Greater omentum gastrointestinal stromal tumor with PDGFRA-mutation and hemoperitoneum

Abstract

Although gastrointestinal stromal tumor (GIST) occurs generally in the digestive tract, omental GIST is very rare. We report the first case of an adult greater omental GIST with a new platelet-derived growth factor receptor α gene (PDGFRA)-mutation with hemoperitoneum. A 43-year-old man was admitted to our hospital complaining of acute abdominal pain. Abdominal contrast-enhanced computed tomography revealed a huge mass in the right abdominal cavity, and a large accumulation of fluid in the pelvic cavity, suggesting hemoperitoneum. We diagnosed the rupture as an intra-abdominal tumor, and an emergency tumorectomy was performed with resection of the greater omentum. This tumor was located in the distal right side of the greater omentum, and showed no continuity with the gastric wall. The tumor occurred primarily in the greater omentum. The resected tumor was about 19 cm × 12 cm × 14 cm in diameter, and weighed 1529 g. Histologically, the tumor was composed of epithelioid-shaped cells with high cellularity, and was positive for CD117 and CD34, and negative for S-100, α-smooth muscle actin. The mitosis was 6/50 under high power field. This case showed exon 18 mutation of PDGFRA with 846 (Asp to Glu) substitution, 848 (Asn to Lys) substitution. This is the first report of this PDGFRA mutation in omental GIST, and this might play an important role in the tumorigenesis of this case. Based on these findings, the tumor was diagnosed as high risk GIST primarily occurring in the greater omentum. The patient was treated with imatinib at a dose of 400 mg/d as adjuvant chemotherapy, and has been followed up for 24 mo with no evidence of recurrence.

Key Words: Gastrointestinal stromal tumor, Greater omentum, Hemoperitoneum, Platelet-derived growth factor receptor α gene, KIT

INTRODUCTION

Gastrointestinal stromal tumors (GISTs), while relatively rare, are the most common mesenchymal tumors of the digestive tract and are believed to originate from the interstitial cells of Cajal (ICCs) or from their precursors in the gastrointestinal tract[1,2].

GISTs can occur anywhere in the gastrointestinal tract where ICCs are present, including the stomach (40%-60%), small intestine (30%-40%), anorectum (7%), colon, and esophagus, as well as in extra-gastrointestinal locations such as mesentery, omentum, and peritoneum[1,3]. Omental, mesenteric, and retroperitoneal tumors comprise less than 5%[4-9]. Tumors occurring in the greater omentum are very rare, and the diagnosis of such lesions is difficult. The incidence of primary GIST in the greater omentum has been reported to be less than 1%[10-12].

The majority of adult GIST cases (83.6%-88.2%) have activation mutations in the KIT gene, which codes for the KIT receptor tyrosine kinase. These mutations result in constitutive activation of the receptor, and subsequently cell survival and proliferation, in the absence of ligand binding. In addition, platelet-derived growth factor receptor α gene (PDGFRA) mutations account for a further 2.6%-4.7% of cases while the remaining 7.1%-13.8% are wild types for either receptor[13,14].

Here we report the first case of the greater omentum GIST with the new PDGFRA-mutation and hemoperitoneum.

