In our first molecular analysis of PTSMT, we analysed the expression of miR-146a and miR-155 because these microRNA are known to be involved in the pathobiology of EBV infection of B cells [
2,
34,
35]. In our current analysis, we could confirm our previous results and found no significant deregulation of these two microRNA using a microarray technique. Furthermore, it is remarkable that several EBV-related microRNA, other than miR-146a and miR-155, are not deregulated in PTSMT. In particular, miR-10b, miR-21, miR-29b, miR-34a and miR-127, which are increased in EBV
+ nasopharyngeal carcinoma and high grade B cell lymphomas/Burkitt lymphomas [
20‐
33], were expressed at low or very low levels in PTSMT. In non-PTSMT EBV
+ carcinomas and lymphomas miR-200b, miR-203 and miR-429 are generally expressed at low levels [
20‐
33]. In PTSMT, but also in EBV
- leiomyomas, these microRNAs were also expressed at low levels, indicating no specific EBV-related decreased expression in smooth muscle tumours. It has to be taken into account that different cell and tumour types can react in different manners to EBV infection. Furthermore, depending on the latency type, EBV induces expression of different virus proteins, which interfere in the host cell cycle. Latent membrane protein 1 (LMP1) is one of these EBV proteins and it has been shown that LMP1 alone can induce altered microRNA expression in nasopharyngeal carcinomas and lymphomas [
20‐
33]. We and others have found previously that EBV
+ PTSMT express EBNA2 and EBNA3 while LMP1 expression is weak or not detectable [
2,
3,
36,
37]. Thus, despite EBV infection in PTSMT, our finding of no major changes in the microRNA expression profile is likely to be related to lack of LMP1 expression.
Leiomyomatous phenotype-associated microRNA expression in PTSMT
MicroRNA expression analyses in uterine leiomyomas and leiomyosarcomas were introduced only a few years ago (Additional file
4: Table S4) [
6‐
17]. The majority of studies have evaluated patient-derived leiomyomas as a model disease of neoplastic smooth muscle proliferation. Among different studies and different analytical methods, microRNA expression patterns of patient-derived tumour samples showed a set of microRNA which are recurrently deregulated in comparison to normal uterus wall cells. Similar to PTSMT, decreased expression of miR-150, miR-200c and miR-221 and increased expression of miR-21 and let-7 family members have become particularly evident in leiomyomas [
6‐
17]. It has been demonstrated that mesenchymal cells which differentiate into smooth muscle cells
in vitro change their microRNA expression patterns, e.g. down-regulation of 13q31.3-clustered miR-17/miR-18a/miR-20a and up-regulation of miR-181a/miR-181c paralogs [
16]. Furthermore, in smooth muscle cells, it has been shown that the 5q32-encoded miR-143/miR-145 cluster is co-expressed [
38,
39]. Both microRNA target a network of factors to promote vascular smooth muscle cell differentiation and repress proliferation [
38,
39]. We could previously demonstrate high expression of miR-143 and even higher expression of miR-145 in pulmonary vessel wall cells [
40]. A similar expression pattern of high miR-143 and higher miR-145 could also be found in PTSMT but also in leiomyomas. Because PTSMT can be found next to vessels (e.g. manifestation in cerebral sinus), it is thought that the aberrant founder cells might be derived from a vessel wall [
2,
41]. However, due to very similar miR-143/miR-145 expression patterns in PTSMT, uterus wall-derived leiomyomas and pulmonary vessels, the high expression of these two microRNA does not prove a vessel wall origin of PTSMT but reflects the smooth muscle differentiation.
In PTSMT and leiomyomas, many microRNA are expressed at low or very low levels, which makes it likely that protein translation of potential target mRNA types is not inhibited. The problem for target prediction, but simultaneously the hallmark of microRNA/mRNA biology, is the characteristic semi-complementary binding of the seven nucleotides at the 3′-end of the mature microRNA (so-called seed sequence) to corresponding mRNA-nucleotides of the 5′-UTR [
4,
5]. This semi-complementary binding is sufficient to induce a biological effect, the inhibition of mRNA/protein translation. As a result, one microRNA can bind to several 5′-UTR-mRNA and
vice versa one 5′-UTR-mRNA can be targeted by several microRNA. In our previous analysis, we have evaluated the expression of several mRNA transcripts in PTSMT and leiomyomas, including MYC, vascular endothelial growth factor A (VEGFA), nuclear factor of kappa light polypeptide gene enhancer in B-cells 1 (NFKB1), tumour protein p53 (TP53), transforming growth factor, beta receptor II (TGFBR2) and transforming growth factor, beta 1 (TGFB1) [
2].
Among several EBV-associated human factors, we found only MYC to be significantly increased in PTSMT [
2]. The mRNA of this transcription factor can be regulated by miR-150, miR-143 and miR-145 [
42,
43] but no PTSMT-specific inverse correlation was found.
VEGFA can be negatively regulated by miR-200c and other microRNA. In leiomyomatous cell lines, miR-200c interaction with VEGFA has been shown [
7] and accordingly, in leiomyomas as well as in PTSMT, very low levels of miR-200c correlate with increased levels of VEGFA [
2].
In many different tumour types, miR-21 is aberrantly expressed, because this microRNA can target several signal networks, either directly by binding to different types of mRNA from similar signal cascades or indirectly via deregulation of factors down/up-stream to the factors directly suppressed by miR-21 [
44,
45]. In particular, miR-21 is a negative regulator of TP53 signalling and simultaneously a promoter of NFKB1 signalling [
44,
45]. Increased expression of miR-21 has been previously demonstrated in leiomyomas [
6,
10,
15,
16] which we could confirm in our analysis. In smooth muscle cells, miR-21 is involved in regulation of apoptosis and TGFBR2/TGFB1 signalling [
6,
10,
13]. TGFBR2-3′UTR has an miR-21 binding site and can therefore directly be regulated by miR-21 in smooth muscle cells [
10].
In vitro studies also suggested an indirect regulatory interaction between miR-21 and TGFB1; of note, TGFB1 is not a direct target of miR-21 [
10]. Furthermore, inhibition of miR-21 expression in smooth muscle cells indirectly increases caspase 3 and caspase 7 activity
in vitro; both caspases have no miR-21 binding site [
6,
13]. The miR-21 expression was lower in PTSMT than leiomyomas but the difference was not significant. In addition, in our previous analysis we found no differences in the expression of miR-21-related NFKB1, TP53, TGFBR2 or TGFB1 between PTSMT and leiomyomas [
2]. Therefore, our
in situ-derived results do not reveal a PTSMT-specific deregulated miR-21 signal cascade, but an expression pattern related to smooth muscle phenotype.
Members of the let-7 family are increased in leiomyomas [
15] and smooth muscle cell lines, e.g. let-7b [
13,
16]. We found that in PTSMT and leiomyomas, the strongest expressed let-7 paralog was let-7b. In leiomyomas and leiomyosarcomas, let-7c shows an inverse correlation with its target mRNA high mobility group AT-hook 2 (HMGA2) [
11,
17]. Both genes, let-7c and HMGA2, are expressed in association with size of leiomyomas [
11], while in PTSMT, irrespective of the size, let-7c was almost absent.
In summary, in addition to leiomyomas and leiomyosarcomas, PTSMT is the third smooth muscle tumour type in which the microRNA expression profile could be evaluated. The expression pattern of microRNA in PTSMT is not associated with EBV infection (presumably due to lack of strong LMP1 expression) but reflects the leiomyomatous differentiation of the tumour cells.