title: Mapping mitochondrial heteroplasmy in a Leydig tumor by laser capture micro-dissection and cycling temperature capillary electrophoresis.
abstract: BACKGROUND
  The growth of tumor cells is accompanied by mutations in nuclear and mitochondrial genomes creating marked genetic heterogeneity. Tumors also contain non-tumor cells of various origins. An observed somatic mitochondrial mutation would have occurred in a founding cell and spread through cell division. Micro-anatomical dissection of a tumor coupled with assays for mitochondrial point mutations permits new insights into this growth process. More generally, the ability to detect and trace, at a histological level, somatic mitochondrial mutations in human tissues and tumors, makes these mutations into markers for lineage tracing.
  
  
  METHODS
  A tumor was first sampled by a large punch biopsy and scanned for any significant degree of heteroplasmy in a set of sequences containing known mutational hotspots of the mitochondrial genome. A heteroplasmic tumor was sliced at a 12 μm thickness and placed on membranes. Laser capture micro-dissection was used to take 25000 μm2subsamples or spots. After DNA amplification, cycling temperature capillary electrophoresis (CTCE) was used on the laser captured samples to quantify mitochondrial mutant fractions.
  
  
  RESULTS
  Of six testicular tumors studied, one, a Leydig tumor, was discovered to carry a detectable degree of heteroplasmy for two separate point mutations: a C → T mutation at bp 64 and a T → C mutation found at bp 152. From this tumor, 381 spots were sampled with laser capture micro-dissection. The ordered distribution of spots exhibited a wide range of fractions of the mutant sequences from 0 to 100% mutant copies. The two mutations co-distributed in the growing tumor indicating they were present on the same genome copies in the founding cell.
  
  
  CONCLUSIONS
  Laser capture microdissection of sliced tumor samples coupled with CTCE-based point mutation assays provides an effective and practical means to obtain maps of mitochondrial mutational heteroplasmy within human tumors.
authors.affiliation: Chair of Applied Statistics, Ecole Polytechnique Federale de Lausanne, Lausanne, Switzerland.
authors.forename: Paulo
authors.identifier: 0000-0002-9327-5649
authors.initials: P
authors.lastname: Refinetti
authors.affiliation: Institute for Cancer Research, The Norwegian Radium Hospital, Oslo, Norway.
authors.forename: Christian
authors.initials: C
authors.lastname: Arstad
authors.affiliation: Laboratory in Metakaryotic Biology, Massachusetts Institute of Technology, Cambridge, MA USA.
authors.forename: William G
authors.initials: WG
authors.lastname: Thilly
authors.affiliation: Chair of Applied Statistics, Ecole Polytechnique Federale de Lausanne, Lausanne, Switzerland.
authors.forename: Stephan
authors.initials: S
authors.lastname: Morgenthaler
authors.affiliation: Institute for Cancer Research, The Norwegian Radium Hospital, Oslo, Norway.
authors.forename: Per Olaf
authors.initials: PO
authors.lastname: Ekstrøm
keywords: Capillary electrophoresis; Heteroplasmy; Homoplasmy; LCM; Mitochondrial mutations; Tumor heterogeneity