DNA instability in replicating Huntington's disease lymphoblasts.
BACKGROUND
The expanded CAG repeat in the Huntington's disease (HD) gene may display tissue-specific variability (e.g. triplet mosaicism) in repeat length, the longest mutations involving mitotic (germ and glial cells) and postmitotic (neurons) cells. What contributes to the triplet mutability underlying the development of HD nevertheless remains unknown. We investigated whether, besides the increased DNA instability documented in postmitotic neurons, possible environmental and genetic mechanisms, related to cell replication, may concur to determine CAG repeat mutability. To test this hypothesis we used, as a model, cultured HD patients' lymphoblasts with various CAG repeat lengths.


RESULTS
Although most lymphoblastoid cell lines (88%) showed little or no repeat instability even after six or more months culture, in lymphoblasts with large expansion repeats beyond 60 CAG repeats the mutation size and triplet mosaicism always increased during replication, implying that the repeat mutability for highly expanded mutations may quantitatively depend on the triplet expansion size. None of the investigated genetic factors, potentially acting in cis to the mutation, significantly influence the repeat changes. Finally, in our experiments certain drugs controlled triplet expansion in two prone-to-expand HD cell lines carrying large CAG mutations.


CONCLUSIONS
Our data support quantitative evidence that the inherited CAG length of expanded alleles has a major influence on somatic repeat variation. The longest triplet expansions show wide somatic variations and may offer a mechanistic model to study triplet drug-controlled instability and genetic factors influencing it.
Cannella
Milena
M
Neurogenetics Unit, IRCCS Neuromed, Pozzilli, IS, Italy. genetics@neuromed.it
Maglione
Vittorio
V
Martino
Tiziana
T
Ragona
Giuseppe
G
Frati
Luigi
L
Li
Guo-Min
GM
Squitieri
Ferdinando
F
D002455:Cell Division / Q000187:drug effects / Q000235:genetics; D002460:Cell Line; D003432:Cross-Linking Reagents / Q000494:pharmacology; D004996:Ethidium / Q000494:pharmacology; D005020:Ethyl Methanesulfonate / Q000494:pharmacology; D005260:Female; D006801:Humans; D006816:Huntington Disease / Q000235:genetics* / Q000473:pathology; D007364:Intercalating Agents / Q000494:pharmacology; D008214:Lymphocytes / Q000187:drug effects / Q000473:pathology*; D008297:Male; D053842:Microsatellite Instability / Q000187:drug effects; D016685:Mitomycin / Q000494:pharmacology; D009030:Mosaicism; D009154:Mutation; D019680:Trinucleotide Repeat Expansion / Q000187:drug effects
D003432:Cross-Linking Reagents; D007364:Intercalating Agents; D016685:Mitomycin; D005020:Ethyl Methanesulfonate; D004996:Ethidium