Succinate dehydrogenase and MYC-associated factor X mutations in pituitary neuroendocrine tumours

Abstract

Pituitary neuroendocrine tumours (PitNETs) associated with paragangliomas or phaeochromocytomas are rare. SDHx variants are estimated to be associated with 0.3-1.8% of PitNETs. Only a few case reports have documented the association with MAX variants. Prolactinomas are the most common PitNETs occurring in patients with SDHx variants, followed by somatotrophinomas, clinically non-functioning tumours and corticotrophinomas. One pituitary carcinoma has been described. SDHC, SDHB and SDHA mutations are inherited in an autosomal dominant fashion and tumorigenesis seems to adhere to Knudson's two-hit hypothesis. SDHD and SDHAF2 mutations most commonly have paternal inheritance. Immunohistochemistry for SDHB or MAX and loss of heterozygosity analysis can support the assessment of pathogenicity of the variants. Metabolomics is promising in the diagnosis of SDHx-related disease. Future research should aim to further clarify the role of SDHx and MAX variants or other genes in the molecular pathogenesis of PitNETs, including pseudohypoxic and kinase signalling pathways along with elucidating epigenetic mechanisms to predict tumour behaviour.

Keywords: MAX; SDH; paraganglioma; phaeochromocytoma; pituitary neuroendocrine tumour; succinate dehydrogenase.

Figure 1

The SDH complex and its relationship to the mitochondrial membranes and mitochondrial cristae. Together the SDH subunits make up respiratory complex II. The hydrophobic SDHD and SDHC subunits anchor the complex within the inner mitochondrial membrane, while the hydrophilic SDHA and SDHB subunits catalyse the oxidation of succinate to fumarate as part of the tricarboxylic cycle. SDHAF2, also known as SDH5, is known to have roles in the flavination of SDHA and research into its roles and structure is ongoing (Sharma et al. 2020). Electrons generated by the tricarboxylic acid cycle (e-) reduce FAD to FADH₂ in SDHA before proceeding through Fe-S clusters in SDHB. These electrons then reduce ubiquinone (Q) to ubiquinol (QH₂) before being transported to the adjacent respiratory complex III. Mitochondrion image created with Biorender.com

Figure 2

(A) The Knudson hypothesis. A germline mutation of SDHA, SDHB or SDHC is inherited and a second hit such as a somatic mutation or chromosomal loss is acquired resulting in loss of heterozygosity (LOH). (B) The Hensen hypothesis is applicable to SDHD (Hensen et al. 2004). SDHD is a maternally imprinted gene located on the long arm of chromosome 11. The presence of a paternally imprinted tumour suppressor gene (S) on the short arm is hypothesised. In the usual paternal transmission of SDHD, the mutated paternal SDHD together with the lack of expression of a tumour suppressor gene and loss of maternal chromosome 11 can result in tumorigenesis (B middle panel). If the SDHD mutation is on the maternal chromosome, usually no tumour is observed, as even if the paternal chromosome with the normal SDHD is lost, the lack of functional SDHD is counteracted by an expressed tumour suppressor gene from the short arm of the maternal chromosome 11 (B middle panel). In the rare maternal transmission of SDHD mutations, two steps are required. The first is suggested to be a chromosomal recombination resulting in transfer of the maternal SDHD mutation to the paternal allele harbouring the imprinted tumour suppressor gene. The second is loss of the maternal chromosome 11. The tumour suppressor gene S located on the short arm of chromosomal 11 is currently unknown. SDHAF2 also shows evidence of paternal imprinting (Kunst et al. 2011).

Figure 3

Long-range enhancer–promoter contacts in SDHD gene expression. On the paternal allele, an enhancer can influence an SDHD promoter and thereby increase SDHD transcription. This occurs via a UPGL promoter, which remains unmethylated due to the competitive binding of a transcription factor (TF) preventing cohesin from engaging with a CpG island (CPI). On the maternal allele, the UPGL promoter is methylated (CH3), preventing the TF binding, which enables cohesin to bind to the CpG island and block the enhancer–promoter activity on SDHD. Consequently, the enhancer binds to an alternative promoter and there is downregulation of SDHD transcription.

Figure 4

(A) All cases of PitNETs reported in association with SDHx variants are summarised. Prolactinomas account for a significant proportion (59%). It is notable that there have only been eight cases where evidence consistent with a causative role for SDHx variant reported in the literature and all are macro PitNETs (B). The average age at diagnosis in this sub-cohort is 44 years (range 31–60). One patient had a mixed somatotroph–lactotroph tumour with clinical acromegaly (Xekouki et al. 2012); 75% of this subgroup had prolactin-expressing tumours. It is also possible that prolactinomas will be under-represented in B as they are not routinely managed with surgery.

Figure 5

The role of MAX in tumorigenesis: MAX heterodimerises with MAD transcription factors and acts to repress the oncogenic MYC protein. MAX mutations impair heterodimerisation resulting in downstream unchecked MYC activity. Germline variants of RET, NF1, PTEN and TMEM127 have been implicated in PPGL, but not in PitNETs. As shown in the figure, the proteins they transcribe have roles in the MYC pathway.

Figure 6

(A) Preoperative T1-weighted coronal MRI sequence displaying the extension in the left hemi-sinus. (B) Histopathologically, the tumour shows a biphasic pattern with a component of large cells with weakly eosinophilic cytoplasm (bottom left of the field) and a component with denser cellularity and cells with eosinophilic cytoplasm (top right of the field) (B1, HE – ×20); some tumour cells show vacuolated cytoplasm, indicated by white arrows (B2, HE – ×40); the immunostain for growth hormone highlights neoplastic somatotroph cells (B3, immunoperoxidase, ×20); Neoplastic cells show ubiquitous nuclear expression of the transcription factor PIT1 (B4, immunoperoxidase – ×20). (C) High-resolution NMR spectroscopy analysis shows the patient’s sample in blue in comparison to a control SDHx-mutated PGL sample in red. A succinate peak at 2.4 ppm is seen in the control PGL but is absent in the pituitary case, showing that the maternally inherited SDHD variant has not resulted in SDHD dysfunction.