A couple of important tumor suppressor genes have figured heavily in my past writings and in anti-aging science discussions, P21 and P53. Another tumor suppressor gene may now be coming onto center stage, NRG1.
Paying attention to the general press 10 days ago, one would get the impression that the NRG1 gene was just discovered and represents a breakthrough in cancer genetics with headlines such as Breast cancer gene discovery ‘most important for 20 years, Gene discovery is the biggest cancer success in 20 years, and Scientists find gene that stops some cancers in their tracks. “Scientists have found a faulty gene linked to half of all breast cancers which experts have hailed as the most important discovery in the disease since the 1970s(ref).”
Actually, the NRG1 gene has been known for some time and its role in cancers has been investigated since 1998. A Google search on the gene, Neuroregulin 1 (NRG1), returns 112,000 entries. It is hardly newly-discovered. The evidence for the tumor-suppressing properties of NRG1 is not new either. It has been steadily accumulating and NRG1’s exact role in many cancers is still not well understood.
The NRG1 gene encodes the protein Neuregulin 1 which is also known as NRG1. “It is known that an extraordinary variety of different isoforms are produced from the NRG1 gene by alternative splicing. These isoforms include heregulins (HRGs), glial growth factors (GGFs) and sensory and motor neuron-derived factor (SMDF)(ref).” “The neuregulins are receptor tyrosine kinase ligands that play a critical role in the development of the heart, nervous system, and breast(ref).” “NRG1 encodes growth factors that bind to tyrosine kinases ErbB3 and ErbB4, and can both stimulate cell proliferation and apoptosis. NRG1 is also quite frequently broken by chromosome translocations(ref).” Among its other activities, it has been thought that “Neuregulin 1 (NRG1) is a leading schizophrenia susceptibility gene(ref),” and it may be involved in bipolar disorders(ref).
The October 2009 publication that set off the recent flurry of press reports is The NRG1 gene is frequently silenced by methylation in breast cancers and is a strong candidate for the 8p tumour suppressor gene. “We found that most breast cancer cell lines had reduced or undetectable expression of NRG1. This included cell lines that had translocation breaks in the gene. Similarly, expression in cancers was generally comparable to or less than that in various normal breast samples. Many non-expressing cell lines had extensive methylation of the CpG island at the principal transcription start site at exon 2 of NRG1. Expression was reactivated by demethylation. Many tumours also showed methylation, whereas normal mammary epithelial fragments had none. Lower NRG1 expression correlated with higher methylation. — The short arm of chromosome 8 is frequently lost in epithelial cancers, and NRG1 is the most centromeric gene that is always affected. NRG1 may therefore be the major tumour suppressor gene postulated to be on 8p: it is in the correct location, is antiproliferative and is silenced in many breast cancers.”
I touched on the role of DNA methylation in my blog post DNA methylation, personalized medicine and longevity and in Epigenetics, epigenomics and aging where I pointed out “Already, certain DNA methylation changes are known to be associated with aging and others associated with certain diseases like lupus and scleroderma.” I subsequently added a candidate theory of aging to my Anti-Aging Firewalls treatise EPIGENOMIC CHANGES IN DNA METHYLATION AND HISTONE ACTYLATION. What is important in the current discussion is that in many cancer lines the expression of the NRG1 gene is shut down by methylation, meaning that no NRG1 tumor-suppressor protein is produced.
The recent publication includes several laboratory displays graphically relating NRG1 expression to methylation in breast cancer and normal cells and concludes “– we suggest that NRG1 may be the principal tumour suppressor gene that leads to the loss of 8p in many breast and other epithelial cancers. NRG1 expression seems to be silenced in most breast cancers compared with the main types of mammary epithelial cell—this could be because tumours arise from a specialized population in which NRG1 is normally silenced, but we prefer the interpretation that NRG1 is silenced by aberrant methylation or other—as yet unknown—events such as promoter mutation. Expression of NRG1 in mammary cells is antiproliferative to the cells that express it, and array-CGH identifies NRG1 as the gene most likely to be a principal 8p tumour suppressor.”
Actually the central results of this study were communicated in a poster presentation with the same title in May 2008.” The story was not picked up by the press until more than a year later.
