Genetics
Overall, germline mutations can be identified in approximately 30 % of patients with head and neck paragangliomas.
33,34 Familial glomus tumors constitute approximately 20% of affected patients, for which the genetic defects are known. One subgroup (10% of all glomus tumors) appears to be caused by sporadic mutations in SDHB and SDHD.
Hereditary susceptibility to paragangliomas, mainly of the head and neck region, was recognized at least two decades ago, and led to the identification through linkage analysis of three loci on chromosomes 11 and 1, named PGL1 on 11q23, PGL2 on 11q11.3, and PGL3 on 1q21-23. Following the discovery of succinate dehydrogenase (SDH) subunit D gene (
SDHD) as the gene responsible for PGL1 in familial head and neck paragangliomas, it was thereafter recognized that two other subunits of this mitochondrial enzyme, SDHC (PGL3) and SDHB (PGL4, 1p36) were associated with heritable pheochromocytoma and/or paraganglioma. To date, the gene for PGL2 has not been identified.
20,35,36,37,38
At present, causative gene mutations are identified in about 32% of paragangliomas-pheochromocytomas.
39 In the hereditary tumor syndromes: MEN2A, MEN2B, Von Hippel-Lindau and NF-1, in which a pheochromocytoma or paraganglioma presents, a causative gene mutation is seen in 17% of cases. Within the mitochondrial SDH complex, paragangliomas account for 15% of mutations.
The molecular basis for tumorigenesis in paragangliomas resides in mutations, germ line, and somatic for the most part in the genes that control SDH. The product of these multiple genes is a 4-dimer protein that catalyzes succinate to fumarate in mitochondria as the first step of oxygen-dependent glucose breakdown via the respiratory pathway. The enzyme is composed of 4 dimers, namely, SDHA, SDHB, SDHC, and SDHD, and is anchored in the internal bilipid membranes of the mitochondria. SDHA and SDHB are the proteomic components that anchor the enzyme in the membrane, while SDHC and SDHD are the proteomic components that catalyze the reaction from succinate to fumarate. This reaction is crucial to entering glucose breakdown from anaerobic to aerobic metabolism.
34 SDH or succinate-ubiquinone reductase is the complex II of the mitochondrial respiratory chain located in the mitochondrial matrix. SDH couples the oxidation of succinate to fumarate in the Krebs cycle with electron transfer to the terminal acceptor ubiquinone, thus leading to the generation of ATP.
40
SDH is an enzyme complex composed by four subunits encoded by four nuclear genes (
SDHA, SDHB, SDHC, and SDHD).
SDHC (cybL, 15 kDA, 169 amino acids) and
SDHD (cybS, 12 kDa, 159 amino acids) subunits are hydrophobic and provide membrane anchor and the binding site for ubiquinone.
SDHA (flavoprotein, 70 kDa, 664 amino acids) and
SDHB (iron-sulfur protein, 27 kDa, 280 amino acids) are hydrophilic, with the former involved in substrate binding and oxidation and the latter in electron transfer. Both
SDHB (35.4 kb, 8 exons) and
SDHC (50.3 kb, 6 exons) genes are located on chromosome 1, in the short and long arm, respectively.
SDHD, located on 11q23.1, spans 8.9 kb and contains four exons whereas
SDHA lies on the short arm of chromosome 5 (5p15) and is composed of 15 exons spread in a genomic region of 38.4 kb. Whereas homozygote germline mutations affecting
SDHA cause Leigh syndrome, a subacute necrotizing encephalomyelopathy during infancy,
SDHD, SDHB, and
SDHC heterozygous mutations cause a genetic
predisposition to head and neck paragangliomas and adrenal/extra-adrenal pheochromocytomas.
41 This inherited tumorigenic predisposition is transmitted in an autosomal dominant fashion with age-dependent and incomplete penetrance. However, for
SDHD located on chromosome 11q, a parent-of-origin effect is revealed as the disease is manifest almost exclusively when the mutation is transmitted from the father. A maternal imprinting has therefore been postulated, but despite the pattern of inheritance,
SDHD shows bi-allelic expression in normal tissues and neural crest-derived tissues.
20,31
All of these genes are tumor-suppressor genes showing loss of heterozygosity (LOH): the loss of the normal allele in the tumor, in conjunction with germline mutation. This results in loss of a protein subunit that destabilizes the SDH complex and alters or abolishes its enzymatic activity.
42
Inactivation of the SDH complex leads to a hypoxic state with accumulation of succinate resulting in the stabilization of HIFla (hypoxia-induced factor la). Oxygen normally facilitates the degradation of HIFla through prolyl hydroxylases. As oxygen levels decline HIFla enters the nucleus and initiates the transcription of a host of genes known to be involved in tumorigenesis, including Vascular endothelial growth factor (VEGF).
43 VEGF and platelet-derived endothelial growth factor (PD-ECGF), as well as endothelin-1, has been found in the majority of specimens examined. It has been postulated that this was consistent with a paracrine mechanism for tumor development.
44,45
Recent hypotheses for the mechanism of a tumorigenesis link a decrease in apoptosis and the activation of a pseudohypoxic pathway via these mechanisms.
46 This peudohypoxia is known as the Warburg effect and is the basis for tumor imaging by F-16 PET/FDG.
47
Paraganglioma Syndrome 1: SDH Subunit D. SDH Subunit D (SDHD) is the second anchoring peptide for mitochondrial complex II in the inner mitochondrial membrane. The gene resides at 11q23 and was the first mitochondrial gene to be linked to tumorigenesis.
