrointestinal stromal tumor KIT/PDGFRA wild-type (WT GISTs) by massively parallel sequencing.

10046 Background: Recently SDH variations were identified in KIT and PDGFRA wild type GISTs. However the causative genomic alterations of these GISTs still remain unknown. Massively parallel sequencing allows identifying novel putative variants. Methods: Whole transcriptome paired-end RNA sequencing was performed by Illumina GAIIx system using a 75 bases paired-end strategy on tumor samples of two young adults patients (P1 and P2) affected by gastric WT-GIST (age 28 and 30 years). Sequences were aligned with BWA against exon+junction references. SNVmix2 was used for SNP calling, identifying by this 2045 and 1780 coding non-synonymous novel single nucleotide variants (SNVs) in P1 and P2, respectively. After checking misalignments by SAMTools, the variants were filtered to increasing SNPcall confidence: SNVs with quality read score > 30 (error probability of 0.1%), total coverage > 40, and ratio between coverage of alterate base and total coverage >0.3 were labeled high confidence. Single point mutations were translated at the protein level and their likelihood of being disease-associated was computed with SNP&GO and confirmed with Sanger sequencing. Results: In both patients common mutations in SDHA and different private SNV were highlighted. Among the private disease-related SNVs we identified mutations in MYH9 (myosin heavy chain 9) and TPI1 (triosephosphate isomerase 1) in patient P1, and a mutation in OGDHL (oxoglutarate dehydrogenase-like) in P2. MYH9 is involved in cell motility and cytokinesis, while OGDHL is an enzyme of the Krebs cycle, as the SDH complex, and TPI1 is involved in glycolysis. Ongoing validation by Sanger sequencing on DNA from tumor and peripheral blood (PB) already confirmed the presence of heterozygous MYH9 K1775E in P1 and OGDHL V815M in P2, both in tumor and PB, showing that these mutations are germinal. Conclusions: Massively parallel RNA sequencing, followed by data analysis, allows to discover novel single nucleotide variants and to identify new potential target genes in WT-GIST.

Clinicoradiological manifestations of paraganglioma syndromes associated with succinyl dehydrogenase enzyme mutation

BackgroundParagangliomas are rare tumours derived from the autonomic nervous system that have increasingly been recognised to have a genetic predisposition. Mutations of the enzyme succinyl dehydrogenase (SDH) have proven to result in paraganglioma formation. There are four subunits (A through D) that form the enzyme complex and are associated with different genophenotypic expressions of disease. SDHB and SDHD mutations are more common, whereas SDHA and SDHC mutations are rare. Patients with SDHB mutations are prone to extra-adrenal pheochromocytomas, malignant disease and extra-paraganglial neoplasia, whereas SDHD mutations have a greater propensity for multiple, benign head and neck paragangliomas.MethodsDiagnosis of a sporadic paraganglioma or pheochromocytoma should lead to a full genetic workup of the patient and family if SDH mutations are found.ResultsFurther annual screening will be required depending on the mutation, which can have a significant impact on radiologists and the resources of the radiology department.ConclusionWe present our imaging experience with a series of patients with proven SDH mutations resulting in paragangliomas with a review of the literature.

SDHA is a tumor suppressor gene causing paraganglioma

Mitochondrial succinate-coenzyme Q reductase (complex II) consists of four subunits, SDHA, SDHB, SDHC and SDHD. Heterozygous germline mutations in SDHB, SDHC, SDHD and SDHAF2 [encoding for succinate dehydrogenase (SDH) complex assembly factor 2] cause hereditary paragangliomas and pheochromocytomas. Surprisingly, no genetic link between SDHA and paraganglioma/pheochromocytoma syndrome has ever been established. We identified a heterozygous germline SDHA mutation, p.Arg589Trp, in a woman suffering from catecholamine-secreting abdominal paraganglioma. The functionality of the SDHA mutant was assessed by studying SDHA, SDHB, HIF-1alpha and CD34 protein expression using immunohistochemistry and by examining the effect of the mutation in a yeast model. Microarray analyses were performed to study gene expression involved in energy metabolism and hypoxic pathways. We also investigated 202 paragangliomas or pheochromocytomas for loss of heterozygosity (LOH) at the SDHA, SDHB, SDHC and SDHD loci by BAC array comparative genomic hybridization. In vivo and in vitro functional studies demonstrated that the SDHA mutation causes a loss of SDH enzymatic activity in tumor tissue and in the yeast model. Immunohistochemistry and transcriptome analyses established that the SDHA mutation causes pseudo-hypoxia, which leads to a subsequent increase in angiogenesis, as other SDHx gene mutations. LOH was detected at the SDHA locus in the patient's tumor but was present in only 4.5% of a large series of paragangliomas and pheochromocytomas. The SDHA gene should be added to the list of genes encoding tricarboxylic acid cycle proteins that act as tumor suppressor genes and can now be considered as a new paraganglioma/pheochromocytoma susceptibility gene.

