Introductory Paragraph
Soft tissue sarcomas, which encompass approximately 10,700 diagnoses and 3800 deaths
per year in the US1, exhibit remarkable histologic diversity, with more than 50 recognized
subtypes2. However, knowledge of their genomic alterations is limited. We describe
an integrative analysis of DNA sequence, copy number, and mRNA expression in 207 samples
encompassing seven major subtypes. Frequently mutated genes included TP53 (17% of
pleomorphic liposarcomas), NF1 (10.5% of myxofibrosarcomas and 8% of pleomorphic liposarcomas),
and PIK3CA (18% of myxoid/round-cell liposarcomas). PIK3CA mutations in myxoid/round-cell
liposarcomas were associated with AKT activation and poor clinical outcomes. In myxofibrosarcomas
and pleomorphic liposarcomas, we found both point mutations and genomic deletions
affecting the tumor suppressor NF1. Finally, we found that shRNA-based knockdown of
several genes amplified in dedifferentiated liposarcoma, including CDK4 and YEATS4,
decreased cell proliferation. Our study yields a detailed map of molecular alterations
across diverse sarcoma subtypes and provides potential subtype-specific targets for
therapy.
Current knowledge of the key genomic aberrations in soft tissue sarcoma is limited
to the most recurrent alterations or translocations. Subtypes with simple, near-diploid
karyotypes bear few chromosomal rearrangements but have pathognomonic alterations:
translocations in myxoid/round-cell liposarcoma (MRC) [t(12;16)(q13;p11), t(12;22)(q13;q12)]
and synovial sarcomas (SS) [t(X;18)(p11;q11)]; activating mutations in KIT or PDGFRA
in gastrointestinal stromal tumors (GIST)3,4. The discovery of the latter mutations
led to the clinical deployment of imatinib for the treatment of GIST5, providing a
model for genotype-directed therapies in molecularly defined sarcoma subtypes. Conversely,
sarcomas with complex karyotypes, including dedifferentiated and pleomorphic liposarcoma,
leiomyosarcoma, and myxofibrosarcoma, have no known characteristic mutations or fusion
genes, although abnormalities are frequently observed in the Rb, p53, and specific
growth-factor signaling pathways6.
Recent large-scale analyses7–10 have established a standard for cancer genome studies,
but soft tissue sarcomas have not yet been a focus of this type of effort. Given the
urgent need for new treatments for the ~4000 patients who die each year in the US
of soft tissue sarcoma1, we sought to identify novel genomic alterations that could
serve as therapeutic targets. Here, we describe complementary genome and functional
genetic analyses of seven subtypes of high-grade soft tissue sarcoma (Table 1 and
Supplementary Table 1) to discover subtype-specific events. Several of our findings,
detailed below, could have nearly immediate therapeutic implications.
To study the genomic alterations in sarcomas, we initially analyzed 47 tumor/normal
DNA pairs encompassing six soft tissue sarcoma subtypes by sequencing 722 protein-coding
and microRNA genes, followed by verifying discovered mutations with mass spectrometry-based
genotyping (see Methods, Supplementary Figure 1A, and Supplementary Table 2). The
results revealed 28 somatic non-synonymous coding point mutations and 9 somatic insertions/deletions
(indels) involving 21 genes in total (Table 2 and Supplementary Figure 1B). No mutations
were detected in microRNAs genes. We extended the analysis to an additional 160 tumors,
where we genotyped each of the mutations found above and re-sequenced exons of NF1
and ERBB4 in pleomorphic liposarcoma and myxofibrosarcoma, PIK3CA and KIT in myxoid/round
cell liposarcoma, and CDH1 in dedifferentiated liposarcoma; this revealed nine additional
mutations (Table 2 and Supplementary Table 3).
KIT was frequently mutated in GISTs and unexpectedly, in one myxoid/round cell liposarcoma
sample (Supplementary Note). The next most frequently mutated genes observed within
specific sarcoma subtypes were PIK3CA, in 18% of myxoid/round cell liposarcomas, TP53
in 17% of pleomorphic liposarcomas (interestingly, the only subtype in which mutations
of this gene were found), and NF1 in 10.5% of myxofibrosarcomas and 8% of pleomorphic
liposarcomas (Table 2 and Figure 1). Additional genes, including protein and lipid
kinases, as well as known or candidate tumor suppressor genes, were found mutated
in just one sample for each sarcoma subtype (Table 2, Figure 1, and Supplementary
Note). Further studies will be needed to establish the functional impact of these
mutations in sarcoma.
Below, we focus on three major specific genomic findings with therapeutic implications:
point mutation and deletion of NF1 in a subset of soft tissue sarcomas, point mutation
of PIK3CA in myxoid/round cell liposarcoma, and the complex pattern of amplification
of chromosome 12q in dedifferentiated liposarcoma.
