Loss of the inactive X chromosome and replication of the active X in BRCA1-defective and wild-type breast cancer cells.
In females, X chromosome inactivation (XCI) begins with the expression of the XIST gene from the X chromosome destined to be inactivated (Xi) and the coating of XIST RNA in cis. It has recently been reported that this process is supported by the product of the BRCA1 tumor suppressor gene and that BRCA1-/- cancers show Xi chromatin structure defects, thus suggesting a role of XCI perturbation in BRCA1-mediated tumorigenesis. Using a combined genetic and epigenetic approach, we verified the occurrence of XCI in BRCA1-/- and BRCA1wt breast cancer cell lines. It was ascertained that the Xi was lost in all cancer cell lines, irrespective of the BRCA1 status and that more than one active X (Xa) was present. In addition, no epigenetic silencing of genes normally subjected to XCI was observed. We also evaluated XIST expression and found that XIST may be occasionally transcribed also from Xa. Moreover, in one of the BRCA1wt cell line the restoring of XIST expression using a histone deacetylase inhibitor, did not lead to XCI. To verify these findings in primary tumors, chromosome X behavior was investigated in a few BRCA1-associated and BRCA1-not associated primary noncultured breast carcinomas and the results mirrored those obtained in cancer cell lines. Our findings indicate that the lack of XCI may be a frequent phenomenon in breast tumorigenesis, which occurs independently of BRCA1 status and XIST expression and is due to the loss of Xi and replication of Xa and not to the reactivation of the native Xi.
Sirchia
Silvia M
SM
Genetica Medica, Farmacologia, Dipartimento di Medicina, Chirurgia e Odontoiatria, Università di Milano, Italy. silvia.sirchia@unimi.it
Ramoscelli
Lisetta
L
Grati
Francesca R
FR
Barbera
Floriana
F
Coradini
Danila
D
Rossella
Franca
F
Porta
Giovanni
G
Lesma
Elena
E
Ruggeri
Anna
A
Radice
Paolo
P
Simoni
Giuseppe
G
Miozzo
Monica
M
D001943:Breast Neoplasms / Q000235:genetics* / Q000378:metabolism; D045744:Cell Line, Tumor; D002843:Chromatin / Q000235:genetics; D041321:Chromosomes, Human, X / Q000235:genetics*; D005260:Female; D015972:Gene Expression Regulation, Neoplastic / Q000235:genetics*; D020868:Gene Silencing; D019398:Genes, BRCA1 / Q000502:physiology*; D006720:Homozygote; D006801:Humans; D017404:In Situ Hybridization, Fluorescence; D062085:RNA, Long Noncoding; D022661:RNA, Untranslated / Q000096:biosynthesis / Q000235:genetics*; D014158:Transcription, Genetic
D002843:Chromatin; D062085:RNA, Long Noncoding; D022661:RNA, Untranslated; C077885:XIST non-coding RNA