microRNA – Drug Discovery Opinion

Loss of microRNA in Malignant Mesothelioma

Scanning electron micrograph showing asbestos fibres; Source – Wikimedia Commons

MicroRNAs (miRNAs) are small (21-23 nucleotides), single-stranded RNA molecules that function as regulators of gene expression. The human genome encodes several hundred miRNAs and abnormal expression of these has been associated with cancer progression. We have previously reported on miRNA involvement in cholesterol regulation, amyotrophic lateral sclerosis and liver cancer. Now a collaboration between Rosetta Genomics, NYU Langone Medical Center and Vanderbilt School of Medicine has identified the potential utility of miR-33 for development of therapies targeting malignant mesothelioma (MM).

MM is a rare cancer that has been associated with exposure to asbestos dust (although a small proportion of patients have never been exposed). Taking anywhere between 20 and 50 years for symptoms to develop, the cancer affects the mesothelium, a thin layer of tissue surrounding the internal organs. The most common form of MM is pleural, involving the lining of the lungs, but it may also affect other tissues such as the peritoneum. Current treatment options, including surgery and chemotherapy, are limited and often confounded by late diagnosis because of the absence of symptoms.

This latest study found that MM cell lines derived from patients with aggressive disease failed to express miR-31, a microRNA that has also been linked to suppression of breast cancer metastasis. Functional studies, where miR-31 was reintroduced to the cells, showed that the microRNA could inhibit proliferation, migration, invasion, and clonogenicity of mesothelioma cells. miR-31 suppressed expression of a number of factors associated with maintenance of DNA replication and cell cycle progression, including the pro-survival phosphatase PPP6C. The mRNA for PPP6C, which contains three miR-31-binding sites in its 3′-UTR, was down-regulated when miR-31 was present and up-regulated in clinical MM specimens compared to matched normal tissues.

Whilst the study, published in the Journal of Biological Chemistry, reveals a key role for miR-31 in MM, considerable challenges remain to exploit this finding for therapy of the disease.

Role for miRNA in Amyotrophic Lateral Sclerosis

ALS sufferer, Professor Stephen Hawking, in zero gravity Photo credit: NASA — ALS sufferer, Professor Stephen Hawking, in zero gravity

Photo credit: NASA

Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig’s disease, is one of the most common neuromuscular diseases worldwide and attacks the neurons responsible for controlling voluntary muscles. Few treatment options are currently available for ALS patients but researchers at UT Southwestern Medical Center and Harvard University have shown that microRNA-206 (miR-206) plays a crucial role in the progression of ALS. miRNAs are small non-coding RNA molecules which down-regulate gene expression and dysfunction of miRNAs has been associated with a number of diseases. In ALS, as the affected neurons stop signalling to muscle cells, the muscles atrophy. Although the skeletal muscles attempt to reinnervate themselves by signalling to healthy neurons via miRNA-206, eventually the surviving neurons are unable to cope, the muscle cells die and the ability of the brain to control voluntary movement is lost.

Mice that are genetically deficient in miRNA-206 form normal neuromuscular synapses during development but, in the ALS mouse model, disease progression is faster in mice that are deficient in miRNA-206. miRNA-206 is required for efficient regeneration of neuromuscular synapses after acute nerve injury and is dramatically induced in the mouse model of ALS. The effects of miRNA-206 in slowing ALS were suggested to be mediated, at least in part, through histone deacetylase 4 and fibroblast growth factor signalling pathways. miRagen Therapeutics hope to exploit the newly discovered role for miRNA-206 in neuromuscular maintenance to develop treatments for patients suffering from ALS and other neuromuscular diseases. Because miR-206 is only produced by skeletal muscles, such treatments may have a limited risk of side effects.

The study is published in the journal Science.

Suppressing ROCK May Prevent Breast Cancer Metastases

Nearly half of all patients treated for apparently localised breast cancer develop metastatic disease. Although there are treatment options that prolong survival and improve quality of life for patients with metastatic breast cancer, these treatments rarely lead to long-term survival without disease recurrence. The most common sites of metastases are the bones, the liver and the lungs: breast cancer is the most common origin of metastatic deposits in the skeleton.

Researchers from Tufts University have now identified Rho-associated kinase (ROCK) as a potential target to reduce breast cancer metastasis. Signalling through ROCK plays a key role in regulating cell adhesion and motility and aberrant expression of ROCK has been linked to metastasis. In the present study, the team showed that ROCK expression is increased in metastatic human mammary tumours and breast cancer cell lines compared with non-metastatic tumours and cell lines. A metastatic phenotype could also be induced in a cell line that is not normally metastatic by over-expression of ROCK.

In a novel mouse model of “human breast cancer metastasis to human bone”, inhibiting ROCK in the earliest stages of breast cancer decreased metastatic tumour mass in bone by 77% and overall frequency of metastasis by 36%. ROCK function could be effectively blocked by either ROCK-targeting short hairpin RNA (shRNA) or by the specific ROCK inhibitor, Y27632. Expression of the microRNA cluster, c-Myc-regulated miR-17-92 was found to be elevated in metastatic breast cancer cells compared with non-metastatic cells and reduced by treatment with Y27632. Blockade of miR-17 was further shown to decrease breast cancer cell invasion/migration in vitro and metastasis in vivo. The authors suggest that the effects of ROCK may be mediated by modulating the c-Myc pathway, including c-Myc-dependent microRNAs. They propose that inhibition of ROCK, or the pathway it stimulates, may represent a novel approach for treatment of breast cancer metastases.

The study is published in the journal Cancer Research.

Replacement miRNA Treats Liver Cancer in Mice

MicroRNAs (miRNAs) are small (21-23 nucleotides in length) single stranded RNA molecules which are not translated into proteins but whose main function is to regulate gene expression. Almost all types of tumour have abnormal – usually reduced – miRNA expression, and scientists at Johns Hopkins Medical School, Nationwide Children’s Hospital, and Ohio State University have now shown that replacing missing miRNAs in mice with liver cancer can rapidly kill the tumour cells whilst leaving healthy cells untouched. The team found that hepatocellular carcinoma (HCC) cells had reduced levels of miR-26a, a miRNA that is normally expressed at high levels in a variety of tissues. In vitro, expression of miR-26a in liver cancer cells was shown to cause cell-cycle arrest associated with direct targeting of cyclins D2 and E2. Systemic administration of miR-26a to mice with HCC using an adeno-associated virus vector inhibited the proliferation of the cancer cells and protected the animals from disease progression. Normal liver cells were unaffected by the treatment. After three weeks, 8 out of 10 mice treated with the miRNA showed only small tumours or a complete absence of tumours, whereas 6 out of 8 sham-treated mice experienced aggressive disease progression.

The study shows that liver cancer can be successfully treated in an animal model of disease by replacing ‘missing’ miRNA using a delivery vector that is suitable for use in the clinic. Although more work is needed before the technique could be used in patients, it could provide an effective and safe treatment for human HCCs, which account for 80 – 90% of all liver cancers and generally have a poor prognosis. If suitable delivery systems can be developed, the technique could also be used to treat other diseases caused by ‘missing’ miRNA.

The study is published in the June 12^th edition of Cell.