Associated disorders

The SDHB gene is associated with autosomal dominant gastrointestinal stromal tumors (GIST) (MedGen UID: 116049), paragangliomas (PGL) (MedGen UID: 349380), pheochromocytomas (PCC) (MedGen UID: 18419), renal cancer, and autosomal recessive mitochondrial complex II deficiency. Other SDHB-related conditions have been reported (OMIM: 185470).

The SDHB gene is a tumor suppressor that encodes an iron-sulfur protein, one of the four nuclear-encoded peptide subunits in complex II. Complex II, also known as succinate dehydrogenase or succinate-ubiquinone oxidoreductase, is a complex of the mitochondrial respiratory chain.

SDHB: hereditary paraganglioma pheochromocytoma
MedGen UIDs: 116049, 349380, 18419

Clinical condition
Single pathogenic variants in the SDHB gene are associated with the development of paragangliomas (PGL) and pheochromocytomas (PCC) as seen in hereditary paraganglioma-pheochromocytoma syndrome (PMID: 24903423, 24893135, 20301715, 23512077), gastrointestinal stromal tumors (GIST) (PMID: 25741136), Carney-Stratakis syndrome (PMID: 23036227, 24899893, 24903423, 20301715, 25741136, 17667967), and renal cell carcinoma (PMID: 18728283, 22544211, 23083876, 19802898). It is unclear if pathogenic SDHB variants are associated with other cancers as the data are currently limited and emerging. The clinical presentation is highly variable among those with a pathogenic variant in SDHB and may be difficult to predict. An individual with a SDHB pathogenic variant will not necessarily develop cancer in their lifetime, but the risk for cancer is increased over that of the general population.

Paraganglioma/pheochromocytoma (PGL/PCC)
PGLs are rare, adult-onset, typically benign neuroendocrine tumors that arise from paraganglia. Paraganglia are a collection of neuroendocrine tissues that are distributed throughout the body, from the middle ear and skull base to the pelvis. PGLs that develop in the head and neck are called head-and-neck paragangliomas (HNP). PGLs located outside the head and neck most commonly occur in the adrenal glands and are called pheochromocytoma (PCC) (also known as chromaffin tumors). PCCs are catecholamine-secreting PGLs that are confined to the adrenal medulla, as defined by the The World Health Organization Tumor Classification; however, this term may also be used to refer to catecholamine-producing PGLs regardless of whether they are adrenal or extra-adrenal (PMID: 24899893). These lesions can cause excessive production of adrenal hormones, resulting in hypertension, headaches, tachycardia, anxiety, and sweaty or clammy skin in some individuals (PMID: 21771581, 24893135, 20301715). Most cases of PGL and PCC are sporadic, but approximately one-third are familial and due to an identifiable pathogenic variant in a susceptibility gene, such as SDHB, that can result in hereditary paraganglioma-pheochromocytoma syndrome (PMID: 24903423, 24893135, 20301715, 23512077).

Pathogenic SDHB variants are associated with large, highly aggressive extra-adrenal sympathetic PGLs (mostly in the abdomen, pelvis, or thorax) that have a 31%–72% risk of malignancy and poor prognosis (PMID: 26347711, 15328326). PCCs and HNPs occur less frequently, 15-31% and 28%, respectively, and have a lower rate of malignant transformation (PMID: 21771581, 24893135, 20301715, 15328326, 16317055). Pathogenic variants in this gene lead to metastatic disease in at least 40% of cases (PMID: 24893135). Affected individuals tend to present at younger ages, are more likely to have multiple synchronous neoplasms, and multifocal, bilateral, recurrent disease (PMID: 24899893, 24893135). The mean age of tumor presentation is approximately 30 years, although cases diagnosed under the age of 10 have been reported (PMID: 21771581). Approximately 50% of individuals with malignant PGL have an SDHB pathogenic variant (PMID: 26347711). It has been suggested that SDHB pathogenic variants may be associated with pituitary adenomas and pancreatic neuroendocrine tumors, however, the evidence is preliminary and emerging (PMID: 26113606, 24096523, 26259135, 25494863).

Gastrointestinal stromal tumors (GIST) are rare, typically adult-onset sarcomas that may be either benign or malignant. They can occur sporadically or due to a heritable pathogenic variant in a susceptibility gene, such as SDHB (PMID: 25741136). GIST can develop anywhere along the GI tract, which includes the esophagus, stomach, gallbladder, liver, small intestine, colon, rectum, anus, and lining of the gut. GISTs arise from a specific type of cell called interstitial cells of Cajal (ICC), which line the walls of the GI tract. More than half of GISTs start in the stomach. The next-largest proportion of cases start in the small intestine, followed by the omentum and peritoneum (PMID: 26113606, 17193819, 25741136, 17804857).