The triad of paragangliomas, gastric stromal tumours and pulmonary chondromas (Carney triad), and the dyad of paragangliomas and gastric stromal sarcomas (Carney–Stratakis syndrome): molecular genetics and clinical implications

Carney triad (CT) describes the association of paragangliomas (PGLs) with gastrointestinal stromal tumours (GISTs) and pulmonary chondromas (PCH). A number of other lesions have been described in the condition including pheochromocytomas, oesophageal leiomyomas and adrenocortical adenomas; CT is a novel form of multiple endocrine neoplasia (MEN), a genetic condition with a female predilection. Inactivating mutations of the mitochondrial complex II succinate dehydrogenase (SDH) enzyme subunits SDHB, SDHC and SDHD have been found in familial and sporadic PGLs, and gain-of-function mutations of the oncogenes c-kit (KIT) and platelet-derived growth factor receptor A (PDGFRA) cause sporadic and familial GISTs. We recently reported an international series of patients with CT, 34 females and three males (median age of presentation 21 years) who did not carry SDHA, SDHB, SDHC, SDHD, KIT or PDGFRA gene mutations. Comparative genomic hybridization revealed a number of DNA copy number changes. The most frequent and greatest contiguous change was a deletion within the 1pcen13-q21 region, which harbours the SDHC gene. Another frequent change was loss of 1p. Although GISTs showed more frequent losses of 1p than PGLs, the pattern of chromosomal changes was similar in the two tumours despite their different tissue origin and histology; the findings were consistent with a common genetic aetiology of these two tumours in CT. In a separate condition, in which the association (or dyad) of GISTs with PGLs is inherited in an autosomal dominant manner (Carney-Stratakis syndrome, CSS), germline mutations of the SDHB, SDHC and SDHD genes (but not KIT or PDFGRA) were found; GISTs in this condition were caused by SDH deficiency. We conclude that CT is a novel MEN syndrome whose genetic defect remains elusive. CSS is caused by SDH defects, suggesting that sarcomas (GISTs) can be caused by defective mitochondrial oxidation, consistent with recent data implicating this enzyme in a variety of endocrine and other tumours. The above have clinical implications (i) for patients with GISTs that are cKIT- and PDGFRA-mutation negative: these tumours are usually resistant to treatment with currently available tyrosine kinase inhibitors and may be part of a syndrome such as CT or CSS; and (ii) for patients with an inherited PGL syndrome, family history should be explored to identify any other tumours in the family, and in particular other endocrine lesions and GISTs.

Germline Mutations and Variants in the Succinate Dehydrogenase Genes in Cowden and Cowden-like Syndromes

Individuals with PTEN mutations have Cowden syndrome (CS), associated with breast, thyroid, and endometrial neoplasias. Many more patients with features of CS, not meeting diagnostic criteria (termed CS-like), are evaluated by clinicians for CS-related cancer risk. Germline mutations in succinate dehydrogenase subunits SDHB-D cause pheochromocytoma-paraganglioma syndrome. One to five percent of SDHB/SDHD mutation carriers have renal cell or papillary thyroid carcinomas, which are also CS-related features. SDHB-D may be candidate susceptibility genes for some PTEN mutation-negative individuals with CS-like cancers. To address this hypothesis, germline SDHB-D mutation analysis in 375 PTEN mutation-negative CS/CS-like individuals was performed, followed by functional analysis of identified SDH mutations/variants. Of 375 PTEN mutation-negative CS/CS-like individuals, 74 (20%) had increased manganese superoxide dismutase (MnSOD) expression, a manifestation of mitochondrial dysfunction. Among these, 10 (13.5%) had germline mutations/variants in SDHB (n = 3) or SDHD (7), not found in 700 controls (p < 0.001). Compared to PTEN mutation-positive CS/CS-like individuals, those with SDH mutations/variants were enriched for carcinomas of the female breast (6/9 SDH versus 30/107 PTEN, p < 0.001), thyroid (5/10 versus 15/106, p < 0.001), and kidney (2/10 versus 4/230, p = 0.026). In the absence of PTEN alteration, CS/CS-like-related SDH mutations/variants show increased phosphorylation of AKT and/or MAPK, downstream manifestations of PTEN dysfunction. Germline SDH mutations/variants occur in a subset of PTEN mutation-negative CS/CS-like individuals and are associated with increased frequencies of breast, thyroid, and renal cancers beyond those conferred by germline PTEN mutations. SDH testing should be considered for germline PTEN mutation-negative CS/CS-like individuals, especially in the setting of breast, thyroid, and/or renal cancers.

