p53 Arg72Pro and MDM2 309 SNPs in hereditary retinoblastoma

The roles of mouse double minute 2 (MDM2) oncoprotein in ocular diseases: A review

Mouse double minute 2 (MDM2), an E3 ubiquitin ligase and the primary negative regulator of the tumor suppressor p53, cooperates with its structural homolog MDM4/MDMX to control intracellular p53 level. In turn, overexpression of p53 upregulates and forms an autoregulatory feedback loop with MDM2. The MDM2-p53 axis plays a pivotal role in modulating cell cycle control and apoptosis. MDM2 itself is regulated by the PI3K-AKT and RB-E2F-ARF pathways. While amplification of the MDM2 gene or overexpression of MDM2 (due to MDM2 SNP T309G, for instance) is associated with various malignancies, numerous studies have shown that MDM2/p53 alterations may also play a part in the pathogenetic process of certain ocular disorders (Fig. 1). These include cancers (retinoblastoma, uveal melanoma), fibrocellular proliferative diseases (proliferative vitreoretinopathy, pterygium), neovascular diseases, degenerative diseases (cataract, primary open-angle glaucoma, age-related macular degeneration) and infectious/inflammatory diseases (trachoma, uveitis). In addition, MDM2 is implicated in retinogenesis and regeneration after optic nerve injury. Anti-MDM2 therapy has shown potential as a novel approach to treating these diseases. Despite major safety concerns, there are high expectations for the clinical value of reformative MDM2 inhibitors. This review summarizes important findings about the role of MDM2 in ocular pathologies and provides an overview of recent advances in treating these diseases with anti-MDM2 therapies.

The mouse double minute 2 309T>G polymorphism and retinoblastoma risk: A meta-analysis

PURPOSE:

Mouse double minute 2 (MDM2) homolog is a protein that in humans is encoded by the MDM2 gene. It is expressed in retinoblastoma (Rb) cells and acts as a key negative regulator of the p53 tumor suppressor gene. Several studies have investigated the association of Rb with MDM2 309T>G polymorphism, but the results were conflicting. To derive a more precise estimation of the association, we performed a meta-analysis of the relationship between MDM2 309T>G polymorphism with Rb in all published studies.

METHODS:

Published literature from PubMed and other databases were retrieved. All the reported studies evaluating the association between MDM2 309T>G polymorphism and Rb risk were included. The pooled odds ratio (OR) and 95% confidence interval (CI) were calculated using the fixed-effect model. A total of four case–control studies, including 520 cases and 745 controls were included.

RESULTS:

This meta-analysis found that MDM2 309T>G polymorphism was significantly associated with Rb risk in the dominant model, TG+GG versus TT (OR = 1.43, 95% CI = 1.11–1.84, P = 0.006).

CONCLUSION:

The present meta-analysis suggested that MDM2 309T>G polymorphism has a significant association with increased Rb risk.

Genetic Predisposition to Solid Pediatric Cancers

Progresses over the past years have extensively improved our capacity to use genome-scale analyses—including high-density genotyping and exome and genome sequencing—to identify the genetic basis of pediatric tumors. In particular, exome sequencing has contributed to the evidence that about 10% of children and adolescents with tumors have germline genetic variants associated with cancer predisposition. In this review, we provide an overview of genetic variations predisposing to solid pediatric tumors (medulloblastoma, ependymoma, astrocytoma, neuroblastoma, retinoblastoma, Wilms tumor, osteosarcoma, rhabdomyosarcoma, and Ewing sarcoma) and outline the biological processes affected by the involved mutated genes. A careful description of the genetic basis underlying a large number of syndromes associated with an increased risk of pediatric cancer is also reported. We place particular emphasis on the emerging view that interactions between germline and somatic alterations are a key determinant of cancer development. We propose future research directions, which focus on the biological function of pediatric risk alleles and on the potential links between the germline genome and somatic changes. Finally, the importance of developing new molecular diagnostic tests including all the identified risk germline mutations and of considering the genetic predisposition in screening tests and novel therapies is emphasized.

