Gastrointestinal stromal tumors (GIST) are the most common tumors of mesenchymal origin in the gastrointestinal tract and are a separate clinical entity, distinct from sarcomas of myogenic or neurogenic origin in the gastrointestinal tract.The concept of GIST was first introduced by Mazur et al. in 1983, and its molecular features were discovered by Hirota et al. a decade ago. In the last decade, there has been deep research and faster progress on the origin, gene expression and mutation, immunohistochemical method (immunohistochemistry) detection, histological diagnosis and diagnosis and treatment of benign and malignant GIST, and prognosis.
1.Progress in molecular biology
1.1 Histological definition
GIST is currently defined as histologically enriched with spindle cells, epithelioid cells, occasional or pleomorphic cells in a fascicular, diffuse arrangement, immunophenotypically expressing KIT protein (CD117), nestin, and proteins of unknown function (discovered on GIST 1, DOGl), genetically present with frequent c-kit genes and platelet-derived growth factor receptor (PDGFR-α) mutations, with a broad spectrum of biological behaviors, probably originating from cajal cells or naive mesenchymal cells (mesenchymal stem cells) that are more primitive than cajal cells and differentiated toward cajal [1]. cajal cells can also express CD34, and when CD117 is not expressed CD34 positivity has diagnostic reference value. It has been reported that protein kinase C (protein kinase C-theta, PKC-θ) protein is also highly specific for GIST, even when c-kit and PDGFRα mutations are negative [2, 3].
1.2 Gene mutations and pathogenesis
Mutations in GIST include mutations in the proto-oncogene c-kit and PDGFRα genes. Most GISTs occur from c-kit gene mutations. c-kit mutations mainly occur in exon 11 (exon 11) in the proximal membrane region, followed by exon 9 (exon 9) in the outer membrane region, and exons 13, 14, and 17 (exon 13, 14, and 17) in the tyrosine region can also be mutated. GIST with exon 11 mutations can be located in the small intestine or in the stomach, and the pathological pattern is more often spindle-shaped cells and mixed cell types. Exon 9 mutations account for about 3% to 21% of cases, and exon 9 mutations are mainly located in the small intestine. In general, there is only 1 locus of c-kit mutation in a given tumor, and cases with 2 or more different mutated loci present simultaneously are extremely rare. c-kit mutations are also present in GISTs smaller than 1 cm and found incidentally, and there is debate about the early oncogenic role of the mutation in the progression of these GISTs [4]. The prognostic role of c-kit mutations is unclear due to conflicting results. From recent data, typical mutations are observed in approximately 8% to 50% of large tumor GISTs, with a mutation frequency of approximately 35%, much higher than previously reported [5, 6].
The presence of activating mutations in the PDGFRα gene in nearly 35% of c-kit mutation-negative GISTs may be another cause of GIST development, especially playing an important role in tumor formation in c-kit mutation-negative tumors. mutations in PDGFRα are functionally acquired or non-functional, and mutations can occur not only in tumor tissue but also in normal tissue. mutations in PDGFRα PDGFRα mutations are mainly located in exons 12 and 18, which are mutually exclusive with c-kit mutations. c-kit mutations are absent in GIST with PDGFRα mutations. PDGFRα mutations mostly occur in the stomach, and the pathological morphology is mostly in epithelial and mixed cells, which are less malignant.
About 10%-15% of GISTs have no mutations in either c-kit or PDGFRα genes, i.e., “wild-type” GISTs, and the mechanism of their occurrence is unknown. It is thought that in these GISTs, tyrosine kinase is activated despite the absence of c-kit mutation.
1.3 Biological behavior
In the early days of the GIST concept, many literatures considered GISTs to be classified as benign or malignant. However, as it was found in the clinic that GIST considered benign could also recur or metastasize, the benign-malignant classification was discarded by most scholars. Currently, GIST is considered to have no absolute benignity and is a tumor with potentially malignant behavior. Its biological behavior is difficult to predict, and the most valuable reference indicators are the size, nuclear division index and anatomical location of the tumor. Since the grading criteria proposed by Fletcher are widely used in clinical practice, tumor size and nuclear division number are still the most recognized indicators to determine the malignancy of GIST. In recent years, studies have found that tumor site is also an independent predictor of recurrence after primary GIST resection, and the malignancy of GIST at different sites with the same tumor size and nuclear division number is not exactly the same (the highest recurrence rate after small intestine GIST), and new grading criteria combining the site of GIST occurrence have been proposed and gradually accepted by clinicians.
