Validation of Next-Generation Sequencing of Entire Mitochondrial Genomes and the Diversity of Mitochondrial DNA Mutations in Oral Squamous Cell Carcinoma.
BACKGROUND
Oral squamous cell carcinoma (OSCC) is mainly caused by smoking and alcohol abuse and shows a five-year survival rate of ~50%. We aimed to explore the variation of somatic mitochondrial DNA (mtDNA) mutations in primary oral tumors, recurrences and metastases.


METHODS
We performed an in-depth validation of mtDNA next-generation sequencing (NGS) on an Illumina HiSeq 2500 platform for its application to cancer tissues, with the goal to detect low-level heteroplasmies and to avoid artifacts. Therefore we genotyped the mitochondrial genome (16.6 kb) from 85 tissue samples (tumors, recurrences, resection edges, metastases and blood) collected from 28 prospectively recruited OSCC patients applying both Sanger sequencing and high-coverage NGS (~35,000 reads per base).


RESULTS
We observed a strong correlation between Sanger sequencing and NGS in estimating the mixture ratio of heteroplasmies (r = 0.99; p<0.001). Non-synonymous heteroplasmic variants were enriched among cancerous tissues. The proportions of somatic and inherited variants in a given gene region were strongly correlated (r = 0.85; p<0.001). Half of the patients shared mutations between benign and cancerous tissue samples. Low level heteroplasmies (<10%) were more frequent in benign samples compared to tumor samples, where heteroplasmies >10% were predominant. Four out of six patients who developed a local tumor recurrence showed mutations in the recurrence that had also been observed in the primary tumor. Three out of five patients, who had tumor metastases in the lymph nodes of their necks, shared mtDNA mutations between primary tumors and lymph node metastases. The percentage of mutation heteroplasmy increased from the primary tumor to lymph node metastases.


CONCLUSIONS
We conclude that Sanger sequencing is valid for heteroplasmy quantification for heteroplasmies ≥10% and that NGS is capable of reliably detecting and quantifying heteroplasmies down to the 1%-level. The finding of shared mutations between primary tumors, recurrences and metastasis indicates a clonal origin of malignant cells in oral cancer.
Division of Genetic Epidemiology, Medical University of Innsbruck, Innsbruck, Austria.
Anita
A
Kloss-Brandstätter
Division of Genetic Epidemiology, Medical University of Innsbruck, Innsbruck, Austria; Department of Database and Information Systems, Institute of Computer Science, Leopold-Franzens University of Innsbruck, Innsbruck, Austria.
Hansi
H
Weissensteiner
Division of Genetic Epidemiology, Medical University of Innsbruck, Innsbruck, Austria.
Gertraud
G
Erhart
Department of Urology, Medical University of Innsbruck, Innsbruck, Austria.
Georg
G
Schäfer
Division of Genetic Epidemiology, Medical University of Innsbruck, Innsbruck, Austria; Department of Database and Information Systems, Institute of Computer Science, Leopold-Franzens University of Innsbruck, Innsbruck, Austria.
Lukas
L
Forer
Division of Genetic Epidemiology, Medical University of Innsbruck, Innsbruck, Austria; Department of Database and Information Systems, Institute of Computer Science, Leopold-Franzens University of Innsbruck, Innsbruck, Austria.
Sebastian
S
Schönherr
Division of Genetic Epidemiology, Medical University of Innsbruck, Innsbruck, Austria; Department of Database and Information Systems, Institute of Computer Science, Leopold-Franzens University of Innsbruck, Innsbruck, Austria.
Dominic
D
Pacher
Department of Urology, Medical University of Innsbruck, Innsbruck, Austria.
Christof
C
Seifarth
Division of Genetic Epidemiology, Medical University of Innsbruck, Innsbruck, Austria.
Andrea
A
Stöckl
Division of Genetic Epidemiology, Medical University of Innsbruck, Innsbruck, Austria.
Liane
L
Fendt
Department of Urology, Medical University of Innsbruck, Innsbruck, Austria.
Irma
I
Sottsas
Department of Urology, Medical University of Innsbruck, Innsbruck, Austria.
Helmut
H
Klocker
Institute of Analytical Chemistry and Radiochemistry, Leopold-Franzens University of Innsbruck, Innsbruck, Austria.
Christian W
CW
Huck
Department for Cranio-, Maxillofacial and Oral Surgery, Medical University of Innsbruck, Innsbruck, Austria.
Michael
M
Rasse
Division of Genetic Epidemiology, Medical University of Innsbruck, Innsbruck, Austria.
Florian
F
Kronenberg
Department for Cranio-, Maxillofacial and Oral Surgery, Medical University of Innsbruck, Innsbruck, Austria.
Frank R
FR
Kloss
D002294:Carcinoma, Squamous Cell / Q000235:genetics* / Q000473:pathology; D004272:DNA, Mitochondrial / Q000235:genetics; D054629:Genome, Mitochondrial; D059014:High-Throughput Nucleotide Sequencing / Q000379:methods; D006801:Humans; D008969:Molecular Sequence Data; D009062:Mouth Neoplasms / Q000235:genetics* / Q000473:pathology; D009154:Mutation; D009364:Neoplasm Recurrence, Local; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D017422:Sequence Analysis, DNA / Q000379:methods
D004272:DNA, Mitochondrial