Biomarkers in retinoblastoma

Retinoblastoma (RB) is the most common intraocular malignancy of childhood caused by inactivation of the Rb genes. The prognosis of RB is better with an earlier diagnosis. Many diagnostic approaches and appropriate clinical treatments have been developed to improve clinical outcomes. However, limitations exist when utilizing current methods. Recently, many studies have identified identify new RB biomarkers which can be used in diagnosis, as prognostic indicators and may contribute to understanding the pathogenesis of RB and help determine specific treatment strategies. This review focuses on recent advances in the discovery of RB biomarkers and discusses their clinical utility and challenges from areas such as epigenetics, proteomics and radiogenomics.

p53, stem cell biology and childhood blastomas

PURPOSE OF REVIEW

Childhood blastomas, unlike adult cancers, originate from developing organs in which molecular and cellular features exhibit differentiation arrest and embryonic characteristics. Conventional cancer therapies, which rely on the generalized cytotoxic effect on rapidly dividing cells, may damage delicate organs in young children, leading to multiple late effects. Deep understanding of the biology of embryonal cancers is crucial in reshaping the cancer treatment paradigm for children.

RECENT FINDINGS

p53 plays a major physiological role in embryonic development, by controlling cell proliferation, differentiation and responses to cellular stress. Tumor suppressor function of p53 is commonly lost in adult cancers through genetic alterations. However, both somatic and germline p53 mutations are rare in childhood blastomas, suggesting that in these cancers, p53 may be inactivated through other mechanisms than mutation. In this review, we summarize current knowledge about p53 pathway inactivation in childhood blastomas (specifically neuroblastoma, retinoblastoma and Wilms' tumor) through various upstream mechanisms. Laboratory evidence and clinical trials of targeted therapies specific to exploiting p53 upstream regulators are discussed.

SUMMARY

Despite the low rate of inherent TP53 mutations, p53 pathway inactivation is a common denominator in childhood blastomas. Exploiting p53 and its regulators is likely to translate into more effective targeted therapies with minimal late effects for children. (see Video Abstract, Supplemental Digital Content 1, http://links.lww.com/COON/A23).

Association between MDM2 rs2279744, MDM2 rs937283, and p21 rs1801270 polymorphisms and retinoblastoma susceptibility

Retinoblastoma (Rb) is the most common intra-ocular malignancy in children. The association of rs2279744, and rs937283 in MDM2 gene, and p21 rs1801270 polymorphism and RB development have been demonstrated. To provide a comprehensive assessment of and to clarify associations between the 3 SNPs (MDM2 rs2279744, MDM2 rs937283, and p21 rs1801270) and the risk of RB, we performed a meta-analysis of all the eligible case-control studies. We searched English databases include PubMed, Embase, Google Scholar, and Cochrane Library, using an upper date limit of January 1, 2018. The association between MDM2 rs2279744, MDM2 rs937283, and p21 rs1801270 polymorphisms and the risk of RB were estimated by calculating a pooled OR and 95% CI under a homozygote comparison, heterozygote comparison, dominant model, and recessive model. The statistical power analysis was performed using G*Power. Our meta-analysis showed a significant association between RB susceptibility and MDM2 rs2279744 recessive model (OR = 1.427, 95%CI: 1.107-1.840, P = .006, I = 0%). Moreover, a significant link was observed between RB risk and MDM2 rs937283 homozygote comparison (OR = 0.471, 95%CI: 0.259-0.858, P = .014, I = 0%) and recessive model (OR = 0.587, 95%CI: 0.410-0.840, P = .004, I = 0%). However, no significant relationship between the p21 rs1801270 polymorphism and RB susceptibility was detected in any of the 4 models (P > .05). In conclusion, we found that significant association between the MDM2 rs2279744 polymorphism and increased RB risk, while MDM2 rs937283 polymorphism was associated with significantly decreased RB risk. However, as to the P21 rs1801270 polymorphism, a statistically significant association was not identified for RB.

A Functional Polymorphism (rs937283) in the MDM2 Promoter Region is Associated with Poor Prognosis of Retinoblastoma in Chinese Han Population