It was found that different mutation sites were also associated with malignancy of GIST, with higher recurrence rates after surgery for GIST with c-kit exon 9 and 11 mutations, and intermediate recurrence rates for wild type. The apoptosis-regulating genes IGF and IGFR are hot spots in oncology research in recent years, and Bracconi et al [13] reported that high-risk GISTs with insulin-like growth factor 1/2 (IGF1 and IGF2) expression had higher recurrence rates after surgery. In China, Wang Lin et al. reported significant differences in the positive rates of IGF1 and IGF2 between the benign, low-grade malignant and malignant groups, suggesting that high IGF expression suggests increased malignant potential.
In general, among many possible influencing factors, nuclear division count is the best indicator to predict the risk of tumor malignancy, and combined with tumor size and location will be more accurate in predicting the malignancy of GIST. As new molecular markers will continue to be discovered and refined, the biological behavior of GIST will be further understood. Although we agree that all GISTs are potentially malignant, this agreement may now change with progress.
2. Clinical progress
2.1 Preoperative biopsy
The diagnosis of GIST is usually obtained on preoperative biopsy or at the time of emergency surgery. Transabdominal surgical biopsy should eventually be considered when histologic samples are not available, especially when small submucosal tumors are not available at the time of endoscopic biopsy. The conventional wisdom has been that GIST is mostly discouraged from biopsy due to the possibility of tumor breakdown and spread caused by biopsy. However, with the advent of the effective targeted drug Imatinib, percutaneous transluminal biopsy and intraoperative frozen section biopsy are feasible when the tumor is estimated to be unresectable. Biopsies can provide a definitive diagnosis or detect c-Kit and PDGFRα to provide a pathological and pharmacological basis for the next step of imatinib treatment [15].The 2007 edition of the NCCN guidelines suggests that treatment with imatinib after biopsy can be considered as long as it is done to avoid functional impairment caused by surgery. Although opposition still exists [16], more and more surgeons have slowly started to accept and apply preoperative biopsy.
2.2 Surgical treatment
As conventional chemotherapy and radiotherapy are extremely ineffective, with a general efficacy of less than 5%, surgical resection is the only curative method for GIST. The first surgery must be a complete resection of the tumor, and non-contact surgical resection must be performed intraoperatively following the principle of tumor-free operation, as well as preventing tumor rupture and obtaining negative margins (R0 resection). Since lymph node metastases are rare in GIST and many retrospective studies suggest that lymph node dissection does not improve survival and reduce recurrence rates, and there have been no reports of jumping lymph node metastases in GIST, surgery for GIST is not advocated for routine extended resection or regional lymph node dissection [17, 18]. Whether patients with positive cut margins (R1 resection) should be reoperated or treated with imatinib adjuvant therapy has also been debated.
The adoption of minimally invasive surgery including intra-laparoscopic resection and combined laparoscopic and endoscopic resection techniques is still controversial. Domestic and foreign scholars have made attempts in this area, and it has been reported in the literature that the mean operative time and mean hospital stay for laparoscopic surgery are significantly shorter than those for open surgery, and the long-term follow-up recurrence rate is essentially the same as that for conventional open surgery [19, 20]. However, in 2005, the European Society of Medical Oncology suggested that in order to avoid the high risk of tumor rupture, peritoneal dissemination, and postoperative peritoneal metastasis, laparoscopic surgical resection should be considered only when the tumor is ≤2 cm in diameter [18]. Therefore, the clinical work should decide the surgical approach according to the actual situation of the patient.
The debate of en-bloc resection lies in the safety margin, i.e., how many centimeters from the tumor is appropriate, because there are still issues of surgical comorbidity, postoperative function, and patient tolerance to be considered, and the scope of surgery should be appropriate, so the choice is different for different sites.
If the tumor invades or infiltrates the neighboring organs, it is not advisable to separate the tumor in order to remove it completely, and only joint organ resection can be performed. Some scholars believe that as long as the resection is clean, the postoperative local recurrence rate of combined visceral resection and local resection is similar, and the overall survival rate of multiple visceral resection is also reduced. Some even propose resection of the greater omentum or peritoneal dissection for complete surgery.
