Discussion:
In the management of retinoblastoma, identifying high-risk features is important, as a failure of treatment can lead to a 24% risk of metastasis. [5-9]. Conversely, relapse in low-risk patients is rare with five-year event free survival (EFS) of >95% [10].
At our institution over the past 25 years, 4 additional patients with high-risk features, developed extraocular metastatic retinoblastoma within one year of diagnosis (Table 1B). An observational study involving 519 patients, noted that 402 patients with low-risk features did not develop metastatic disease [9], which makes this case unique as he had low risk disease at presentation.
The marked LDH level in our patient (Figure 1A) reflected the increased proliferative index of his tumor. LDH is an oxoreductase that is important in generating ATP under states of hypoxia. The rapid proliferation of the tumor likely contributed to its chemotherapy sensitivity and the development of tumor lysis syndrome and acute renal failure during the first cycle of retrieval chemotherapy and achieved a near complete remission (minimal residual FDG uptake in an axillary lymph node) (Figure 1B, D&E).
To characterize the genomic landscape and tumor progression in this patient, targeted sequencing was performed on the eye (September, 2009), bone marrow (February, 2018), and brain (October, 2018) (Table 1A). The patient’s normal DNA from the eye tissue revealed a germline pathogenic variant in the RB1 tumor suppressor gene, and in all three tumors biallelic inactivation of RB1, acquired by somatic inactivation of the remaining allele through copy-neutral LOH of RB1. While the initiation of tumorigenesis in retinoblastoma begins with RB1 inactivation, additional genetic alterations may be required for continued tumor growth. A set of high-confidence somatic mutations and copy number changes were present in each of the three tumor samples [Figure 1C, Table 1A] Similar to many pediatric neoplasms including retinoblastomas [11,12], the overall somatic mutation burden in these tumors was low and all the mutations were non-recurrent with unknown functional impact. Aside from RB1, recurrent gene mutations are rare in retinoblastoma [12,13]. Copy number analysis of the primary cancer, biopsied in 2009, demonstrated copy neutral LOH of 13q and losses on 3q. Two metastatic samples, biopsied in 2018, shared these events in the primary cancer, with additional gains on chromosomes 1q, 2p, 6p, 12, and 15. The brain biopsy showed further gains of chromosome 19 and copy loss on 17p (Figure 1C). This is consistent with prior reports of several highly recurrent copy number alterations in retinoblastoma, such as gain of 1q, 2p, 6p, and loss of 13q and 16q [12,13].
Cancers may recur after decades of latency. This phenomenon may be due to an acquired ability of cancer cells, possibly brought on by the stress of chemotherapy, to manipulate the tumor microenvironment and evade the immune system, metastasizing to sanctuary tissues and enter a state of cellular senescence where they are metabolically active, but do not proliferate until stimulated to “reawaken” and divide [14-17]. This may explain the late relapse in our patient.
Of the mutations found [Table 1A], CYP21A2, CD276, FANC2D , are associated with PolyPhen scores predicting that they may be pathogenic. Interestingly, these three have been previously reported in tumor migration, invasion, and cell cycle control [18-26], and may have played a role in this patient’s tumor dormancy.
The observed mutational changes support the notion that due to the low burden of somatic mutation, the mechanism of tumorigenesis/evasion may be related to another mechanism, however, the observed mutations should not be dismissed and their role in tumor dormancy should be further investigated.