RIZ1 is epigenetically inactivated by promoter hypermethylation in thyroid carcinoma.
BACKGROUND
Allelotype studies have suggested that chromosome 1p is frequently lost in thyroid cancers, thus suggesting that there is an important tumor suppressor at this location. RIZ1 (PRDM2), located on 1p36, is a recently described tumor suppressor gene and is a member of the protein methyltransferase superfamily. RIZ1 expression is lost in a variety of tumors, primarily by means of epigenetic mechanisms that involve promoter hypermethylation.


METHODS
RIZ1 expression was examined in a panel of thyroid tumor cell lines and primary thyroid tissues (14 normal, 19 benign, and 31 cancerous) by using real-time polymerase chain reaction (PCR). Methylation status of the RIZ1 promoter was studied using bisulfite sequencing and methylation-specific PCR.


RESULTS
The authors demonstrated that RIZ1 expression is lost in thyroid tumor cell lines and is also significantly reduced in thyroid carcinomas, when compared with normal thyroid tissues (P < .0001) and benign tumors (P = .0003). The current study results also showed that loss of RIZ1 is mediated by aberrant cytosine methylation of the RIZ1 promoter. One hundred percent of carcinomas were methylated, compared with 33% of normal thyroid tissues (P = .001). RIZ1 mRNA expression was significantly higher (P = .02) in unmethylated (1.22 +/- 1.2, mean +/- standard deviation [SD]), compared with methylated tissues (0.37 +/- 0.42, mean +/- SD). Last, treatment with a DNA methyltransferase inhibitor led to reactivation of RIZ1 expression in cell lines that had negligible RIZ1 expression at baseline.


CONCLUSIONS
The current study suggested an important role for RIZ1 expression in thyroid tumorigenesis and identified a potential novel therapeutic target for tumors unresponsive to other therapies.
Lal
Geeta
G
Department of Surgery, Division of Surgical Oncology, University of Iowa, Iowa City, Iowa, USA.
Padmanabha
Lakshmi
L
Smith
Brian J
BJ
Nicholson
Rachael M
RM
Howe
James R
JR
O'Dorisio
M Sue
MS
Domann
Frederick E
FE
D000964:Antimetabolites, Antineoplastic / Q000494:pharmacology; D001374:Azacitidine / Q000494:pharmacology; D002277:Carcinoma / Q000235:genetics*; D045744:Cell Line, Tumor; D019175:DNA Methylation; D004268:DNA-Binding Proteins / Q000235:genetics / Q000502:physiology*; D044127:Epigenesis, Genetic / Q000187:drug effects; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects; D011495:Histone-Lysine N-Methyltransferase; D006801:Humans; D009687:Nuclear Proteins / Q000235:genetics / Q000502:physiology*; D011401:Promoter Regions, Genetic / Q000187:drug effects; D012333:RNA, Messenger / Q000032:analysis / Q000378:metabolism; D013964:Thyroid Neoplasms / Q000235:genetics*; D014157:Transcription Factors / Q000235:genetics / Q000502:physiology*
D000964:Antimetabolites, Antineoplastic; D004268:DNA-Binding Proteins; D009687:Nuclear Proteins; D012333:RNA, Messenger; D014157:Transcription Factors; D011495:Histone-Lysine N-Methyltransferase; C093563:PRDM2 protein, human; D001374:Azacitidine