CASE REPORT

A 43-year-old man was admitted to our hospital for acute abdominal pain. He had no past medical or surgical history, was taking no medications, and never had a previous colonoscopy or upper endoscopy. There was no family history of any gastrointestinal malignancies. His height was 173 cm, weight was 108 kg (BMI: 36). The only positive findings on examination were right lower abdominal pain, tenderness, and defense. Hematological examination found slight anemia with a hemoglobin level of 6.8 g/dL and high levels of C-reactive protein of 4.46 mg/dL. The levels of CEA, CA19-9, and AFP were within the normal ranges. A contrast-enhanced computed tomography (CT) scan of the abdomen and pelvis showed a huge mass in the right abdominal cavity. The interior was heterogeneous, with some high density area, but comprised mostly of low density regions which seemed to be a cystic component or interstitial mucous (Figure 1A and B). Moreover a large amount bloody ascites was found in the pelvic cavity, suggesting hemoperitoneum. We diagnosed the rupture as an intra-abdominal tumor, and an emergency tumorectomy was performed. This tumor was located in the distal right side of omentum. The disconnect was successfully ligated to the greater omentum mass insertion, and the tumor showed no continuity with the gastric wall. The tumor size was oval, 19 cm × 12 cm × 14 cm in diameter, and the weighed 1529 g. There was about 850 mL of bloody asictes in the abdominal cavity. On gross examination, the resected tumor was a dark-red, relatively hard mass, and mixed with partly bleeding blood clots. The cut surface of the tumor showed cystic degeneration, and focal hemorrhage (Figure 2A and B). The tumor occurred primarily in the greater omentum. Tumor invasion to adjacent organs, liver metastasis, peritoneal dissemination, and omental lymph node swelling were not observed. Haematoxylin and eosin (HE) staining and immunohistochemistry were carried out on paraffin wax sections of the formalin-fixed tissues[10]. HE staining showed that the tumor cells were composed of epithelioid-shaped cells with high cellularity (Figure 3A and B). The mitosis was 6/50 under high power field. Immunohistochemical staining showed that the tumor cells were positive for CD117 (Figure 3C), positive for CD34 (Figure 3D), and negative for S-100 and α-smooth muscle actin (α-SMA), consistent with a GIST. Therefore, these features strongly indicate clinical malignancy with abdominal hemorrhage, most likely GIST[4].

Figure 1 Contrast-enhanced axial (A) and coronal (B) abdominal computed tomography. A huge mass was seen in the right abdominal cavity showing internal heterogeneity, with some high density area, but comprised mostly of low density regions which seemed to be a cystic component or interstitial mucous. The high density area was not enhanced, thus hemorrhage was suggested. Moreover a large amount of bloody ascites was seen in the pelvic cavity.

Figure 2 Gross appearance of the resected specimen. A: The tumor appeared as a dark-red, relatively hard mass mixed with partly bleeding blood clots; B: The cut surface of the tumor showed cystic degeneration, and focal hemorrhage.

Figure 3 Histopathological findings of the resected specimen of the tumor. A and B: The tumors were characterized by epithelioid-shaped cells and showed high cellularity (HE staining, A: × 40, B: × 200); C: Immunostaining of CD117 (KIT) was positive; D: Immunostaining of CD34 was positive (C, D: × 200).

Genomic DNA was extracted from the formalin-fixed, paraffin wax-embedded tissues using the QIAamp DNA Mini Kit (QIAGEN, Valencia, CA, USA), and Exon 9, 11, 13, 17 of KIT and exon 12, 14 or 18 of PDGFRA, were amplified using polymerase chain reaction (PCR), as described previously[15] and PCR primers listed in Table 1. This analysis of extracted genomic DNA revealed no mutations of exons 9, 11, 13, 17 of KIT and exons 12 or 14 of PDGFRA. There was, however, exon 18 mutation of PDGFRA with 846 (Asp to Glu) substitution and 848 (Asn to Lys) substitution (Figure 4). Informed consent for the present analysis was previously obtained. Based on these findings, the tumor was diagnosed as high risk GIST primarily occurring in the greater omentum. The patient was treated with imatinib at a dose of 400 mg/d as adjuvant chemotherapy, and has been followed up for 24 mo with no evidence of recurrence.

Table 1 Polymerase chain reaction primers used to analyze c-kit and PDGFRA gene exons.

