This study has demonstrated the feasibility of detecting 2HG and quantifying its relative abundance in IDH1
-mutated gliomas using 1
H HR-MAS spectroscopy. There was a strong concordance established between IDH1
-mutant tissue samples acquired from patients and the presence of 2HG. These data corroborate the previously identified relationship between IDH1
mutation and the aberrant production of this metabolite that was validated using liquid chromatography–mass spectrometry analysis (10
). Because our study design constrained the acquisition of targeted biopsies to relatively small regions with homogeneous in vivo imaging characteristics, the sampled tissue was in some cases not ideal for spectroscopic analysis. Despite this limitation, it was possible to obtain an accurate assessment of the presence and absence of 2HG for most of the patients evaluated. When present, the levels of 2HG were on par with those of the chemically related metabolites Glu and Gln, which are readily detected using 1
H HR-MAS spectroscopy.
We noted several important correlations between 2HG levels in human tissue and parameters that are associated with gliomas. Relationships between levels of 2HG and other ex vivo metabolites may aid in our understanding of the altered metabolic state of IDH
-mutated glioma and in determining whether 2HG is a contributing factor. The standing hypothesis that 2HG is a tumor-promoting oncometabolite (3
) currently lacks full mechanistic support; however, there are advantages to elucidating the biochemical pathways that are influenced by 2HG and may lead to improved survival for patients with IDH
mutations. The correlations between histopathology parameters and levels of 2HG that were observed in our study indicated that lesions with IDH1
-mutant enzymes have increased mitotic activity, with elevated tumor scores and cell density. The strong correlation between these parameters is consistent with there being higher cellularity in IDH
-mutant gliomas where 2HG is present. This implies that in vivo levels of 2HG may be able to contribute not only to the classification of glioma but also to characterizing the spatial extent of infiltrative lesions.
The negative correlation between 2HG and normal delicate vascularity may assist in designing strategies for treating lesions with mutant IDH1 enzymes. 2HG has been associated in the literature with the competitive inhibition of oxygen-sensing prolyl hydroxylases (PHDs) and histone demethylases (17
). Restraining PHD activity is hypothesized to result in an up-regulation of proangiogenic growth factors, including a common chemotherapy target, vascular endothelial growth factor (VEGF). It is therefore of clinical interest whether patients harboring IDH mutations may be ideal candidates for therapies that target 2HG production as well as antiangiogenic agents that promote vascular normalization. With regard to histone demethylases as chromatin-modifying enzymes, their inhibition could alter the regulation of gene expression such that oncogenesis is provoked (18
), another potential reason to therapeutically reduce levels of 2HG.
We also found a significant relationship between IDH1 mutants—as identified via 2HG levels in tissue—and in vivo MR diffusion parameters, which supports the potential role of diffusion imaging in evaluating whether the tissue architecture of mutant gliomas is distinct from that of other neoplasms. The elevation of tCHO that we observed from the ex vivo spectra of IDH1-mutant gliomas suggests that in vivo levels of tCHO may also be important for evaluating these lesions. Because MR diffusion and spectroscopy measures are readily obtained on most clinical MR scanners, they may be useful in defining regions of interest for studying variations in levels of 2HG.
The fact that the observed levels of 2HG from tissue samples expressing IDH1 mutations were similar to those of other routinely detected metabolites, such as Gln and Glu, supports the potential clinical use of 2HG as a noninvasive biomarker for IDH1. Given that tissue samples taken from disparate regions of tumor were homogeneous in their mutation status, it is reasonable to assume that 2HG will be present throughout lesions expressing aberrant enzymes. The elevated levels of 2HG in gliomas that had converted to a higher grade at the time of recurrence were attributable to differences in cellularity. This is an important finding because it suggests that the amount of 2HG produced per tumor cell remains unchanged during malignant transformation. From the correlation between levels of 2HG, tumor score, and tissue cellularity, it seems likely that 2HG will contribute to determining the extent of recurrent tumor in an in vivo setting. This will be valuable in differentiating tumor from treatment effects, especially in difficult cases where ambiguous anatomic imaging complicates interpretation by a neuroradiologist.
This study has provided important information that should be considered carefully in the development of technology for translating 2HG detection into an in vivo setting. One of the main challenges is providing a robust in vivo method for resolving 2HG from neighboring metabolites that are of a similar concentration and will have overlapping resonances at clinical field strengths. There have been three preliminary studies that have reported on the detection of in vivo 2HG peaks in patients with IDH1
-mutated gliomas (19
). These used different acquisition parameters and reported varying accuracies for estimated metabolite levels based on the Cramar-Rao bounds derived using the LCModel algorithm for spectral quantification. Spectral editing and more sophisticated data acquisition methods, including variants of the 2D experiment used in our study, may be useful in separating these components and verifying individual resonances (21
). From our data, it appears that focusing on the unobstructed spectral feature created by the α-proton may be of interest for improving the specificity and accuracy of detecting 2HG in vivo.
Considering the survival benefits associated with IDH1/2
mutations, the presence of 2HG may have significant prognostic value for patients with low-grade gliomas. The characterization of IDH
-mutated lesions using MR methods could also hold implications for the study of other human cancers that share these genetic abnormalities, including colorectal cancer, prostate cancer, and acute myeloid leukemia, which shows an adverse prognosis for IDH1/2
mutations, in contradistinction with gliomas (22
). Recent evidence suggests that many of these cancers carry chemosensitive properties that could enhance treatment efficacy and inform clinicians on how to stratify patients for different therapies (5
). If D-2HG truly bears oncogenic properties (3
), then testing for mutations in the IDH
gene will probably not be sufficient for the clinical management of glioma patients; additional monitoring of 2HG levels would become important, especially for therapies targeting the IDH1
). Finally, as a potential marker for tumor in vivo, 2HG may prove especially helpful to clinicians attempting to discern disease recurrence from treatment effects in patients whose radiographic imaging is considered suspect. This information may help clinicians identify candidates for the most appropriate therapies or clinical trials and assist in the monitoring of treatment.