Incomplete penetrance of a novel SDHD variation causing familial head and neck paraganglioma

Abstract Objective Identification of variations in tumour suppressor genes encoding the tetrameric succinate dehydrogenase (SDHx) mitochondrial enzyme complex may lead to personalised therapeutic concepts for the orphan disease, familial paraganglioma (PGL) type 1‐5. We undertook to determine the causative variation in a family suffering from idiopathic early‐onset (22 ± 2 years) head and neck PGL by PCR and Sanger sequencing. Design Prospective genetic study. Setting Tertiary Referral Otolaryngology Centre. Participants Twelve family members. Main outcome measures Main outcomes were clinical analysis and SDH genotyping Results and Conclusions A novel heterozygous c.298delA frameshift variation in exon 3 of SDH subunit D (SDHD) was associated with a paternal transmission pattern of PGL in affected family members available to the study. Family history over five generations in adulthood indicated a variable penetrance for PGL inheritance in older generations. The c.298delA variant would cause translation of a 34‐residue C‐terminus distal to lysine residue 99 in the predicted transmembrane domain II of the full‐length sequence p.(Thr100LeufsTer35) and would affect the translation products of all protein‐coding SDHD isoforms containing transmembrane topologies required for positional integration in the inner mitochondrial membrane and complex formation. These results underly the importance of genetic screening for PGL also in cases of unclear inheritance, and variation carriers should benefit from screening and lifelong follow‐up.


| INTRODUC TI ON
Paragangliomas (PGL) are neuroendocrine tumours of the peripheral nervous system arising in cells derived from the neural crest. PGL are rare and occur with an estimated incidence of 1-9 cases per million persons per year and are registered as an orphan disease by the U.S.
Office of Rare Diseases (GARD) and European Orphanet (classification number ORPHA29072).
PGL are classified as adrenal PGL originating in the adrenal medulla as pheochromocytoma (PCC) or extra-adrenal PGL that may occur in the abdomen, thorax and head and neck regions. 1 The most common locations are carotid, jugulotympanic and vagal head and neck (hn)PGL. Very rarely laryngeal PGL can occur. 2 Although PGL at any site can release catecholamines, this is more common for adrenal tumours and approximately 95% of hnPGL are nonsecretory. 3 A genetic cause can be found in approximately 40% of hnPGL patients, carrying a variation in one of more than 20 genes. 4 Around half of PGL and PCC cases show germline variations in the succinate dehydrogenase complex genes (SDHx) [4][5][6] and are classified as PGL syndromes PGL1-5 (table 1). PGL1 (OMIM #168000) is caused by variations in the succinate dehydrogenase complex subunit D gene (SDHD, OMIM *602690). 7 SDHD variants cause over 97% of cases of hnPGL 4 summarised in table S1. Furthermore, PGL1 patients show a high occurrence of multifocal and reoccurring tumours in contrast to patients with causative variations in other SDHx genes. [8][9][10][11] The mean onset of disease in familial hnPGL is below 45 years of age. [8][9][10]12 The transmission in PGL1 patients demonstrates a parent-of-origin effect and is mostly transmitted paternally 5,10,13-15 but also maternally. [16][17][18][19] Both loss of heterozygosity at 11q22-23 20 and somatic loss of the entire maternal chromosome 11 have been observed in 85% of SDHD caused PGL. 21 SDHD is the smallest unit of the succinate dehydrogenase (SDH) complex located in the mitochondrial membrane and involved in both the Krebs cycle and the respiratory chain catalysing the oxidation of succinate to fumarate coupled with the reduction of ubiquinone to ubiquinol. PGL tumourigenesis is believed to be initiated through the accumulation of succinate and reactive oxygen species (ROS) due to reduced SDH complex efficiency, increased reliance on glycolysis, protection against apoptosis and triggering a chronic pseudohypoxic response including increased angiogenesis. 7,22 In SDHD variation carriers, this abnormal vascularisation may increase the risk of ischaemic stroke following preoperative embolisation. 23 Succinate accumulation can also inhibit α-ketoglutaratedependent dioxygenases leading to large-scale hypermethylation and epigenetic reprogramming induced by inhibition of DNA and histone demethylases. 24 Here we identify a novel pathogenic, paternally inherited SDHD   • Genetic testing is highly recommended in cases of hnPGL. Patients with a SDHD variation should benefit from complete clinical screening and lifelong follow-up.
tomography (CT) scans were performed. If blood pressure levels were elevated, metanephrine values were assessed.

| Ethical considerations
Informed consent was obtained prior to study inclusion.

| RE SULTS
In a non-consanguineous Caucasian family from Austria (population 8.5 million), patients (n = 3/12) were diagnosed with hnPGL with a median age of onset of 22 ± 2 years (Figure 1). HnPGL patients included in the study were tumour-resected. Proband II.

| Synopsis of key/new findings
In this study, we describe a novel SDHD variation (c.298delA) as-

| Transmission of SDHD variations
The c.298delA variation is not present in the Human Gene Mutation In summary, we identify a novel c.298delA frameshift variation in SDHD associated with paternal transmission of hnPGL1. Genetic testing is highly recommended in hnPGL, and the unusual paternal inheritance patterns should be considered. Patients with a pathogenic SDHD variation should benefit from complete screening and lifelong follow-up.

ACK N OWLED G EM ENTS
The authors thank the family for participation in the study.

CO N FLI C T O F I NTE R E S T
There are no conflicts of interest.

AUTH O R CO NTR I B UTI O N S
MK, TP, CS, FL, PF, KF and TL were involved in study concepts and design. MK, TP, AF, EF, FL, PF, KF and TL were involved in data acquisition and interpretation. MK, TP, KF and TL were involved in manuscript preparation and editing. All authors were involved in manuscript review.