2.3 Targeted therapy
With imatinib (Imatinib) and sunitinib (Sunitinib) becoming the standard first- and second-line therapeutic agents for GIST, respectively, GIST has become a paradigm of targeted therapy for solid tumors. Because imatinib is highly effective in advanced GIST, several phase III trials have now been conducted for adjuvant treatment with imatinib after resection of the primary GIST. Other than imatinib and sunitinib, the NCCN guidelines do not recommend other agents for GIST treatment.
2.3.1 Imatinib for advanced/progressive GIST
Before the advent of imatinib, the prognosis of GIST was poor, with a median OS of only 9 to 18 months in most studies, and the chance of cure was almost nil even with complete resection of the lesion. The adoption of imatinib has been a fantastic change in the natural history of recurrence and metastasis in GIST resistant to radiation and chemotherapy. Imatinib has become the new standard of care for patients with advanced/progressive GIST.
Based on the results of North American S0033 and European EORTC62005 [21, 22], the NCCN guidelines recommend an initial dose of 400 mg/d for advanced/progressive GIST, and treatment with imatinib should be continued until disease progression or intolerance, even in patients who obtain CR. It is worth noting that although patients with c-kit exon 11 mutations are more sensitive to imatinib, there is no significant benefit of dose adjustment in this group of patients, whereas patients with c-kit exon 9 mutations can benefit significantly from imatinib dose increases [15]. However, there is controversy as to whether imatinib dose adjustment or direct switch to sunitinib is preferred. By detecting mutation types such as c-kit and PDGFRA, drug therapy for progressive GIST will be more targeted to minimize the proportion of primary drug resistance.
Several studies have shown that the effect of surgical treatment is positive for patients with progressive GIST after neoadjuvant therapy with TKI agents [23-26].NCCN guidelines also emphasize the application of imatinib for neoadjuvant therapy and the possibility of surgical treatment for some patients. It is currently considered that all patients without extensive progression after neoadjuvant therapy should be aggressively treated with surgery. Even those with local progression (secondary drug resistance) should have aggressive surgery. Of course, combined with bleeding, perforation, obstruction, etc., there are indications for emergency surgery.
The optimal timing of surgery after neoadjuvant therapy has also been debated. Since secondary drug resistance usually appears about 2 years after imatinib treatment, surgery should usually be performed within 2 years of imatinib treatment. Most scholars recommend that surgery should be considered only after 6-12 months of disease stabilization or remission after imatinib treatment, i.e. after the appearance of maximal effect, but it is difficult to judge the point of maximal effect in clinical practice, and most surgeons emphasize early resection when they can, so as to avoid poor surgical effect after drug resistance, or even loss of surgical opportunity due to drug resistance outbreak.
Patients with progressive GIST can also try to take targeted drugs while performing local treatment such as intervention and radiofrequency. The author had a patient who had mild shrinkage of the lesion and stabilized it for another 4 months after imatinib 600mg/d treatment plus interventional embolization. If liver metastasis occurs, complete resection of the primary and metastatic foci is the primary principle, and local treatment such as combined radiofrequency and hepatic artery embolization is considered for unresectable ones according to the specific situation.
2.3.2 Adjuvant treatment of primary GIST with imatinib
Although approximately 85% of patients with primary GIST can undergo radical surgical resection, the recurrence rate is higher than 50% within 2 years after surgery, and the 5-year survival rate is around 50%. How to reduce the recurrence rate after radical resection has been a difficult and hot topic in the field of GIST treatment. Although it is inconclusive whether imatinib is routinely applied as adjuvant therapy after GIST, it should be used as adjuvant therapy in patients with high risk of recurrence.
Three large international phase III trials led by ACOSOG, EORTC and SSG/AIO are underway. The Z9001 trial by ACOSOG in the United States showed a 97% one-year survival rate in the dosing group compared to 83% in the control group. In the subgroup analysis, the difference was significant in the medium- and high-risk group with tumors >6 cm, and the application of imatinib for postoperative adjuvant therapy resulted in a more significant benefit in RFS (96% vs. 67%-86%) [28, 29], so postoperative adjuvant therapy has become a consensus. The same results were obtained in 16 units of adjuvant imatinib in China organized by Professor Zhan Wenhua [30], and the historical control data reported abroad also suggest the importance of adjuvant therapy [31], so it is correct to apply adjuvant therapy to postoperative high-risk patients. Despite the remarkable, these results are still preliminary and it is not fully accepted whether adjuvant therapy with imatinib becomes the current standard choice.
