Highlighting Interstitial Telomeric Spots in interphasic cells (HITS-FISH)

Telomeric fusions underscore telomeric binding proteins disfunctions. Fusions occur in senescent cells between short telomeres, they are detected in metaphasic cells by dicentric chromosomes most generally with no telomeric spots at the fusion point.

In some SV40 transformed fibroblast cell line, interstitial telomeric spots (ITS) are as bright as terminal telomeric spots, this indicates some uncoupling between telomere lenght and their capacity to block telomeric fusions. 

ITS observed in a precrisis human SV40 transformed cell line (Ducray et al., oncogene 1999)

From metaphasic chromosomes observation, fusions retaining visible telomeric spots are rare events but may be more frequent in interphasic cells.

ITS from a SV40 transformed cell line

Bal31 in-situ:

Several nucleases or DNA polymerases  were used in assays requiring fixed material (chromosome banding+restriction endonuclease, in-situ nick translation, fluorescent labelling with TUNEL, in-situ PCR). The exonuclease Bal31 removes totally terminal telomeric sequences of naked DNA after some hours of digestion (R C Allshire, M Dempster, and N D Hastie, NAR 1989; de Lange et al, MCB 1990) . In-situ,  the terminal telomeric sequences, should be removed by Bal31 digestion.  If telomeric spots can be detected in nuclei after Bal31 digestion, those spots should be  ITS. Monitoring ITS may be a tool of interest  to probe telesome functions at the scale of unique cell, or to monitor the state of a tissue.

Pig cells as model cells?

On human metaphasic chromosomes, a bright ITS indicates an  abnormal cell but for cheaper DAPI stained chromosomes rearrangements too. Normal pig karyotype  shows interstitial telomeric spots:

Normal diploid pig metaphase with a chromosomes pair having ITS.

After QFISH on interphasic pig nuclei, there are two ITS of same magnitude than terminal spots, these two pig ITS remain hidden in the forest of terminal telomeric spots:

  

Pig nuclei (25x magnification)

Some conditions of Bal31 digestion should retain these two ITS and remove totally terminal telomeric spots yielding the possibility to observe telomeric fusions in nuclei.

Assay:

Tuning the conditions of Bal31 digestion is a prerequisite step. Supposing that the exonuclease Bal31 can remove terminal sequence from fixed material, when this enzyme is usually used to digest purified DNA in solution. 

If pig cells are not available, performing Bal31 digestion on control human cells such lymphocytes should tell if Bal31 can remove telomeres in-situ.

From conventionnal cytogenetic preparation of pig cells (fibroblasts,lymphocytes), digestion time, numbers of Bal31 units could be set up as follow : 

Possible protocol for HITS FISH

The protocol modified from Lansdorp et al. (Hum Mol Genet. 1996 May;5(5):685-91.) could be also adapted to liquid hybridization for flow FISH. Under optimal conditions pig cells could be used as internal control for Bal 31 digestion by mixing human and pig cells in the same cytogenetic preparation (9:1 ratio).

Human cells can be then distinguished from pig cells with an additionnal oligo probe. In the following example (25x magnification), simultaneous hybridization with Cy3-PNA-(CCCTAA)3 (displayed in green) and Cy5-DNA (displayed in red) oligo probe was performed (unpublished data):

Candidate cells to test the assay, may be SV40 precrisis cells with highly rearranged chromosomes and numerous dicentrics, or better cells with controlled mutations.