Carney-Stratakis syndrome
Carney-Stratakis syndrome (CSS) is an autosomal dominant condition that can be caused by pathogenic variants in SDHB. This rare condition is characterized by the development of PGL, GIST, or both (PMID: 23036227, 24899893, 24903423, 20301715, 25741136, 17667967). CSS is very similar to but distinctly different from Carney triad, a typically sporadic condition associated with pulmonary chondromas, GIST, and PGL (PMID: 20119652, 26173966).

SDHB also has preliminary evidence for an association with breast cancer (PMID: 21979946) and thyroid cancer (PMID: 19802898, 24096523, 25694510). Therefore, this gene is available as a “preliminary-evidence” gene on Invitae’s Breast Cancer Panel, Breast and Gyn Cancers Panel, and Thyroid Cancer Panel. Preliminary-evidence genes are selected from an extensive review of the literature and expert recommendations, but the association between the gene and the specific condition has not been completely established. This uncertainty may be resolved as new information becomes available, and therefore clinicians may continue to order these preliminary-evidence genes.

Gene information
SDHB is a tumor-suppressor gene that is involved in complex II of the mitochondrial electron transport chain (PMID: UniProtKB – P21912 (SDHB_HUMAN); Accessed September 2015). The succinate dehydrogenase enzyme, which is part of complex II, is composed of four subunit proteins encoded by SDHA, SDHB, SDHC, and SDHD. These are nuclear genes whose transcripts are then imported into the mitochondria, where they are modified, folded, and assembled (PMID: 21771581). Lack of any component of mitochondrial complex Ⅱ will result in the instability of the entire complex (PMID: 25741136). If there is a pathogenic variant in this gene that prevents it from functioning normally, the risk of developing certain types of cancers may be increased.

Pathogenic variants in the SDHB gene are inherited in an autosomal dominant manner. This means that an individual with a pathogenic variant has a 50% chance of passing the condition on to their offspring. With this result, it is now possible to identify at-risk relatives who can pursue testing for this specific familial variant. Many cases are inherited from a parent, but some cases can occur spontaneously (i.e., an individual with a pathogenic variant has parents who do not have it). An individual with a variant in SDHB has a 50% risk of passing that variant on to offspring.

It is suggested that individuals with hereditary paraganglioma-pheochromocytoma syndrome, along with their at-risk relatives, have regular clinical monitoring by a physician or medical team with expertise in the treatment of hereditary GIST and PGL/PCC syndromes. A consultation with an endocrine surgeon, endocrinologist, and otolaryngologist is also recommended to establish an individualized care plan (PMID: 20301715).

Screening should begin between five and ten years of age, or at least ten years before the earliest age at diagnosis in the family (PMID: 24903423, 20301715, 24523625). Although there is currently no clear consensus regarding a screening and surveillance protocol for individuals with pathogenic SDHB variants, lifelong annual biochemical and clinical surveillance is suggested (PMID: 24893135, 20301715, 24523625, 25385035):

  • Physical exam and blood pressure at the time of diagnosis and then every 6-12 months
  • 24-hour urinary excretion of fractionated metanephrines and catecholamines and/or plasma fractionated metanephrines at least annually to detect metastatic disease, tumor recurrence, or development of additional tumors
    • Follow up with imaging by CT, MRI, 123I-MIBG (metaiodobenzylguanidine) scintigraphy, or FDG-PET if the fractionated metanephrine and/or catecholamine levels become elevated, or if the original tumor had minimal or no catecholamine/fractionated metanephrine excess
  • Periodic (e.g., every 6–24 months) abdominal, thorax, and pelvic MRI or CT to detect PGL
  • MRI or CT of skull base and neck every 2 to 4 years
    • In order to minimize radiation exposure, MRI may be the preferable imaging modality with CT and nuclear imaging reserved to further characterize detected tumors
  • Periodic (e.g., every 2–4 years) 123I-MIBG scintigraphy or PET to detect PGLs or metastatic disease that may be undetectable by MRI or CT
    • Imaging modalities should be at the discretion of the managing provider due to conflicting data regarding the utility and efficacy of the various options (PMID: 25385035, 24893135)
  • Evaluate cases with extra-adrenal sympathetic PGL and PCC for blood pressure elevations, tachycardia, and other signs and symptoms of catecholamine hypersecretion
  • Consider evaluation for GISTs in individuals (especially children, adolescents, or young adults) who have unexplained gastrointestinal symptoms (e.g., abdominal pain, upper gastrointestinal bleeding, nausea, vomiting, difficulty swallowing) or who experience unexplained intestinal obstruction or anemia
  • Consider screening for renal cell carcinoma (PMID: 23083876, 24523625)
  • Medical genetics consultation