The optimal duration of imatinib adjuvant therapy is still unknown, and the SSG/AIO trial with randomized 1- and 3-year treatment may help to explore the duration issue. 62024 trial with 2-year duration treatment will provide important information on this issue.
Since there are better prognostic factors than size and nuclear split count that have a greater impact on postoperative recurrence, it is crucial that adjuvant therapy is further investigated in terms of how patients are to be selected. For example, large tumors of the stomach (including >10 cm) with low nuclear fractionation (≤5/50 HPF) are now also considered to have a low risk of recurrence (≤10%) and may not require adjuvant therapy; whereas duodenal tumors and small bowel tumors with tumors >5 cm or nuclear fractionation >5/50 HPF, <5 cm but high nuclear fractionation phase may be recommended for adjuvant therapy with imatinib. This requires further meta-analysis.
2.3.3 Evaluation of the efficacy of imatinib
CT and PET/PET-CT are mostly used to evaluate the efficacy of imatinib. CT is generally preferred, but the onset of imatinib is mostly 3-6 months, with a median time of about 4 months, and the median time for CT to detect tumor shrinkage also takes 3-4 months, while PET can detect the tumor response to imatinib within hours or days, which is often consistent with the remission of clinical symptoms, so it is considered that PET/PET-CT is considered to be more preferable to assess the efficacy 2~4 weeks before and after treatment.
Currently, CT evaluation is mostly performed before and about 1 month after targeted therapy in clinical practice, and individualized imaging data of patients are established for subsequent comparison, and thereafter every 3 months as an examination cycle, which should be reviewed promptly when symptoms or signs worsen. When 2 consecutive CT results do not suggest further improvement, it is suggested that Imatinib treatment has reached its maximum effect, and this is a more appropriate time for surgery. At present, the Choi criteria have largely replaced the RE-CIST (Response Evaluation Criteria in Solid Tumors) criteria, which were mainly evaluated by the change of tumor diameter measured by CT, but did not reflect the efficacy of TKI well; while the Choi criteria integrated tumor diameter and tumor density (CT value) to judge the efficacy of TKI, which can better correspond to the results of PET and predict the progression and survival of the disease. The Choi criterion, however, combines tumor diameter and tumor density (CT value) to determine TKI efficacy, which can better correspond to PET results and predict disease progression and survival [32, 33].
2.3.4 Imatinib resistance studies
Resistance to imatinib exists in both advanced and postoperative GISTs, although most patients are initially treated effectively.Resistance to imatinib in GISTs is both primary and secondary.
Primary resistance is rare, i.e. tumor growth occurs within the first 6 months of treatment, and is 10%-15%. primary resistance to imatinib may be related to the mechanism of occurrence of GIST. the B2222 trial also showed that patients with mutations in wild-type GIST, c-kit exon 9 and PDGFRα exon 18 (D842V) are prone to primary resistance. Secondary mutations leading to primary resistance are relatively rare, and about 10% of primary resistance is due to new mutations in addition to the original c-kit or PDGFRα mutations [34]. Some scholars believe that there may be a KIT activation pathway that does not require kinase involvement.
Secondary resistance occurs after 6 months of treatment, and approximately 50% of GIST patients initially sensitive to imatinib develop secondary resistance after 2 years of dosing. The main reason for its occurrence is acquired secondary c-kit or PDGFRα gene mutations. The secondary mutations are mostly located near the ATP binding site or KIT kinase activation loop encoded from c-kit, and these secondary mutations alter the KIT construct, hiding the imatinib binding site and leading to drug resistance. Secondary mutations mainly occur in GIST with exon 11 mutations, while GIST with exon 9 mutations are relatively uncommon. A study [35] showed that 73% of GISTs resistant to imatinib developed new mutations, and the new mutations were mainly missense mutations in the tyrosine kinase region, with the main mutation sites located in c-kit exons 13, 14, 17 or 18.
Bauer et al [36] suggested that abnormal activation of KIT is another important cause of secondary resistance to imatinib. burger et al [37] found that long-term oral imatinib can reduce intracellular imatinib concentration by more than 50% by upregulating cellular expression of ABC transporter proteins ABCG2 and ABCB1, which upregulate cellular drug pump expression and thus contribute to intracellular imatinib efflux. Tarn et al [38] found the presence of insulin-like growth factor 1 receptor (IGF-1R) amplification in imatinib GISTs, and targeting IGF-1R has the potential to provide new targets for the treatment of GISTs, especially those insensitive to imatinib. Other mechanisms such as plasma glycoprotein acids and increased expression of multidrug resistance genes may be associated with imatinib resistance.