Sequence variation in human succinate dehydrogenase genes: evidence for long-term balancing selection on SDHA.

BackgroundBalancing selection operating for long evolutionary periods at a locus is characterized by the maintenance of distinct alleles because of a heterozygote or rare-allele advantage. The loci under balancing selection are distinguished by their unusually high polymorphism levels. In this report, we provide statistical and comparative genetic evidence suggesting that the SDHA gene is under long-term balancing selection. SDHA encodes the major catalytical subunit (flavoprotein, Fp) of the succinate dehydrogenase enzyme complex (SDH; mitochondrial complex II). The inhibition of Fp by homozygous SDHA mutations or by 3-nitropropionic acid poisoning causes central nervous system pathologies. In contrast, heterozygous mutations in SDHB, SDHC, and SDHD, the other SDH subunit genes, cause hereditary paraganglioma (PGL) tumors, which show constitutive activation of pathways induced by oxygen deprivation (hypoxia).ResultsWe sequenced the four SDH subunit genes (10.8 kb) in 24 African American and 24 European American samples. We also sequenced the SDHA gene (2.8 kb) in 18 chimpanzees. Increased nucleotide diversity distinguished the human SDHA gene from its chimpanzee ortholog and from the PGL genes. Sequence analysis uncovered two common SDHA missense variants and refuted the previous suggestions that these variants originate from different genetic loci. Two highly dissimilar SDHA haplotype clusters were present in intermediate frequencies in both racial groups. The SDHA variation pattern showed statistically significant deviations from neutrality by the Tajima, Fu and Li, Hudson-Kreitman-Aguadé, and Depaulis haplotype number tests. Empirically, the elevated values of the nucleotide diversity (% π = 0.231) and the Tajima statistics (D = 1.954) in the SDHA gene were comparable with the most outstanding cases for balancing selection in the African American population.ConclusionThe SDHA gene has a strong signature of balancing selection. The SDHA variants that have increased in frequency during human evolution might, by influencing the regulation of cellular oxygen homeostasis, confer protection against certain environmental toxins or pathogens that are prevalent in Africa.

Functioning Paraganglioma and Gastrointestinal Stromal Tumor of the Jejunum in Three Women

Functioning paraganglioma and gastrointestinal stromal tumor (GIST) are uncommon tumors that occur mostly in a sporadic and isolated form, occasionally as components of multiple neoplasia syndromes, either separately or together. Separately, they occur in several inherited syndromes including multiple endocrine neoplasia 2, and the GIST, lentigines, and mast cell tumor syndrome. Together, they are variably prominent components of three syndromes: the familial paraganglioma and gastric GIST syndrome, neurofibromatosis type 1, and the Carney triad. The two former conditions are inherited as autosomal dominant traits; the latter does not appear to be inherited and affects young women predominantly. This article reports the nonfamilial occurrence of functioning paraganglioma and GIST of the jejunum in 3 women, 1 young (22 years) at initial presentation. The occurrences were unexpected because of the infrequency of the tumors. The neoplasms, respectively, did not show germline SDHA, SDHB, SDHC, and SDHD, and KIT mutations associated with familial paraganglioma and familial GIST. The paraganglioma-jejunal GIST combination may be the harbinger of a rare genetic syndrome, a variant of the Carney triad or the paraganglioma-gastric stromal sarcoma syndrome, or be coincidental.

