Previous studies have shown that members of the miR-371~373 and miR-302/367 clusters are frequently and aberrantly expressed in extracranial GCT. Therefore, analysis of these GCT-specific miRNAs in iGCT may be an innovative and promising diagnostic tool. Here, we present the to date largest study on miRNAs as liquid biomarkers in iGCT delineating GCT-specific miRNAs in serum and CSF.
In our study miR-371a, miR-372, miR-367, miR-302a and miR-302d in serum discriminated between iGCT of different histology and controls comprising mostly healthy donors. This is in line with our [10] and others’ miRNA analyses in patients suffering from testicular GCT including a total of >1000 serum samples [12, 18, 19]. In iGCT, relative miR-371a levels in two smaller case series were similar to our study but formerly only detectable in the serum of 2/5 patients [15, 20].
The analysis of a panel of miRNAs may enhance the diagnostic accuracy. Thus, in our study serum miR-302d also distinguished iGCT patients from healthy controls with a comparable expression level to miR-371a. Although miR-302d is detectable in GCT tissues of both pediatric and adult patients and is overexpressed in YST compared to germinoma [21], data on its usefulness as liquid biomarker have been lacking. Serum miR-302d has not been investigated in testicular cancer patients focusing on the miR-371~373 cluster [11, 12, 18, 19] or has been excluded from detailed analysis in extracranial GCT as its levels ranged only within two Ct values between patients and non-tumor control [15]. Here we document profound expression of both miR-371a and miR-302d in all iGCT sera including serum of biomarker-negative patients. Thus, miR-302d added value in the diagnosis of iGCT especially in biomarker-negative patients, complementary to but not outperforming miR-371a.
MiRNA analysis may be influenced by various pre- and post-analytical variables and normalization strategies. The latter requires an endogenous stably expressed reference gene miRNA. MiR-30b-5p fulfilled this purpose in several analyses on testicular GCT in serum [12, 19, 22, 23] and was also employed in two studies on intracranial GCT both in serum as well as in CSF [15, 20]. However, miR-30b-5p was barely detectable in another analysis in 2/4 CSF samples from healthy donors by RT-qPCR [24]. In our study in a total of 13 different CSF samples, Ct values of miR-30b-5p markedly varied within a range of twelve Ct cycles underscoring the need for a more homogeneously expressed reference gene miRNA in CSF. Previous RNAseq-based miRNA examination of 129 samples of 12 different body fluids from healthy donors identified abundant miR-204-5p expression in 11 CSF samples, thus ranking among the top five miRNAs in CSF in this study [25]. Here, we confirm the suitability of miR-204-5p for normalization purposes in miRNA analysis of the CSF by demonstrating its higher expression and lower variability compared to the broadly used reference gene miR-30b-5p.
Of note, under these optimized conditions in our study miR-371a, miR-372, miR-367 and miR-302d in CSF presented as strong indicators of iGCT disease. In addition, miR-302b was beneficial in CSF to differentiate iGCT from other diseases such as low-grade gliomas or pituitary inflammation. In another study analyzing miRNA expression in CSF of two iGCT patients and utilizing miR-30b-5p as housekeeper, only miR-371a and miR-372 discriminated patients from those with a benign brain tumor with miR-302d being not included in the analysis [15]. Also, miR-371a analysis in CSF seems to discriminate iGCT from Langerhans cell histiocytosis (LCH) as shown in a recent case report [20]. Since LCH often presents with similar radiological changes as iGCT such as thickening of the pituitary gland, miRNA analysis may add value to the routine CSF assessment in the differential diagnosis of iGCT.
While healthy donors should be preferred as comparator group for normalization, sampling of CSF remains challenging, since lumbar puncture is mainly performed in symptomatic individuals. This issue was addressed in one study by normalization of CSF-levels against ΔCt values of serum rather than CSF controls [15]. This resulted in comparatively high miR-371a expression up to 2−ΔΔCt levels of 100000 in the CSF of the two iGCT patients [15]. Similarly, normalization of CSF values in our iGCT patients against the serum ΔCt values of our predominantly healthy controls resulted in extraordinary high relative miR-371a CSF levels (3500-470000) in spite of raw CSF Ct values of miR-371a in our iGCT cohort left unchanged. Since miRNA distribution between CSF and serum differed in comparative microRNA analysis of healthy donors by RNAseq [25] and qPCR [26], usage of serum ΔCt control values for normalization of CSF miRNA data remains a matter of debate and selection of appropriate controls is critical. Five of our nine CSF control samples presented an exceedingly low maximum fluorescence in the miR-371a-qPCR although fluorescence in qPCR of the housekeeper and the other miRNAs was sufficient. The amount of miR-371a in these samples seemed to be so low that miR-371a Ct values could not be determined and the samples had to be excluded from the analysis. In the other four control samples valid Ct values could be attributed. Thus, sufficient numbers of CSF control samples for miR-371a analysis are also essential.