Gene	Exon	Forward primer	Reverse primer
c-kit	9	TTCCCTTTAGATGCTCTGCTTCT	CCTTTGTTGTTACCTTTAAATGC
	11	GAGTGCTCTAATGACTGAGA	AAAGGTGACATGGAAAGCCC
	13	GCTTGACATCAGTTTGCCAG	AAAGGCAGCTTGGACACGGCTTTA
	17	GTTTTCTTTTCTCCTCCAAC	TGCAGGACTGTCAAGCAGAG
PDGFRA	12	GGACTTTGGTAATTCACCAG	TGTAAAGTTGTGTGCAAGGG
	14	ACAGGAAGTTGGTAGCTCAG	TCACAACCACATGTGTCCAG
	18	ACCATGGATCAGCCAGTCTT	TGAAGGAGGATGAGCCTGACC

Figure 4 Representative sequencing results of omentum gastrointestinal stromal tumor. This case showed a codon 846 (Asp to Glu) substitution, and a codon 848 (Asn to Lys) substitution within exon 18 of PDGFRA gene.

DISCUSSION

In general, GISTs are the most common mesenchymal tumors of the gastrointestinal tract, occurring mainly in stomach, and small and large intestine. They are considered to be derived from the ICCs[1], and the incidence of the primary GIST lesion in the greater omentum is very rare[10-12].

It has been reported that GIST in the mesentery and greater omentum, structures which lack ICCs, are derived from mesenchymal cells that are less differentiated than ICCs[16]. These may be ICC precursors straying into the abdominal cavity[1], or KIT-positive cells similar to ICCs immediately below mesothelial cells in the greater omentum[11] although the precise etiology remains to be clarified. The present case has shown KIT-positive and CD34 positive cells within specimens from the greater omentum. However, the meaning of this is uncertain.

It has previously been reported that omental GISTs are clinicopathologically heterogeneous[17,18]. Patients with solitary tumors usually show gastric GIST-like morphology and have a better prognosis than those with multiple tumors, whose tumors usually show small intestinal GIST-like histology. Omental GISTs unattached to the gastrointestinal tract often resemble gastric GISTs, suggesting that they may be gastric GISTs directly extending into, or parasitically attached to the omentum, whereas multiple omental GISTs more often resemble small intestinal GISTs, suggesting that they may be metastases or derived from this source. Most single omental GISTs are relatively indolent tumors compatible with long-term patient survival, despite large tumor size. The present case was a solitary omental GIST, suggesting overall similarity with gastric GISTs.

Recent studies have established that activating mutations in the KIT gene are present in up to 92% of GISTs. PDGFRA mutations account for a further 2.6%-4.7% of cases, while the remaining 7.1%-13.8% are WT for either receptor[13,14]. These gain-of-function mutations result in constitutive KIT or PDGFRA activation without ligand stimulation and are considered to be a cause of GISTs[18]. The respective oncoproteins exhibit constitutive tyrosine kinase activity and promote cell growth, and might play a central role in GIST pathogenesis[2,18,19]. Imatinib mesylate, a tyrosine kinase inhibitor known to inhibit the activities of BCR-ABL, KIT, and PDGFR, is currently being used for the treatment of both chronic myeloid leukemia and metastatic GIST. In Table 2 we summarize the collected data on PDGFRA mutation primarily from omental GIST case-reports (Table 2)[5,12,17,20-22]. In addition, Miettinen et al[18] reported PDGFRA mutations in 10 cases. There were exon 18 D842V substitutions in 6 cases, exon 18 deletions of 842-845 in 1 case, exon 18 deletions of 841-845 in 1 case, exon 12 substitution V561D in 1 case, and exon 12 deletion of 566-571 in 1 case. Of the 21 cases available for molecular study, 17 cases (81%) had PDGFRA gene mutations at exon 18 and 4 cases (19%) had PDGFRA gene mutations at exon 12. Only case 5 had both PDGFRA12 and KIT11 mutation. The majority of KIT-negative extra-gastrointestinal stromal omental tumors were characterized by epithelioid cell type, predominant PDGFRA mutation genotype, and showed low mitotic activity and a relatively favorable prognosis, despite a large tumor size[18,22]. However, as molecular genetic data has been reported in only a small number of omental GIST, further studies with a larger number of omental GIST cases will be needed. The present case showed exon 18 mutation of PDGFRA with 846 (Asp to Glu) substitution and 848 (Asn to Lys) substitution. This is the first report of this PDGFRA mutation in an omental GIST, and this might play an important role in the tumorigenesis of this case.

