Examining Potential Markers of Osteosarcoma Stem Cells Is at the Forefront of Research

R. Lor Randall, MD, FACS

ALDH1A1 and CD44 have shown conflicting signals regarding their role as potential prognostic markers in osteosarcoma, underscoring the need for further research into the role of cancer stem cells in the pathogenesis of the disease, according to R. Lor Randall, MD, FACS.

Findings from the publicly available TARGET-OS dataset (n = 85) revealed that regarding ALDH, the univariate HR for overall survival (OS) was 0.78 (95% CI, 0.64-0.95; P = .015) and 0.87 (95% CI, 0.75-1.02; P = .09) for disease-free survival (DFS). For CD44, the univariate HR was 0.74 (95% CI, 0.54-1.00; P = .053) for OS and 0.85 (95% CI, 0.66-1.09; P = .194) for DFS. In the 20-specimen University of California (UC) Davis Health retrospective review cohort, investigators noted that the percentages of CD44/ALDH1A1-positive cells were not significantly associated with hazards of overall mortality or disease recurrence; the H-scores were not either.

“With all of us concentrating on immunomodulation and immuno-targeting, we should not forget about the possibility of cancer stem cells being involved in the pathogenesis and progression of some tumors, particularly sarcomas,” Randall said in an interview with OncLive^®. “Despite our research [at UC Davis in the 20-specimen cohort] not being able to show that, we did see it in the TARGET dataset which was close to 90 specimens. That signal from the TARGET data is exciting.”

In the interview, Randall, the David Linn Endowed Chair for Orthopedic Surgery, the chair of the Department of Orthopedic Surgery, and a professor at UC Davis Comprehensive Cancer Center in Sacramento, California, detailed the research examining ALDH1A1 and CD44 in osteosarcoma stem cells, and how further research may be warranted with cancer stem cells.

OncLive: What was the rationale for this study examining potential markers of osteosarcoma stem cells?

Randall: There are 2 broad categories of sarcomas [with] the translocation driven and non-translocation driven sarcomas. [Diseases] such as Ewing sarcoma and a variety of others [including] some of the rhabdomyosarcoma subtypes and a variety of synovial sarcomas all have translocation-derived pathogenesis. Osteosarcoma, pleomorphic spindle cell sarcoma, and a variety of other sarcomas, which make up approximately two-thirds of all sarcomas, are noted to have a shattered genome and complete karyotypic abnormal findings; no one [fully] understands the pathogenesis.

We know with translocation derived sarcomas that if you knock out the translocation or some of the downstream targets that are upregulated or downregulated, you get quiescence of the phenotype. But with these other sarcomas, particularly osteosarcoma and spindle cell sarcomas, we have no real understanding of the pathogenesis.

Therefore, stem cell biology has crept into the fore[front] as potentially being the cell type of origin or the cell type associated with progression of these tumors. Particularly cancer stem cells [are of focus] and the 2 markers for cancer stem cells that have gotten the most attention are CD44 and ALDH1A1. I am not a cancer stem cell biologist as my background is more in molecular biology, but here at UC Davis we have a rich stem cell investigative team, and we decided to look at these cancer stem cell markers in osteosarcoma. We know that there are other cancers where they may be prognostic or a therapeutic target, and we thought perhaps this could be true in osteosarcoma.

What were the key findings of the study?

The purpose of our study was to examine the association between the cancer stem cell markers ALDH1A1 and CD44 and OS, metastasis potential, and disease recurrence in [patients with] osteosarcoma from both our institutional cohort and a national database. Our hypothesis was that patients with these markers that stained on biopsy would have decreased survival, increased metastasis, and would be likely to experience disease relapse. We [analyzed] this with tissue microarrays at UC Davis and we looked at the RNA expression data from The Cancer Genome Atlas [TCGA].

Over a period we were able to obtain 20 samples [from patients with biopsy-proven osteosarcoma. We looked at both CD44 and ALDH1A1, Kaplan Meier analyses [of] OS and DFS for our cohort, and then TCGA datasets. We didn’t find anything that was too exciting [with our UC Davis dataset as] the HRs for our 20 samples were around 1 indicating that there was not any correlation with OS or DFS.

Interestingly, with the TARGET dataset, we did find that high ALDH1A1 expression was associated with improved outcomes which was counter to what we thought. We thought that because ALDH1A1 was a cancer stem cell marker, increased expression would at least correlate with a worse outcome, but we found the exact opposite which was a little surprising. [However], we didn’t find that either marker for our [UC Davis] cohort was prognostic.

The questions that remain are: Are cancer stem cells at all involved in osteosarcoma and what is the pathogenesis for these pleomorphic sarcomas? More investigation is warranted in cancer stem cell biology.

Should related pathways or targets be the subject of future research in this area?

In other cancers, both these markers have been determined to be prognostic. Like many things in sarcoma, we learn from the more common tumors whether something is prognostic or therapeutic, and then apply those same sets of analyses to sarcoma tissues and try to find out if the same applies.

Chang et al studied 442 patients with ovarian cancer and found that high ALDH1A1 expression was [associated] with favorable survival. ALDH1A1 expression was also prognostic for malignant melanoma [according to Taylor et al].

Is there anything else you’d like to add on the field of cancer stem cell research?

Cancer stem cell biology is something that has been around for a bit. Stem cell biology has been around for decades, but [investigators] have isolated cancer stem cells in a variety of these more pleomorphic high-grade cancers. The surface markers for stem cell biology or phenotype are expressed on these and they’re able to pull them out of these tumors that are more aggressive. The hypothesis is that these cancer stem cells are repopulating the tumor and driving the progression of the tumor because they escape the therapeutic effects of the non-stem cell population.

The vast majority [of tumors] are going to be non-stem cell and they won’t have any of these markers—CD44 or ALDH1A1—they’ll have whatever the markers are for that type of tumor. But then there’s this subpopulation of cancer stem cells with these markers that [investigators] have seen historically populated in the more aggressive areas of the tumor or metastases.

What is the importance of biorepositories and their association with translational research?

Tissue is the issue and it’s near and dear to my heart as a surgeon because we’re the ones usually harvesting the tissue, particularly the resection specimens. For this study, we did both biopsy and resection specimens; the Holy Grail is the [tissue from the] metastases which we were able to acquire much less frequently because of the nature of not going after metastatic tissue, because it’s not in the interest of the overall treatment of the patient. But having these biorepositories is the only way we’re going to be able to look at disease progression.

Biopsy tissue is a pretherapeutic intervention [and] resection specimens in sarcoma [and] maybe in some other solid tumors are usually after some sort of neoadjuvant therapy—you have a selective population of tumors. In the metastatic setting, those lesions are going to have inherently different biology from both the primary and the pretreatment biopsy tissue. If we have these repositories of tissue through the whole spectrum of disease, we are much more likely to create treatments tailored to the metastatic phenotype.

What is the main takeaway from this research?

The concepts about tissue throughout the chronological spectrum of disease would certainly apply to cancer stem cells as well—if [they are present] in the biopsy, and they are different proportionally in the resected specimen and in metastatic disease that would implicate cancer stem cells as being somewhat of an involved either bystander or driver for progression of disease. The lens right now is on immunomodulation, PD-1, and a variety of other areas of investigation, but we at UC Davis and others have a particular interest in cancer stem cell biology; there may be targetable areas within this field as well.

Reference

Haffner MR, Saiz AM Jr, Darrow MA, et al. Effect of ALDH1A1 and CD44 on survival and disease recurrence in patients with osteosarcoma. Cureus. 2024;16(1):e52404. doi:10.7759/cureus.52404