The effect of single nucleotide polymorphisms (SNPs) at MDM2 has been investigated in several cancer types. Three MDM2 SNPs(rs937283, rs2270744 and rs769412) have previously been suggested to be positively correlated with cancer. In this study, we aimed to explore the association of rs937283, rs2270744 and rs769412 polymorphisms with retinoblastoma (RB) risk, clinicopathological characteristics, and prognosis. Compared with wild-type genotype AA at rs937283, individuals carrying AG and GG genotype had a significantly increased risk for developing RB (OR = 1.86, 95% CI 1.13–3.08; OR = 2.48, 95% CI 1.10–5.62, respectively). RB patients with allele G at rs937283 were more susceptible to invasion and high tumor aggression (OR = 2.42, 95% CI 1.43–4.11; OR = 2.15, 95% CI 1.27–3.64, respectively). Kaplan-Meier curves and log-rank results revealed that RB patients harboring genotype GG and G allele at rs937283 had worse survival (P < 0.02 and P < 0.01, respectively). In addition, the A to G substitution at rs937283 significantly enhanced the transcription activity of the MDM2 gene in vitro. In vivo, we found that MDM2 mRNA and protein were overexpressed in individuals who carried the G allele at rs937283. This study suggested that the MDM2 rs937283 polymorphism is a novel functional SNP both in vitro and in vivo as well as a biomarker for poor prognosis in RB.

Association of p53 rs1042522, MDM2 rs2279744, and p21 rs1801270 polymorphisms with retinoblastoma risk and invasion in a Chinese population

Single nucleotide polymorphisms (SNPs) of p53 rs1042522, MDM2 rs2279744 and p21 rs1801270, all in the p53 pathway, which plays a crucial role in DNA damage and genomic instability, were reported to be associated with cancer risk and pathologic characteristics. This case-control study was designed to analyse the association between these SNPs and retinoblastoma (RB) in a Chinese Han population. These SNPs in 168 RB patients and 185 adult controls were genotyped using genomic DNA from venous blood. No significant difference was observed in allele or genotypic frequencies of these SNPs between Chinese RB patients and controls (all P > 0.05). However, the rs1042522 GC genotype showed a protective effect against RB invasion, as demonstrated by event-free survival (HR = 0.53, P = 0.007 for GC versus GG/CC). This effect was significant for patients with a lag time >1 month and no pre-enucleation treatment (P = 0.007 and P = 0.010, respectively), indicating an interaction between p53 rs1042522 and clinical characteristics, including lag time and pre-enucleation treatment status. Thus, the rs1042522 SNP may be associated with RB invasion in the Han Chinese population; however, further large and functional studies are needed to assess the validity of this association.

RB1 mutation spectrum in a comprehensive nationwide cohort of retinoblastoma patients

BACKGROUND

Retinoblastoma (Rb) is a childhood cancer of the retina, commonly initiated by biallelic inactivation of the RB1 gene. Knowledge of the presence of a heritable RB1 mutation can help in risk management and reproductive decision making. We report here on RB1 mutation scanning in a unique nationwide cohort of Rb patients from the Netherlands.

METHODS

From the 1173 Rb patients registered in the Dutch National Retinoblastoma Register until January 2013, 529 patients from 433 unrelated families could be included. RB1 mutation scanning was performed with different detection methods, depending on the time period.

RESULTS

Our mutation detection methods revealed RB1 mutations in 92% of bilateral and/or familial Rb patients and in 10% of non-familial unilateral cases. Overall an RB1 germline mutation was detected in 187 (43%) of 433 Rb families, including 33 novel mutations. The distribution of the type of mutation was 37% nonsense, 20% frameshift, 21% splice, 9% large indel, 5% missense, 7% chromosomal deletions and 1% promoter. Ten per cent of patients were mosaic for the RB1 mutation. Six three-generation families with incomplete penetrance RB1 mutations were found. We found evidence that two variants, previously described as pathogenic RB1 mutations, are likely to be neutral variants.

CONCLUSIONS

The frequency of the type of mutations in the RB1 gene in our unbiased national cohort is the same as the mutation spectrum described worldwide. Furthermore, our RB1 mutation detection regimen achieves a high scanning sensitivity.

The genomic landscape of retinoblastoma: a review

Retinoblastoma is a paediatric ocular tumour that continues to reveal much about the genetic basis of cancer development. Study of genomic aberrations in retinoblastoma tumours has exposed important mechanisms of cancer development and identified oncogenes and tumour suppressors that offer potential points of therapeutic intervention. The recent development of next-generation genomic technologies has allowed further refinement of the genomic landscape of retinoblastoma at high resolution. In a relatively short period of time, a wealth of genetic and epigenetic data has emerged on a small number of tumour samples. These data highlight the inherent molecular complexity of this cancer despite the fact that most retinoblastomas are initiated by the inactivation of a single tumour suppressor gene. This review outlines the current understanding of the genomic, genetic and epigenetic changes in retinoblastoma, highlighting recent genome-wide analyses that have identified exciting candidate genes worthy of further validation as potential prognostic and therapeutic targets.