Table 2 Clinicopathologic findings in 11 cases of omental gastrointestinal stromal tumor.

Case No.	Age (yr)/sex	Size (cm)	Mitosis/50HPF	Risk group	Cell type	Mutation	Follow up (mo)	Outcome	Ref.
1	73/F	4	31	Low	Myxoid Ep	Exon18 del D842V	4	NED	[5]
2	52/M	> 20	41	High	Ep	Exon18 D842_H845del	13	NED	[5]
3	49/F	17	1	3b	Ep	Exon18 D842_H845del	48	NED	[12]
4	65/M	20	2	3b	Myxoid Ep	Exon12 V561D	6	NED	[20]
5	64/M	15	115	6b	Sp	Exon12 polymorphism Q567P	NA	NED	[21]
6	62/F	11	3	3b	Ep	Exon 18 D842V	87	NED	[22]
7	41/M	19	0	3b	Ep	Exon 18 D842V	27	NED	[22]
8	69/M	8.5	1	3a	Ep	Exon18 del IMHD843-846	20	NED	[22]
9	54/F	20	18	6b	Ep	Exon 18 del DIMH842-845	15	DOD	[22]
10	59/M	33	1	3b	Ep	Exon 18 D842Y	2	DOD	[22]
11	43/M	19	6	6b	Ep	Exon18 D846E, N848K	24	NED	Our case

¹MIB-1-labeling index (%). Tumors were divided into risk groups described previously[18,22]. Case 1 and 2 were graded as high, intermediate, or low risk as described previously[22]. Ep: Epithelioid; Sp: Spindle; NED: No evidence of disease; DOD: Die of disease; NA: Not available.

The mean diameter of omental GISTs has been reported to be about 15.35 cm[17] and was 19cm in the present case. Moreover, mitotic activity, cellularity and presence of necrosis have been found to be associated with worse outcomes. A high mitotic rate (> 5/50 HPF) and a high Ki-67 labeling index (> 10%) indicate a significantly poorer outcome[4]. In the present case, the large size, high mitotic counts and extensive hemorrhagic necrosis were malignant features, so the tumor was diagnosed as malignant GIST. Although peritoneal metastasis was not seen in the present case, we must pay attention to tumor recurrence because the tumor ruptured. PDGFRA may become a molecular therapeutic target of imatinib in at least some populations of omentum GIST. The preliminary clinical data and in vitro studies have demonstrated that GISTs with a PDGFRA D842V substitution were resistant to imatinib, but some populations of other PDGFRA mutants were sensitive[8,23-25]. Todoroki et al[20] and Kim et al[21] used STI-571 as adjuvant postoperative treatment. Because the PDGFRA genotype in this case might be sensitive to imatinib, the patient was treated with adjuvant chemotherapy imatinib at a dose of 400 mg/d before possible tumor recurrence was identified. This patient remains alive without disease 24 mo after surgery. We will carefully follow up with CT and expect to conduct further clinical investigation.

In conclusion, we have reported the first case of the greater omentum GIST with a new type of PDGFRA-mutation and hemoperitoneum. Both immunohistochemical evaluation and molecular analysis are necessary not only to confirm the diagnosis, but also to determine the therapeutic strategy. The existing data on greater omentum GIST is not sufficient to make a firm conclusion on the prognosis and survival. Further studies are necessary to distinguish between the PDGFRA-mutation and clinicopathologic factors or biologic behaviors.

ACKNOWLEDGMENTS

We greatly appreciate Hikari Kishimoto for assisting with the preparation of the manuscript.