20 The proportion of mutation carriers at
SDHD gene that will develop a tumor is 87% to 100% (very high penetrance).
48 SDHD is responsible for PGL1 syndrome. Patients with PGL1 commonly have multifocal tumors and very rarely malignant ones. Maternal imprinting (absence of disease transmitted by the mother) suggesting sex-specific epigenetic modifications.
34 Higher altitudes in SDHD/PGL1 patients show increased phenotypic severity as well as increased likelihood of developing a pheochromocytoma.
49 In a similar manner, nonsense/splicing mutations show earlier presentation of head and neck paragangliomas as well as increased incidence of pheochromocytomas.
48 It has been proposed that
SDHD is a critical component of a cellular oxygen-sensing system. Mutations in
SDHD may incapacitate the oxygen-sensing mechanism, leading to an apparent or real hypoxic state accompanied by chronic hypoxic stimulation and cell proliferation. Support for hypoxia-induced hyperplasia comes from evidence obtained in high-altitude physiological studies. Cows, guinea pigs, rabbits, and dogs experience carotid body hyperplasia when living at high altitudes, which exposes them to a hypoxic condition, and this has also been described in humans. Another clinical observation lending support to these theories is the finding that patients suffering from conditions resulting in hypoxemia, such as cystic fibrosis, cyanotic heart disease, and chronic obstructive pulmonary disease, experience carotid body hyperplasia, and those suffering from chronic obstructive pulmonary disease have a higher rate of carotid body tumors.
20,21,35,36,37,38
Paraganglioma Syndrome 2: SDH Assembly Factor 2. SDH Assem bly Factor 2 (SDHAF2) is essential for the correct flavination of
SDHA and thus the function of the SDH complex.
40 This is referred to as the PGL2 locus. Mutations in SDHAF2 have been described to produce only head and neck paragangliomas, and these make a small contribution to the genetic burden of this condition.
50 SDHAF2 gene maps to chromosome 11q13 and as with the
SDHD mutations there are parent-of-origin effects on expression, such that tumor development only occurs after paternal inheritance. A striking aspect of mutations on
SDHAF2 is the very high level of penetrance and the development of tumors at an early age.
40
Paraganglioma Syndrome 3: SDH Subunit C. SDH Subunit C (SDHC) is one of two peptides anchoring mitochondrial complex II in the inner mitochondrial membrane. The gene resides at 1q23.3. Only 10 index cases of families with head and neck paragangliomas have been identified worldwide.
51 It accounts for <1 % of all head and neck paragangliomas and is responsible for PGL3 syndrome.
39
Paraganglioma Syndrome 4: SDH Subunit B. SDH Subunit B (SDHB4) is the iron sulfur protein catalytic subunit of complex II. The gene resides at 1p36.1-p35.
52 It has been identified as the susceptibility gene for PGL4 (paraganglioma syndrome 4—OMIM: #115310). Patients with head and neck paragangliomas due to mutations of
SDHB gene show a high rate of malignancy at 20% to 54%. These mutations have a low level of penetrance of 25% to 40%.
21,40,53 Only 37% of
SDHB mutation-positive index cases report a family history at presentation.
34 SDHB is known to be a significant cause of adrenal pheochromocytomas.
Succinate Dehydrogenase Subunit A
SDHA is a flavoprotein and the main catalytic subunit of SDH. Head and neck paragangliomas had not been associated with mutations in the gene for SDHA. However, recently a patient with an SDHA mutation and a catecholamine-secreting abdominal paraganglioma was described.
54 Congenital deficiency due to homozygous recessive mutations of
SDHA gene have been described, and these patients are severely affected with early cardiomyopathy, but no development of paragangliomas.
52,55,56 Other
SDHA mutations produce Leigh syndrome, a mitochondrial encephalopathy.
55 These mutations are generally missense.
52 It has been postulated that complete loss of
SDHA function as would occur with a truncating mutation would not be compatible with life.
Neurofibromatosis type 1 is also associated with both pheochromocytomas and jugulotympanic paragangliomas. Pheochromocytomas are also associated with multiple endocrine neoplasia type II with RET gene mutations, and with Von Hippel-Lindau syndrome and mutations in VHL gene.
In patients with successive generations of a family harboring the mutations, tumors develop at progressively younger ages. This finding is a good example of genetic anticipation, in which the mutation appears to be more severe with succeeding generations.
Succinate Dehydrogenase Complex Assembly Factor 2 (SDHAF2). This protein is responsible for the addition of the flavin-adenine dinucleotide (FAD) prosthetic group to form the SDHA flavoprotein. Studied mutations in the gene that codes for SDHAF2 show that this is related to high penetrance for head and neck paragangliomas.
20
In summary the genetic associations in these paraganglioma syndromes are as follows:
-
Patients with paraganglioma develop tumors at a younger age than sporadic cases
-
In PGL1, PGL2, and PGL3, the genetic transmission is autosomal dominant, with highly variable expressivity and reduced penetrance. Genomic imprinting is seen in PGL1: the
paternally transmitted genes lead to tumor development and the maternally transmitted gene gives carrier status without developing tumors
-
PGL1 and PGL4 show multicentricity and pheochromocytomas
-
PGL1 has a high degree of penetrance whereas PGL4 shows moderate penetrance
-
PGL4 shows a marked increase in malignant paragangliomas
-
PGL3 present exclusively as benign paragangliomas with no multifocality and has no association with pheochromocytomas
-
Other tumor syndromes: Neurofibromatosis type 1, MEN type 2, and Von Hippel-Lindau predispose to paragangliomas