The Mitochondrial SDHD Gene Is Required for Early Embryogenesis, and Its Partial Deficiency Results in Persistent Carotid Body Glomus Cell Activation with Full Responsiveness to Hypoxia

Abstract

The SDHD gene encodes one of the two membrane-anchoring proteins of the succinate dehydrogenase (complex II) of the mitochondrial electron transport chain. This gene has recently been proposed to be involved in oxygen sensing because mutations that cause loss of its function produce hereditary familiar paraganglioma, a tumor of the carotid body (CB), the main arterial chemoreceptor that senses oxygen levels in the blood. Here, we report the generation of a SDHD knockout mouse, which to our knowledge is the first mammalian model lacking a protein of the electron transport chain. Homozygous SDHD ؊/؊ animals die at early embryonic stages. Heterozygous SDHD ؉/؊ mice show a general, noncompensated deficiency of succinate dehydrogenase activity without alterations in body weight or major physiological dysfunction. The responsiveness to hypoxia of CBs from SDHD ؉/؊ mice remains intact, although the loss of an SDHD allele results in abnormal enhancement of resting CB activity due to a decrease of K ؉ conductance and persistent Ca 2؉ influx into glomus cells. This CB overactivity is linked to a subtle glomus cell hypertrophy and hyperplasia. These observations indicate that constitutive activation of SDHD ؉/؊ glomus cells precedes CB tumor transformation. They also suggest that, contrary to previous beliefs, mitochondrial complex II is not directly involved in CB oxygen sensing. Mitochondrial complex II (succinate dehydrogenase [SDH])plays major biological roles in the Krebs cycle and the mitochondrial electron transport chain, oxidizing succinate to fumarate and transferring electrons to the ubiquinone pool (9, 12). SDH has also been proposed to have an antioxidant function, protecting against superoxide production in mitochondria, and to participate in oxygen (O 2 ) sensing (3,7,22,27). Deficiencies of any of the four subunits of SDH (A, B, C, and D) are associated with a broad spectrum of human diseases, ranging from myo-and encephalopathies to aging and tumor formation. Mutations in SDHA, the matrix-faced flavoprotein, result in dysfunction of the energy metabolism, e.g., Leigh syndrome or exercise intolerance, whereas mutations either in the iron-sulfur protein SDHB or in SDHC and SDHD, the membrane-anchoring proteins that contain one heme and are essential for ubiquinone binding, are more often associated with pheochromocytoma and paraganglioma (1,22,23). Specifically, mutations in SDHD are the main cause of familiar hereditary paraganglioma (PGL) (3) a mostly benign, highly vascularized tumor of the carotid body (CB). SDHD is the first tumor suppressor gene identified which encodes a mitochondrial protein.The CB is the main arterial chemoreceptor that senses blood O 2 concentration. It is a highly irrigated bilateral organ, located at the bifurcation of the carotid artery and derived from the neural crest. Upon exposure to acute hypoxia, neurosecretory CB glomus cells release transmitters which activate afferent sensory fibers connected with brain stem centers to elicit hyperventilation and sympathetic activation ...

Substantia nigra dopaminergic neurons and striatal interneurons are engaged in three parallel but interdependent postnatal neurotrophic circuits

“…Rosa26R‐YFP mice were crossed with Pv Cre/+ and Chat Cre/+ in order to identify cells that have undergone recombination. Pv Cre/+ ; Sdhd –/loxP (PV‐Sdhd) mice were generated from the original Sdhd KO (Piruat, Pintado, Ortega‐Saenz, Roche, & Lopez‐Barneo, 2004) and (Diaz‐Castro et al, 2012) floxed alleles. All the transgenic alleles were genotyped following Jackson Laboratory instructions.…”

Non-pheochromocytoma (PCC)/paraganglioma (PGL) tumors in patients with succinate dehydrogenase-related PCC–PGL syndromes: a clinicopathological and molecular analysis

“…Haploinsufficient effects for the SDHx genes have been suggested in: i) bilateral adrenal medullary hyperplasia associated with a germline SDHB mutation showing retention of heterozygosity (38); ii) PCCs without loss of the WT SDHD allele arising in SDHD-mutated patients (39); and iii) somatic SDHD inactivation being accompanied by consistent reduction of transcript levels in various tumors (40,41). Albeit, in mouse models, no indications were obtained for an SDHB/SDHD haploinsufficient contribution to tumorigenesis (42,43) Table 1), with a further 31 PAs co-occurring with PCCs and/or PGLs pointing to a causative association with SDHx, MEN1, or other yet unidentified predisposing genes (44,45,46). SDHx-mutated patients appear to have a characteristic clinical phenotype with PRL-secreting macroadenomas, and to a lesser extent GH-secreting macroadenomas, usually accompanied with a personal history of PCC/PGL, implying that SDH deficiency might promote tumor growth with lactotrophs being more susceptible to this particular deficient state.…”

Toward an improved definition of the genetic and tumor spectrum associated with SDH germ-line mutations

“…64,65 This suggests that epigenetic silencing of SDH could also be an important mechanism involved in tumor development and could explain SDH deficiency in the absence of germ-line mutations. Tumorigenesis caused by haploinsufficiency seems unlikely because SDHD, SDHD/H19, and SDHB knockout mice, as well as conditional or inducible tissue-specific SDHD knockouts, do not develop tumors or any other genotype-related pathology except for a slight carotid body hyperplasia [66][67][68][69] (Judith Favier, personal communication).…”