Discussion
Overall, we found that long-term survivors of childhood cancer have
decreased gut bacterial microbiome diversity and differently abundant
bacterial taxa, even years after the completion of therapy. Further,
these changes are apparent despite the finding of similar present diet
practices between survivors and controls. Additionally, radiation
therapy, particularly to the abdomen and pelvis, but also cranial
radiotherapy, appears to be associated with reductions in gut microbial
diversity and differently abundant taxa. Interestingly, exposures such
as chemotherapy and antibiotic intensity were not directly associated
with alpha diversity in survivors, though they were associated with
differentially abundant taxa. In this study, alpha diversity itself did
not appear to be associated with components of the metabolic syndrome or
chronic inflammation among childhood cancer survivors, but low
prevalence of metabolic derangements in the study population may have
precluded this finding. Notably, however, among survivors with increased
adiposity, elevated body mass index, and increased inflammation,
differently abundant taxa were present, suggesting a potential
interaction with the microbiome.
Two previous studies in similar populations also demonstrated reduced
microbial diversity in childhood cancer survivors. Among 13 long-term
Hodgkin lymphoma survivors and twin controls (8 dizygotic, 5
monozygotic), survivors had fewer operational taxonomic units in fecal
samples compared to their co-twin control [39]. A recent study of
adult survivors of childhood acute lymphoblastic leukemia from Malaysia
compared survivors to healthy controls [18]. Similar to the present
study, Chua, et al. recruited controls among healthcare works, siblings,
and other volunteers and were matched for sex and ethnicity, though
neither subjects nor controls were excluded for recent antibiotic use.
Chua, et al. also found that survivors had altered composition of taxa
and decreased alpha diversity of bacteria compared to healthy controls,
but used anal swabs for sample acquisition.
Survivors of childhood acute lymphoblastic leukemia have been previously
demonstrated to have increased levels of inflammation compared to
controls [17]. Indeed, Chua, et al. demonstrated modest associations
between bacterial taxa and Interleukin-6 and C-Reactive protein, though
the role of cancer history, chemotherapy and antibiotics in these
associations was unclear [18]. In the present study we were not able
to detect an association between alpha diversity and markers of chronic
inflammation in survivors, but did see differences in taxa abundance
based on levels of C-Reactive protein. As opposed to the work previously
performed, this is the first study to clearly demonstrate alterations in
microbiota in a heterogeneous cohort of long-term survivors based on
multiple different underlying malignancies. Additionally, although
radiation has been shown to lead to short term changes in the microbiome
in oncology patients [40,41], ours is the first study to demonstrate
a relationship between radiation and microbiome diversity many years
after treatment.
Our study has some limitations. First, our cohort did not have high
levels of phenotypic abnormalities. Hyperlipidemia and insulin
resistance were uncommon, and most patients did not possess components
of the metabolic syndrome. This limited our ability to detect an
association of the microbiome with these late effects, and perhaps a
larger or older cohort of patients may have improved that power.
Further, the associations between differently abundant taxa and
phenotypes demonstrated in this study do not necessarily imply
causation, and mechanistic studies would be required to further
characterize these relationships. Additionally, we did not perform
internal transcribed spacer sequencing in this study to detect a
potential interplay between the bacterial microbiome and the gut fungal
microbiome, which has been implicated in obesity and other diseases
[42,43]. Finally, this was a single institution exploratory study,
and microbiome findings may be unique to the patients seen in our clinic
or geography. Regardless, this study still defines important
relationships ripe for further exploration.
Overall, this study demonstrates that long-term survivors of childhood
cancer have decreased gut bacterial microbiome diversity and differently
abundant bacterial taxa, even years after the completion of therapy.
Additionally, radiation may be particularly important in microbial
diversity among this cohort. In future experiments, we aim to further
categorize the metabalome in a similar cohort of patients to better
understand if the difference found on 16S sequencing results in
alterations of metabolic products in the blood-stream. We also aim to
explore potential relationships with the fungal microbiome and gain a
better understanding of mechanisms via gnotobiotic mouse experiments.
Further, in order to detect associations with clinically relevant late
effects, we aim to perform a larger multicenter study using older
subjects with greater prevalence of comorbidities.