In our cohort of iGCT patients we documented a similar GCT-specific miRNA expression pattern between CSF and serum indicating the passage of these miRNAs across the blood-brain barrier. Similarly, in testicular cancer circulating miR-371a is specifically derived from GCT tissue and its tissue levels correlate with levels of the tumor surrounding hydrocele and levels of serum being indicative for miRNA transfer through the blood-testis barrier [27, 28]. However, low total RNA yields and different miRNA proportions of total RNA can affect miRNA analysis in CSF. We employed a well-established, robust and quality-controlled pipeline for miRNA analysis including pre-amplification of samples [15] and increased total RNA yield by performing RNA isolation twice. Nevertheless, miR-371a analysis in CSF was impaired by low amount of miR-371a in CSF control samples and qPCR of miR-302a and miR-302c repetitively revealed non-sigmoidal but linear amplification curves in both patients and controls. As analysis of GCT-specific miRNAs in serum showed a similar pattern to miRNA assessment in CSF, serum may be preferred to CSF as it yields more RNA and can be obtained by a less invasive and easier access in both patients and healthy controls.
In iGCT, an important clinical question is the distinction between pure germinoma and NGGCT as the latter require a more intensive treatment due to inferior outcomes [6, 7]. Known GCT-specific miRNAs such as miR-371-373 do not permit discrimination between these two iGCT subtypes in our and two other recent studies [15, 20]. Thus, we analyzed miR-142-5p and miR-146a-5p in serum of iGCT patients, which were highly expressed in germinoma compared to non-germinoma tissue in a global miRNA analysis of 12 iGCT [16]. Of note, as a key regulator of anti-tumor immunity, miR-142-5p inhibited PD-L1 expression on tumor cells in vitro enhancing tumor infiltration by CD4+- and CD8+-lymphocytes [29]. In germinoma, detection of miR-142-5p may therefore reflect the immunohistochemically documented high lymphocytic infiltration in the tumor microenvironment in the absence of PD-L1 [30]. In our study overexpression of miR-142-5p and miR-146a-5p was detectable in the serum of 1/3 patients who suffered from an iGCT with partly germinoma histology. As miR-142-5p and miR-146a-5p may also be highly expressed in the peripheral blood of healthy subjects due to their regulating role in the immune response, these miRNAs may only discriminate between germinoma and NGGCT in tissue but not in serum.
Perspectively, serum miRNAs of the C19MC cluster may prove beneficial to identify NGGCT, as these miRNAs were indicative for pure embryonal carcinoma histology not only in tissue but also in the serum of patients suffering from testicular GCT [31]. In a large miRNA tissue analysis of 103 testicular GCT, overexpressed miRNAs from the C19MC cluster were also indicative for NGGCT [32].
In the largest data set so far on miRNAs as liquid biomarkers in iGCT, we underlined the utility of miR-371a, miR-372, miR-367 and miR-302d in patient serum for diagnosis of iGCT, a result particularly noteworthy in germinoma patients negative for other biomarkers such as AFP and ß-HCG. Furthermore, we confirmed the suitability of these miRNAs as liquid biomarkers in both serum and CSF of iGCT patients. Using our miRNA protocol, diagnosis of iGCT in serum samples was a feasible and valid approach, particularly as serum can be readily obtained from patients and healthy controls by a less invasive procedure. This approach complements miRNA analysis in CSF, which may be complicated due to low RNA yield. Besides verification of miR-371~373 and miR-302d for iGCT diagnosis in larger cohorts of iGCT patients and control subjects suffering from brain tumors of different histologies, future iGCT studies should also be geared to investigate other miRNAs in serum and/or CSF that enable for discrimination of germinoma from NGGCT. Furthermore, miRNA analysis during therapy may allow to monitor response to therapy as well as detect minimal residual disease.