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.