Several drugs that may be effective in imatinib-resistant GIST (Sunitinib, Nilotinib, etc.) are currently under investigation or in clinical trials. New generation TKI agents are characterized by multi-targeted action and usually possess some anti-neoangiogenic ability. Sunitinib significantly prolongs progression-free survival and improves overall remission rates in patients with GIST who have failed or are intolerant to imatinib therapy [39]. However, sunitinib has more side effects than imatinib and may lead to adverse effects that do not occur with imatinib, such as hypertension, impairment of cardiac function and thyroid function, but is still generally well tolerated by patients.In January 2006, the US FDA approved sunitinib as a second-line agent for the treatment of imatinib-resistant progressive GIST, and sunitinib was also officially launched in China in June 2008. In addition, the application of protein kinase inhibitor PKC-412 and marine biological extract ET-743 in progressive GIST is also under further study.
2.4 Genetic testing
The importance of detecting the type of gene mutation in GIST patients is now increasingly emphasized, both in terms of diagnosis to treatment and evaluation of prognosis. It is more important to perform genetic testing, especially when immunohistochemical results cannot be established, or in cases with definite CD117 negativity, or to diagnose familial GIST, and to evaluate pediatric GIST; the choice of clinical tyrosine kinase inhibitors, treatment response and prognosis differ depending on the gene locus where the mutation occurs. For example, the detection of c-kit exon 9 and 11 mutation status is currently the most important predictor of imatinib resistance. Imatinib was found to have a significantly higher objective remission rate and PFS in patients with c-kit exon 11 mutations than in patients with wild-type and exon 9 mutations in clinical practice, and the NCCN also recommends that patients with exon 9 start immediate treatment with imatinib 800 mg/d; also the genotype of GIST facilitates the choice of surgery, and since imatinib is more effective in wild-type GIST than exon 11 mutation exon 9 mutants are lower, such patients should be surgically resected early; even in CD117(+) GIST, c-kit exon 11 mutants are generally sensitive to imatinib, and exon 9Ala502_Tyr503 mutation and PDGFRα exon 18Asp842Val mutants are not sensitive to imatinib [34, 40].
In conclusion, genetic testing brings us more and more information and should be performed in all patients with GIST. genetic testing is even more essential in patients with CD117-negative and clinical high-risk and malignant, recurrent metastatic GIST in order to guide future treatment and evaluate prognosis. However, the prevalence of genetic testing is too low in China due to the conditions, which affects the treatment specification and case statistics of GIST patients, and the promotion of genetic testing for GIST can not be delayed.
In addition to the aforementioned CD117, nestin, protein of unknown function (DOGl) and protein kinase C (PKC-θ) proteins, there are reports summarizing p53, p27, Ki-67, Bax of Bcl-2 protein family, high mobility group protein-1 (HMGB-1), matrix metalloproteinase 2 (MMP2), cyclooxygenase -2 (COX-2), heat shock protein (Hsp) 90, carbonic anhydrase related protein (CA-RP), anti-human telomerase reverse transcriptase antibody and transcription factor E2F1 may be associated with the prognosis of GIST [41].
3. outlook
The research on GIST has made great progress, although the concept of GIST as a separate clinical entity is clearer, the pathogenesis is initially understood, and the treatment is faster and more effective. However, there are still many problems, the histological origin and gene mutation need to be further studied, immunohistochemical criteria, benign and malignant judgment, pathological clinical staging, evaluation of biological behavior and treatment, prognosis judgment, etc. need to form a standard. GIST in the pediatric population has been poorly reported and also needs further study. In the comprehensive treatment of GIST with preferred surgery, the application of specific targeted drugs has a broad prospect, but their treatment norms need to be further summarized, and their potential effects, optimal dose, optimal time frame and drug resistance issues are still the focus of future research. The detection of patients’ blood concentration levels for guiding drug use is also a new hot spot. New targeted gene therapy regimens for GIST based on the RNAi principle have been proposed and may be combined with tyrosine kinase inhibitors to inhibit the c-kit gene at both the protein and mRNA levels, which may yield desirable therapeutic effects.