Promoter DNA methylation of tumour suppressor microRNA genes in multiple myeloma

Promoter DNA methylation of tumour suppressor microRNA genes in multiple myeloma

黃君揚(Kwan-yeung Wong)

香港大學/Doctor of Philosophy/Medicine/博士(2011年)

並列摘要

Multiple myeloma (MM) is an incurable haematological malignancy. It is

characterized clinically by an asymptomatic precursor stage, known as monoclonal

gammopathy of undetermined significance (MGUS), which will transform into

symptomatic MM at a rate of 1% per year. Gene promoter hypermethylation by

catalytic conversion of cytosine into 5?methylcytosine at promoter?associated CpG

island is an alternative mechanism of gene inactivation. MicroRNA (miRNA) is a class of

short, single?stranded, non?coding RNA molecules, which will repress the expression of

target protein by sequence?specific binding to the three prime untranslated region of

the corresponding messenger RNA. In carcinogenesis, miRNA can be either oncogenic

when tumour suppressor genes are targeted, or tumour suppressive when oncogenes

are targeted. Despite reports of hypermethylation of multiple protein?coding tumour

suppressor genes, little is known about DNA methylation of non?coding tumour

suppressor miRNA genes in MM.

This thesis aimed to investigate the role of promoter hypermethylation of tumour

suppressor miRNA genes in MM using a candidate miRNA approach. Moreover, the

prognostic significance of tumour suppressor miRNA hypermethylation was studied in

a uniformly?treated cohort of MM patients.

The role of DNA methylation at the promoter of miR?203, miR?34a, miR?34b/c,

miR?124?1, miR?129?2 and miR?224 were studied in MM. The tumour suppressor role

of miR?34b/c, miR?124?1, miR?203 and miR?224 were demonstrated in human

myeloma cell lines (HMCLs). In particular, restoration of miR?203 in MM cells was

shown to inhibit cellular proliferation via targeting and hence direct downregulation of

a proto?oncogene, cyclic AMP responsive element binding protein. There are several

observations in primary MM samples. First, there was frequent methylation of

miR?129?2, miR?203 and miR?224 but infrequent methylation of miR?34a, miR?34b/c

and miR?124?1 in MM at diagnosis. Second, tumour?specific hypermethylation of each

of the miR?203 and miR?224 promoters was detected at comparable frequencies in

MGUS, diagnostic and relapsed/progressed MM, and hence implicated as an early

event in myelomagenesis. Thirdly, miR?129?2 methylation was more frequent in

diagnostic MM than MGUS, and hence implicated in MGUS progression to MM. On the

other hand, despite rare miR?34b/c methylation at diagnosis, miR?34b/c methylation

was frequent at relapse/progression, thereby implicating miR?34b/c methylation in

MM relapse/progression. Fourthly, despite frequent miR?124?1 methylation in HMCLs,

miR?124?1 methylation was rare in both diagnostic and relapsed MM marrow samples,

suggesting that miR?124?1 methylation was acquired during in vitro cell culture.

Finally, the prognostic significance of methylation of a panel of tumour

suppressor miRNAs was studied in a uniformly?treated cohort of MM patients, which

revealed that miR?224 hypermethylation as an independent favourable prognostic

factor for survival.

In conclusion, hypermethylation of tumour suppressor miRNAs is implicated in

the pathogenesis (miR?203, miR?129?2, miR?224), progression (miR?34b/c), and

prognostification (miR?224) of MM.

並列關鍵字

DNA - Methylation ； Small interfering RNA ； Antioncogenes ； Multiple myeloma - Genetic aspects