RB1 Germline Variant Predisposing to a Rare Ovarian Germ Cell Tumor: A Case Report

Abstract

Malignant ovarian germ cell tumors (MOGCTs) are neoplasms of the ovary, of which, due to their rarity and heterogeneity, few is reported about genetic background and development. Here, we report a 18-years old patient diagnosed with an ovarian mixed germ cell tumor, without any previous history of malignancies, who has been treated with surgery and chemotherapy and died 4 years later due to peritoneal metastasis complications. Patient's blood DNA was screened for a panel of 52 cancer-related genes in order to identify predisposing aberrations to this rare cancer. The analysis discovered the uncharacterized c.2393G>A variant in RB1, the retinoblastoma gene, leading both to a missense change and a splicing perturbation of the RB1 transcript. The variant was found to be hypomorphic, damaging the C-terminal domain with a partially impaired protein function. The variant is inherited from the unaffected mother. Due to an imprinting mechanism, the maternal allele is ~3-fold more expressed than the paternal one. The parent-of-origin effect combined with the hypomorphic impact of the variant determines a rescue of sufficient tumor-suppressor activity to prevent retinoblastoma development but can predispose to other cancers in the adult age. In order to understand the somatic events acting on the germline predisposition we used the NGS-liquid biopsy covering 77 cancer driver genes. Using this approach, we detected deleterious mutations in TP53, SMAD4, FGFR3, and MSH2, indicative of a dis-regulation of cell cycle and DNA repair mechanisms pathways. In conclusion, we have pinpointed for the first time that an RB1 leaky variant, not leading to retinoblastoma because of its maternal origin, can predispose in adults to a very rare form of ovarian cancer and that the somatic disruption of few genes contributes to the tumor progression and aggressiveness.

Keywords: RB1; hypomorphic variant; ovarian cancer; tumor predisposition; yolk sac tumor.

Figure 1

(A) 4x HPF, H&E stain. Ovarian cystic teratoma consisting of epidermoid elements and mature neuroectodermal tissue with focal morphological characteristics of a yolk sac tumor. (B) 20x HPF, H&E stain. At higher magnification, the limited and focal tumoral area shows typical characteristics of the hepatoid variant of yolk sac tumor. It can usually be found in young females, associated with increased levels of serum α-fetoprotein (AFP). To note, the presence of great polygonal cells with a well represented eosinophilic cytoplasm and abundant presence of hyaline bodies. (C,D) 10 x HPF, H&E stain. Teratoma shows abundant mature neuroectodermal tissue interspersed in the stroma with different and well-differentiated structures such as hair follicles. The presence of these elements indicates a high maturation level of the neoplasia.

Figure 2

(A) Electrophoresis gel of the RT-PCR products of case and control sample with forward primer spanning exon 21-22 junction and reverse spanning exon 24–25 junction. Next to the gel the schemes corresponden to the amplified products. (B) Scheme explaining the mechanism of splicing of intron 22 and exon 23 in presence of the variant c.2393G>A; the figure shows the loss of the use of the canonical sites of exon 22 and 23, the generation of a de novo 3'SS and the activation of a cryptic 5' SS in intron 22 used with the canonical 3'SS of exon 24. This results in an aberrant exons junction with the incluion of part of intron 22 and the exclusion of exon 23 in the mature transcript. (C) Three-dimensional model of the FL and the aberrant spliced RB proteins predicted with EzMol, C-term domain is highlighted in red. (D) Electropherogram represents the sequence of the FL agarose band displayed in (A), c.2393 position is indicated in the rectangle (E) The graph shows the result of the densitometric analysis of the CASE gel bands displayed in (A); sum of the FL and aberrant bands was considered as the total and each value from both transcripts was compared to that. M, DNA molecular marker, CTR, healthy control sample, NTC, no-template control, FL, full length transcript.