Integrated analysis of DNA copy number, expression, and mutation data uncovered diverse
alterations of the Neurofibromatosis type 1 gene (NF1) in several sarcoma subtypes.
While germline and somatic inactivation of NF1 is associated with malignant peripheral
nerve sheath tumors11 and GISTs in Neurofibromatosis type 1 patients12, no somatic
NF1 alterations have been reported in other sarcomas. We detected six point mutations
and twelve genomic deletions encompassing the NF1 locus, occurring in both myxofibrosarcoma
and pleomorphic liposarcoma (Table 2 and Figure 1, 2A–B; copy number analysis discussed
further below). Two of the mutations, R304* and Q369*, were previously reported as
germline mutations in patients with Neurofibromatosis type 113,14, while the other
four mutations (three missense and one nonsense) have not been previously reported.
In some tumors, biallelic inactivation was evident, with heterozygous point mutations
accompanied by deletion of the wild-type allele and correspondingly reduced gene expression
compared to normal adipose tissue15 in most cases (Figure 2B). Together, these data
indicate a diverse pattern of NF1 aberrations in myxofibrosarcomas and pleomorphic
liposarcomas. These results complement recent reports of NF1 alterations in lung cancers
and glioblastomas7,8.
PIK3CA, encoding the catalytic subunit of phosphatidylinositol 3-kinase (PI3K), had
one of the highest somatic mutation frequencies among the genes in this analysis (Table
2). Nucleotide substitutions in PIK3CA were initially detected in 4 of 21 myxoid/round-cell
liposarcomas (MRCs). We measured the frequency of point mutations in PIK3CA in this
subtype by genotyping an independent cohort of 50 MRCs16 for 13 common sites of PIK3CA
mutation, including those discovered in our initial sequencing; mutations were detected
in 9 additional patients (in total, 13 of 71). The mutations were clustered in two
domains, the helical domain (E542K and E545K) and the kinase domain (H1047L and H1047R)
(Table 2); both these domains are also mutated in epithelial tumors17.
MRC patients whose tumors harbored mutations in PIK3CA had a shorter duration of disease–specific
survival than did those with wildtype PIK3CA (p=0.036, log-rank test). Similar to
observations in breast cancers18, patients with helical-domain PIK3CA mutations had
worse outcomes than those with kinase-domain mutations (Figure 3A). However, this
difference was not statistically significant given the small number of cases in our
study.
As both helical- and kinase-domain PIK3CA mutants are believed to activate Akt, although
through different mechanisms19–21, we assessed Akt activation in MRC tumors harboring
wildtype and mutated PIK3CA. Of note, only E545K helical-domain mutations were associated
with increased Akt phosphorylation relative to wildtype, both at serine-473 and threonine-308
(TORC2 and PDK1 phosphorylation sites, respectively), and with increased phosphorylation
of Akt substrates PRAS40 and S6 kinase (Figure 3B). Surprisingly, tumors with H1047R
kinase-domain mutations did not have similar increases in Akt phosphorylation or activation
(Figure 3B). However, H1047R-mutant tumors exhibited variably higher levels of PTEN,
a negative regulator of PI3K activity, which may partly explain lower Akt activity.
In addition, we detected a single MRC tumor with homozygous PTEN deletion and high
Akt phosphorylation levels (data not shown). Further studies are needed to determine
the relationship between activated PI3K signaling (resulting from PIK3CA mutations)
and the pathognomonic t(12;16)(q13;p11) translocation in this subtype.
In addition to sequencing, we characterized the spectrum of genomic aberrations in
soft tissue sarcoma with 250K single nucleotide polymorphism (SNP) arrays for somatic
copy number alterations (SCNAs: n=207; Figure 1 and Supplementary Figure 2A) and loss-of-heterozygosity
(LOH) (n=200; Supplementary Figure 2B) and with oligonucleotide gene expression arrays
(n=149) (see Methods). The patterns of statistically significant SCNAs22,23 (Figure
1) revealed substantial differences between subtypes with simple and complex karyotypes
(Figure 1). Myxoid/round-cell liposarcoma, synovial sarcoma, and GIST had relatively
normal karyotypes compared to dedifferentiated and pleomorphic liposarcoma, leiomyosarcoma,
and myxofibrosarcoma. In addition, only the four complex subtypes harbored significant
copy-neutral LOH (Supplementary Figure 2B and Supplementary Table 4). These types
exhibit varied levels of complexity: both dedifferentiated liposarcoma and leiomyosarcoma
are less complex than pleomorphic liposarcoma and myxofibrosarcoma (Figure 1). The
latter two subtypes were strikingly similar (Figure 1 and Supplementary Figure 2A),
indicating they might appropriately be considered a single entity in a molecular classification,
as previously suggested24.