Familial Paragangliomas: Genetic Grounds

About 10 to 50% of paragangliomas are familial and they may present as a solitary tumor or as multiple tumors, as occurs in 30 to 40% of familial paragangliomas. These tumors are generally confined to the head and neck region, but occasionally they are accompanied by adrenal or extra-adrenal paragangliomas. Paragangliomas also may be part of different syndromes, such as multiple endocrine neoplasia type 2 (over-representation of mutant RET), von Hippel-Lindau disease (VHL germline mutation and wildtype allelic deletion), and neurofibromatosis type 1. It now appears that germline inactivating heterozygous mutations in SDHB, SDHC, and SDHD, which encode subunits of mitochondrial complex II (succinate dehydrogenase, succinate-ubiquinone oxidoreductase), are responsible for 70% of familial cases and approximately 8% of nonfamilial cases. On the other hand, mutations in the subunit SDHA display a phenotype resembling other mitochondrial and Krebs cycle gene defects, which are often characterized by hypotonia, growth retardation, cardiomyopathy, myopathy, neuropathy, and metabolic derangement. In SDHB (1p36)- and SDHC (1q21)-linked families, disease inheritance is autosomal dominant. In SDHD (11q23)-linked families, the disease phenotype is expressed only upon paternal transmission of the mutation, consistent with maternal imprinting or with a somatic genetic mechanism targeting both the SDHD gene on 11q23 and a paternally imprinted gene on 11p15.5. In addition, loss of heterozygosity (LOH) on chromosome 11, mainly in 11q23 (PGL1), has been observed. A two-hit model comprising both LOH and point mutation appears to be the pathophysiologic basis for sporadic tumor development. Loss of chromosome 11 regions, including the deletion of PGL1 and PGL2 loci, may result in a more severe phenotype, as exemplified by the development of multiple tumors. In contrast with SDHD mutation carriers who have more frequent multifocal paragangliomas, SDHB mutation carriers are more likely to develop malignant disease and possibly extraparaganglial neoplasias, including renal cell and thyroid carcinomas.

Succinate dehydrogenase deficiency in human.

Mitochondrial succinate dehydrogenase (SDH) consists merely of four nuclearly encoded subunits. It participates in the electron transfer in the respiratory chain and in succinate catabolism in the Krebs cycle. Mutations in the four genes, SDHA, B, C and D, have been reported, resulting in strikingly diverse clinical presentations. So far, SDHA mutations have been reported to cause an encephalomyopathy in childhood, while mutations in the genes encoding the other three subunits have been associated only with tumour formation. Following a brief description of SDH genes and subunits, we examine the properties and roles of SDH in the mitochondria. This allows further discussion of the several hypotheses proposed to account for the different clinical presentations resulting from impaired activity of the enzyme. Finally we stress the importance of SDH as a target and/or marker in a number of diseases and the need to better delineate the consequences of SDH deficiency in humans.

Phenotypic dichotomy in mitochondrial complex II genetic disorders.

This review presents our current knowledge on the genetic and phenotypic aspects of mitochondrial complex II gene defects. The mutations of the complex II subunits cause two strikingly different group of disorders, revealing a phenotypic dichotomy. Genetic disorders of the mitochondrial respiratory chain are often characterized by hypotonia, growth retardation, cardiomyopathy, myopathy, neuropathy, organ failure, and metabolic derangement. These disorders are transmitted through maternal lineage if the defective gene is located in the mitochondrial genome or may follow a Mendelian pattern if it is in the nucleus. Mitochondrial complex II (succinate:ubiquinone oxidoreductase) is the smallest complex in the respiratory chain and is composed of four subunits encoded by nuclear genes SDHA, SDHB, SDHC, and SDHD. Complex II oxidizes succinate to fumarate in the Krebs cycle and is involved in the mitochondrial electron transport chain. SDHA and SDHB encode the flavoprotein and iron-sulfur proteins, respectively, and SDHC and SDHD encode the two hydrophobic membrane-spanning subunits. While mutations in SDHA display a phenotype resembling other mitochondrial and Krebs cycle gene defects, those in SDHB, SDHC and SDHD cause hereditary paraganglioma. Paraganglioma is characterized by slow-growing vascular tumors of the paraganglionic tissue (i.e., adrenal and extra-adrenal paragangliomas, including those in the head and neck, mediastinum, abdomen, and pheochromocytomas). Paraganglioma caused by SDHD mutations occurs exclusively after paternal transmission, suggesting that genomic imprinting influences gene expression. Association of a mitochondrial gene defect with tumorigenesis expands the phenotypic spectrum of mitochondrial diseases and adds genomic imprinting as a new transmission mode in mitochondrial genetics. The phenotypic features of complex II gene mutations suggest that whereas the catalytic subunit SDHA mutations may compromise the Krebs cycle, those in other structural subunits may affect oxygen sensing and signaling.