Let’s get MDM2 and MDMX out of the shadow of p53

When it comes to cancer, one molecule stands out as being among the most extensively studied: the p53 tumor suppressor protein. p53 can exist in cells in several different forms. When p53 is in its so-called wild-type form, it is capable of activating various responses that contribute to tumor suppression. In their recent review, Columbia postdoc Rafaela Muniz de Quieroz and colleagues summarize the vast scientific literature on two key regulators of p53: MDM2 and MDMX. Both MDM2 and MDMX are known to interact with p53 and disrupt its function. Their absence has been linked not only to increased cancer development, but also to a number of dysfunctions, including embryonic lethality in mice. MDM2 has been shown to negatively regulate p53 by diverse mechanisms spanning from expression of the p53 gene to degradation of the p53 protein or its expulsion from the cellular nucleus, where the protein accomplishes its function. Although very similar to MDM2, MDMX is less well studied. We do know, however, that MDMX is a protein that can work together with the MDM2  in p53 degradation.

While many reviews and studies have pointed to the roles of MDM2, and to a lesser extent of MDMX, in p53 regulation, the current review by Quieroz and her colleagues  puts a larger focus on the myriad of p53-independent activities of MDM2 and MDMX. The authors provide important details about the p53-independent functions of both MDMX alone and as part of a MDM2–MDMX complex. The review discusses some key features in the structure and function of the proteins, including  key parts  that are relevant for their function, for some associated abnormalities, or for the formation of MDM2 and MDMX complexes.

MDM2 and MDMX are regulated on multiple levels within cells. These include regulation on the DNA level, including usage of several alternative promoters (DNA sequences needed to turn a gene on or off). One of the promoters of MDM2 and MDMX is regulated by their target p53, but there are also p53-independent promoters capable of switching on the genes of MDM2 and MDMS regardless of p53. In addition, numerous variations in the DNA sequences, the so-called single nucleotide polymorphisms (SNPs), affect the expression of the two genes and are relevant to different pathologies. Regulation on the RNA level includes co-transcriptional regulation like alternative splicing, as well as post-transcriptional regulation by microRNAs, long non-coding RNAs, circular RNAs, or RNA binding proteins. The review also presents a detailed characterization of the regulation of MDM2 and MDMX at the protein level, by summarizing data on numerous post-translational modifications or interacting partners of the two proteins, with regards to the different p53 contexts of the cited studies. Amongst the presented binding partners are some of the more recently identified interactors of the MDMs, which include proteins involved in the defense against several viruses. Overall, both MDM2 and MDMX stand out as extensively regulated at virtually every known level which according to the authors “attests to their relevance not only as inhibitors of p53 but of myriad other cellular activities and outcomes on their own”.

Since MDM2 and MDMX have majorly been studied in their relation to inhibit wild-type p53, of a particular interest stands a section of the review summarizing numerous processes in which the two proteins have been shown to be involved in cells lacking wild-type p53 (Figure 1).

Figure 1: Nonmalignant disease (left) and cancer-related (right) p53-independent functions of MDM2 and MDMX (adapted from Figure 4 of the review).

As shown in Figure 1, the p53-independent roles of MDM2 and MDMX in cancer and in other pathologies are versatile. That hints to the importance of uncovering molecules that can modulate the deleterious effects associated with dysfunctions of the two MDMs. A summary of numerous molecules that were shown to regulate the two proteins and thus consist of potential therapeutic targets, are also discussed in the review. Again the authors put an emphasis on how such small molecules might be useful in cells that lack wild-type p53. This is important not only because the two proteins have multiple functions other than regulating wild-type p53 which can be studied in such cells, but also because an important percentage of tumors is characterized by absence of wild-type p53.

The last section of the review points out some outstanding questions and directions for future research. If the fascinating questions of the versatile p53-independent roles MDM2 and MDMX have sparked your interest, find out more from the original paper.

Ancestry connects non-cancer causing mutations in cancer patients

The cause of cancer as a disease has been partly attributed to genetics across a diverse range of populations. However, it is unclear whether cancer patients carry additional genetic mutations, also known as variants, in non-cancer causing genes and if these variants are evolutionarily related. Because ancestry-specific variants were more recently generated in evolutionary time, they could have been easily missed in analyses where all patients were cumulatively analyzed without consideration for ancestry. A recent concept proposed by geneticists suggests that people are more likely to develop or be protected from diseases based on recently acquired mutations and are less so due to more distant mutations. This is an interesting theory that scientists can now test using genome information from more than 10,000 cancer patients whose ancestries are known. So far, how mutations affect gene expression – whether they completely abolish the expression of gene products (e.g. protein) or result in the creation of a misshapen protein, have only been reported for variants present in patients with European ancestry. The remaining ancestries are yet to be explored.


Advances in sequencing technology have made it easier for researchers to access genome sequencing information under clinical settings and for healthcare providers to share personalized diagnoses as part of ‘genomic medicine’ to patients. Using publicly available genome sequencing data for cancer patients, Dr. Xiao Fan and colleagues analyzed the variants in non-cancer causing genes and in “medically actionable” genes in 10,389 cancer patients. The authors found 1.46 billion mutations, which were then filtered through rigorous quality testing of sequencing information followed by expert geneticist review, resulting in a final total reliable set of 2,920 non-cancer related pathogenic and likely pathogenic variants. About 750 of these variants were harbored on average within a quarter of the cancer cases, no matter the heritage. A surprising majority (~27%) of the total variants were displayed in patients with European ancestry, followed sequentially by Latinx/Native American (15%), African American (13%) and East Asian (12%) patients.


Because genetic mutations can affect expression of proteins, the authors then dug deep into the variant data to examine whether these variants behaved in an expected manner on a molecular level. When genes contain mutations that cause the protein it encodes to be a shorter version of itself, the mutation is referred to cause a protein “truncation”. Sometimes, a truncating mutation in a gene can trigger a decrease in expression at the messenger RNA (mRNA) level even before the mRNA is used to make the protein. To find out if the variants that produced truncated gene products underwent changes at the mRNA level, the authors measured the gene expression levels of such variants. Of the variants that showed a meaningful difference in gene expression compared to non-cancer patients, a large majority of variants showed a decrease in expression. This result indicated to the authors that truncation-causing variants often work at the mRNA level even before the cells spend energy to make the disease-associated proteins. The authors then examined the behavior of gene variants that do not cause truncations but rather cause just a single swap in the gene sequence, known as “missense” variants. Missense mutations typically only cause a change in one or two building blocks of the protein but do not affect the abundance of the protein itself. Surprisingly, the authors found that the missense variants in their data are unusually regulated in the cancer patients at the mRNA level resulting in a decrease in gene, and therefore, protein expression. This is an uncommon observation, making the authors speculate that missense variants are perhaps controlled by gene-expression independent mechanisms within the cancer patients’ cells.

This study provides a testament to the power of genomic medicine that can be used to complement conventional medical treatment. With a strong sample of ~10,000 cancer patients, this report stands as one of the most comprehensive studies that considers race and ancestry in its analysis. While genomic profiling is becoming more common in medical diagnoses, this study further provides a reason for understanding diseases and invention of medicine based on race, ethnicity and genetic heritage.

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