Our copy number profiling revealed both focal and broad regions of recurrent amplification
(Supplementary Table 5). The alteration with the highest prevalence in any subtype
was chromosome 12q amplification in dedifferentiated liposarcoma (~90%; Figure 1 and
Figure 4A). As amplification is a common mechanism of oncogenic activation, we designed
an RNA interference (RNAi) screen to help identify genes in amplified regions that
are necessary for cancer cell proliferation in this subtype. We performed knockdown
with short hairpin RNAs (shRNA) on 385 genes (Supplementary Table 2) in three dedifferentiated
liposarcoma cell lines (LPS141, DDLS8817, and FU-DDLS-1) with copy number profiles
similar to those observed in primary tumors of this subtype. A total of 2,007 shRNA
lentiviruses, a median of five per gene, were tested for their effects on cell proliferation
after 5 days (see Methods).
Using a statistical method, RSA (see Methods, Supplementary Note, and ref. 25), we
identified 99 genes whose knockdown significantly decreased cell growth in at least
one cell line (nominal p<0.05; Supplementary Table 6). For 91 of the 99 genes, two
or more independent shRNAs had anti-proliferative activity, reducing the likelihood
that our results are due to off-target effects. To determine whether the effect of
gene knockdown on cell proliferation was specific for dedifferentiated liposarcoma,
we compared our results to a pooled shRNA screen of ~9500 genes in 12 cancer cell
lines of different types26 which included 58 of the 99 genes whose knock-down reduced
proliferation. Only one of the 58 genes, PSMB4, was identified as a common essential
gene, for which depletion reduced cell proliferation in ≥8 of 12 cancer cell lines
in the prior study26.
27 of the 99 genes whose knockdown reduced proliferation were amplified in at least
one of the three dedifferentiated liposarcoma cell lines used in our study (Supplementary
Figure 3). Among these 27 genes, the most strongly overexpressed in dedifferentiated
liposarcoma compared to normal fat15 was CDK4, a cell-cycle regulator and a known
oncogene27. We confirmed that sustained knockdown of CDK4 (>10 days) inhibited proliferation
when we assayed two of the three cell lines we screened (see Methods, Figure 4B).
Furthermore, pharmacological inhibition of CDK4 in dedifferentiated liposarcoma cells
with PD0332991, a selective CDK4/CDK6 inhibitor currently in clinical trials28, induced
G1 arrest in the same two cell lines (Figure 4C).
For MDM2, another oncogene found in focal 12q amplifications, knockdown did not significantly
impair proliferation in our arrayed screen in any of the three cell lines tested.
Nevertheless, proliferation was impaired by subsequent knockdown lasting more than
a week when we assayed two of those three cell lines (Figure 4D). Interestingly, another
gene whose knockdown reduced proliferation of cells in which it was amplified was
YEATS4 (GAS41), encoding a putative transcription factor that represses the p53 tumor
suppressor network during normal cell proliferation29. YEATS4, frequently co-amplified
with MDM2 (Figure 4A), was transcriptionally upregulated both in tumors relative to
normal adipose tissue and in tumors with amplification compared to those copy-neutral
for the locus (Supplementary Figure 3). Repeat shRNA experiments confirmed the effect
of YEATS4 knockdown seen in the arrayed screen (Figure 4E), consistent with the hypothesis
that YEATS4 and MDM2 amplification may cooperatively repress the p53 network in dedifferentiated
liposarcoma, as recently suggested30. This finding may have consequences for Nutlin-based
antagonism of the p53-MDM2 interaction15,31 in dedifferentiated liposarcomas. Our
findings lend additional support for YEATS4 serving as a likely key amplified gene
in cancer, as recently suggested through a weight-of-evidence classification scheme
proposed for identifying such amplified cancer genes32.
This dataset provides the most comprehensive database of sarcoma genome alterations
to date, revealing genes and signaling pathways not previously associated with this
group of diseases. The study results are available as a community resource that might
further the biological understanding of sarcomas and, eventually, shed light on additional
strategies to improve patient care. Some of our findings already have potential therapeutic
implications. For instance, the PIK3CA mutations found in MRC constitute the first
report of such mutations in a mesenchymal cancer. These mutations identify a subset
of tumors that might respond to treatment with PI3K inhibitors currently in clinical
trials33. Our results also provide further rationale for use of CDK4 inhibitors in
dedifferentiated liposarcoma and suggest the use of mTOR inhibitors in NF1-deficient
sarcomas, since loss of NF1 function appears to cause mTOR pathway activation34. Finally,
these data lend support for the clinical evaluation of agents targeting the p53/MDM2
interaction in dedifferentiated liposarcoma.
This work argues for the therapeutic importance of genomic alterations in sarcoma
and encourages us to pursue next-generation sequencing strategies that will continue
to define the landscape of genomic aberrations in these deadly diseases.
Methods
Methods and any associated references are available in the online version of the paper
at http://www.nature.com/naturegenetics/.
Supplementary Material
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