It is interesting to look at the history of discovery of the tumor suppressing properties of NRG1. I mention only a few of the many publications which are building blocks for the most-recent study. The first relevant publication article I have seen cited was a 1998 one Neu differentiation factor (NDF), a dominant oncogene, causes apoptosis in vitro and in vivo. “– we find that tumors induced by NDF (NRG1) display extensive apoptosis in vivo. NDF is therefore an oncogene whose deregulation can induce transformation as well as apoptosis.”
The 2000 study Chromosome translocations in breast cancer with breakpoints at 8p12 studied breakpoints and translocations on chromosome 8p in several cancer lines, relevant to the later discovery that these breaks and translocations were in NRG1. This is one of several prior and subsequent studies looking at chromosomal rearrangements on 8p in cancers(ref). One 2000 study states “We conclude that chromosome 8p carries a tumour suppressor gene or genes, the loss of which results in growth advantage of breast tumour cells, especially in carriers of the BRCA2 999del5 mutation.” The recent research strongly suggests that that gene is NRG1.
The discovery of the involvement of two key genes in breast cancer, BRCA1 and BRCA2 was very exciting but led to a hunt for additional genes that might be involved, such as pointed out in the 2002 review article Genes other than BRCA1 and BRCA2 involved in breast cancer susceptibility.
The 2004 publication A Recurrent Chromosome Breakpoint in Breast Cancer at the NRG1/Neuregulin 1/Heregulin Gene states “We previously reported that five breast cancer cell lines have chromosome translocations that break in the NRG1 gene and that could cause abnormal NRG1 expression. — Breaks in NRG1 were detected in 6% (19 of 323) of breast cancers and in some lung and ovarian cancers. In an unselected series of 213 cases with follow-up, breast cancers where the break was detected tended to be high-grade (65% grade III compared with 28% of negative cases). They were, like breast tumors in general, mainly ErbB2 low (11 of 13 were low) and estrogen receptor positive (11 of 13 positive).
The 2005 paper NRG1 gene rearrangements in clinical breast cancer: identification of an adjacent novel amplicon associated with poor prognosis did some applicable legwork looking at rearrangements of the NRG1 gene and how they are specifically implicated in breast carcinoma oncogenesis.
The 2005 study Activation of ErbB2 by Overexpression or by Transmembrane Neuregulin Results in Differential Signaling and Sensitivity to Herceptin looks at the use of herceptin as a cancer treatment targeting NRG1. “Treatment with the anti-ErbB2 receptor antibody Herceptin had an inhibitory effect on proliferation only in cells expressing neuregulin but not on cells overexpressing ErbB2, and its inhibitory activity was accompanied by a decrease in p21. These results suggest that Herceptin may also be of help in the treatment of tumors in which neuregulin feeds the tumoral tissue.”
A 2006 poster publication The NEUREGULIN1 gene and breast cancer is interesting in that it telegraphs the essence of the heralded 2009 results. “Our current work shows that NRG1 expression is silenced in many breast cancer cell lines (17 out of 23 lines), as compared with normal breast cell lines. Western blotting experiments also indicate that NRG1 is downregulated at the protein level. To investigate whether NRG1 maybe repressed by epigenetic mechanisms, we examined DNA methylation at a CpG island present in the promoter and the first exon of the gene using bisulphite sequencing. This region is heavily methylated in 76.5% (13/17) of breast cancer cell lines that have no NRG1 expression. In contrast, the region is relatively unmethylated in normal breast lines, and in cancer cell lines expressing NRG1. Treatment of cancer cell lines with 5-aza-2-deoxycytidine, which abolished DNA methylation, activated the expression of NRG1 by 7–100 times. — These results suggest that DNA methylation is a key mechanism that silences NRG1 expression in breast cancer cells, and our current view is that NRG1 could be the long-sought tumour suppressor on 8p, with the translocations either inactivating the gene or producing aberrant transcripts.”
Paul Edwards, a molecular biologist at Cambridge University, and his team were involved in this 2006 as well as the 2008 and 2009 reporting. And there are a number of other studies over the years related to NRG1 and cancers. A point I am making is that “breakthrough genetic discoveries” reported in the popular press are often the results of years of hard study by many people. I do believe the latest results by Edward’s team represents solid and important progress. However, we still only partially understand the role of NRG1 in other cancers and the likely role of NRG1 as a key tumor suppressor gene must be confirmed. Despite sensational reporting, progress is achieved through many small incremental steps and there is far yet to go.