For individuals with pathogenic SDHB variants, it is recommended that PGLs and PCCs be resected as soon as possible after tumor discovery. Prompt resection is particularly important for extra-adrenal sympathetic PGLs due to the risk of metastasis. Early detection and intervention through surveillance may prevent or minimize complications (PMID: 24893135, 20301715). Due to the complexity in treating SDHB-related PGL, it is suggested that affected individuals seek care through an experienced multi-disciplinary clinic.

Summary of medical management and surveillance recommendations (PMID: 24893135, 24523625, 20301715, 25385035)

Recommendation SDHB screening
Age to begin screening (years) 5–10 years
Physical exam and blood pressure Every 6–12 months
Urinary excretion of fractionated metanephrines and catecholamines in 24 hours Annually
MRI/CT of abdomen, thorax, and pelvis Every 6–24 months
MRI/CT of skull base and neck Every 2–4 years
Periodic 123I-MIBG (metaiodobenzylguanidine) scintigraphy Every 2–4 years
Screening for renal cell carcinoma Consider

It remains unclear whether imaging studies should be conducted as frequently in childhood as in adulthood (PMID: 24523625).

It is important to note that individuals with a pathogenic SDHB variant may be at a greater risk of developing PGL or PCC when living in higher altitudes or when chronically exposed to hypoxic conditions. In general, inactivating mutations in one of the succinate dehydrogenase genes, which includes SDHB, leads to accumulation of succinate, the formation of reactive oxygen species, and the activation of hypoxia-dependent pathways (PMID: 24899893). Avoidance of living at high altitudes and activities that promote long-term exposure to hypoxia and predispose to chronic lung disease (e.g., smoking) is therefore encouraged (PMID: 20301715, 24523625).

Syndromes associated with a predisposition to PGLs and PCCs may be associated with high morbidity and significant complications, which can lead to decreased lifespan and quality of life, therefore early screening and therapeutic interventions are imperative. However, the natural history of hereditary paraganglioma-pheochromocytoma syndrome is variable and continues to evolve. This can often result in significant uncertainty regarding long-term prognosis. Targeted genetic counseling may help patients cope with this diagnosis while keeping them an active participant in the management of their condition (PMID: 24854530). In addition, the clinical manifestations of PGLs and PCCs are broad; many symptoms can mimic minor ailments such as headaches and palpitations. Once a pathogenic variant has been identified, patients should be encouraged to have a low threshold for contacting their healthcare provider for further evaluation of unusual symptoms (PMID: 24854530).

An individual’s cancer risk and medical management are not determined by genetic test results alone. Overall cancer risk assessment incorporates additional factors, including personal medical history, family history, and any available genetic information that may result in a personalized plan for cancer prevention and surveillance.

Even though data regarding SDHB is limited, knowing if a pathogenic variant is present is advantageous. At-risk relatives can be identified, enabling pursuit of a diagnostic evaluation. Further, the available information regarding hereditary cancer susceptibility genes is constantly evolving and more clinically relevant data regarding SDHB are likely to become available in the near future. Awareness of this cancer predisposition encourages patients and their providers to inform at-risk family members, to diligently follow condition-specific screening protocols, and to be vigilant in maintaining close and regular contact with their local genetics clinic in anticipation of new information.

Review date: February 2016

Assay and technical information

Invitae is a College of American Pathologists (CAP)-accredited and Clinical Laboratory Improvement Amendments (CLIA)-certified clinical diagnostic laboratory performing full-gene sequencing and deletion/duplication analysis using next-generation sequencing technology (NGS).

Our sequence analysis covers clinically important regions of each gene, including coding exons, +/- 10 base pairs of adjacent intronic sequence, and select noncoding variants. Our assay provides a Q30 quality-adjusted mean coverage depth of 350x (50x minimum, or supplemented with additional analysis). Variants classified as pathogenic or likely pathogenic are confirmed with orthogonal methods, except individual variants that have high quality scores and previously validated in at least ten unrelated samples.

Our analysis detects most intragenic deletions and duplications at single exon resolution. However, in rare situations, single-exon copy number events may not be analyzed due to inherent sequence properties or isolated reduction in data quality. If you are requesting the detection of a specific single-exon copy number variation, please contact Client Services before placing your order.

Gene Transcript reference Sequencing analysis Deletion/Duplication analysis
SDHB NM_003000.2