Next generation sequencing for characterization of mitochondrial genome in pituitary adenomas

Introduction Disrupted mitochondrial functions and genetic variations of mitochondrial DNA (mtDNA) have been observed in different tumors. Regarding pituitary adenomas mtDNA was evaluated only in oncocytic type using PCR based methods and it showed high prevalence of Complex I variants. Next generation sequencing (NGS) allows high throughput sequencing and it is useful for accurate identification of heteroplasmy of mitochondrial genome as well. Aim We aimed to investigate the entire mitochondrial genome in different adenoma types. Material and methods We collected 22 gonadotroph (GO), 11 growth hormone producing (GH) and 11 null-cell (NC) adenoma specimens from samples removed by transsphenoidal surgery. From fresh frozen tissues DNA extraction was performed using QIAamp Fast DNA Tissue Kit. For library preparation VariantPro™ Amplicon Mitochondrion Panel kit was used. The total mtDNA (16569 bp) was sequenced on Illumina MiSeq Instrument. Following complex bioinformatic analysis Revised Cambridge Reference Sequence (rCRS) of the human mitochondrial DNA was used as reference. Heteroplasmy was determined using 3% cutoff. Results. The whole mitochondrial genome were covered by 630±370 (avg±SE) reads per base. 496 variants were identified in adenomas compared to reference sequence. Overall a low (7.22%) heteroplasmy prevalence was found. Based on mitochondrial sequence variants by hierarchical cluster analysis we could not discriminate different adenoma types. No association between Ki-67 index or recurrent-nonrecurrent status of adenomas and mitochondrial variants were detected. Four variants appeared more often in null-cell adenomas compared to gonadotroph adenomas (chrM_188: 18% vs. 0%, chrM_16093: 18% vs. 0%, chrM_185: 27% vs. 0% and chrM_14798: 36% vs. 5%; Padj=0.0246, 0.0246, 0.01542 and 0.01829, respectively). Of these variants chrM_14798, chrM_4216 and chrM_15452 are non-synonymous polymorphisms leading to amino acid change in MT-CYB (mitochondrially encoded cytochrome b) and in MT-ND1 (mitochondrially encoded NADH dehydrogenase 1) genes. We identified chrM_16189 variant (non-protein coding variant) in 40% (6/15) of nonrecurrent adenomas compared to recurrent ones where this variant was not present (0/11) (p=0.0209). Conclusions Next-generation sequencing is a reliable method for investigating mitochondrial genome and heteroplasmy in pituitary adenomas. In pituitary adenomas the prevalence of heteroplasmy of mitochondrial genome is low suggesting that these alterations may not influence mitochondrial function considerably. Of pituitary tumours only null cell adenomas possess alterations of mitochondrial genome with potential functional consequences suggesting that during the development of this subtype of pituitary tumours mitochondrial function-associated mechanisms may have role

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