Human PDGFA receptor gene maps to the same region on chromosome 4 as the KIT oncogene

Molecular Pathogenesis of Gastrointestinal Stromal Tumor: A Paradigm for Personalized Medicine

Over the past three to four decades, the molecular pathogenesis of gastrointestinal stromal tumors (GISTs) has been elucidated in great detail. In this review, we discuss the biological genesis of GISTs, identification of the various primary activating driver mutations (focusing on KIT and PDGFRA), oncogene addiction and targeted therapies with imatinib and other tyrosine kinase inhibitors, and the subsequent characterization of the various mechanisms of drug resistance. We illustrate how GIST has become a quintessential paradigm for personalized medicine. Expected final online publication date for the Annual Review of Pathology: Mechanisms of Disease, Volume 17 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Inflammatory Fibroid Polyp of the Gastrointestinal Tract: A Systematic Review for a Benign Tumor

BACKGROUND/AIM

Inflammatory fibroid polyp (IFP) is a rare, usually solitary and intraluminal polypoid benign tumor that can affect any part of the gastrointestinal (GI) tract, although in the majority of cases it affects the stomach. This lesion is characterized by proliferation of highly vascular fibrous tissue and infiltration by a variable number of different inflammatory cells. Its etiology is unknown. Our aim was to describe all the reported data concerning IFP.

MATERIALS AND METHODS

An extensive search of the PubMed Index was performed for publications with titles or abstracts containing the terms: "inflammatory fibroid polyp" with/without "Vanek". Results were filtered for publications in English and concerning only humans. One hundred and twenty-four publications were finally included in this review.

RESULTS

IFP has a female predominance. It affects patients in their 5th decade of life, although there are cases of patients from 4 to 84 years of age. IFP usually affects the stomach and more specifically the gastric antrum but can be detected throughout the GI tract. A significant number of cases remain asymptomatic but the most frequent presentations of IFP are abdominal pain, acute abdomen and GI bleeding. Most cases are treated by endoscopic resection of the lesion. No recurrence nor IFP-specific complications have been reported. Histopathology of IFP varies.

CONCLUSION

It is relatively safe to conclude that both the etiology and the timing of diagnosis might change the histopathology, immunohistological staining and tissue structure of IFP. Suggested theories should be taken into consideration with caution as the etiology and pathophysiological mechanisms of IFP are unknown.

Genetics of Gastrointestinal Stromal Tumors: A Heterogeneous Family of Tumors?

Approximately 85-90% of adult gastrointestinal stromal tumors (GISTs) harbor KIT and PDGFRA mutations. The remaining cases, including the majority of pediatric GISTs, lack these mutations, and have been designated as KIT/PDGFRA wild-type (WT) GISTs. Nearly 15% of WT GISTs harbor BRAF mutations, while others arise in patients with type I neurofibromatosis. Recent work has confirmed that 20-40% of KIT/PDGFRA WT GISTs show loss of function of succinate dehydrogenase complex. Less than 5% of GISTs lack known molecular alterations ("quadruple-negative" GISTs). Thus, it is important to consider genotyping these tumors to help better define their clinical behavior and therapy.

Molecular characterisation of gastrointestinal stromal tumours in a South African population

Gastrointestinal stromal tumours (GISTs) are the most common mesenchymal tumours of the digestive tract. Pathogenesis is linked to activating mutations identified in two proto-oncogenes, v-kit Hardy/Zuckerman 4 feline sarcoma viral oncogene homologue KIT (KIT) and the platelet-derived growth factor α (PDGFRα). In addition, these mutations affect response to treatment with tyrosine kinase inhibitors. In the present study, we report on the molecular characterisation of GISTs in the South African population. Tumour DNA was extracted from 46 GIST samples, followed by cycle sequencing of KIT exons 11, 13 and 17 and PDGFRα exons 12, 14 and 18. Fragment length analysis was used to detect a 6-bp duplication in KIT exon 9. Wild-type duplications were analysed further by PCR and sequencing of additional KIT and PDGFRα exons was performed. Overall, 78.3% of the samples had a mutation in KIT or PDGFRα. Of these, mutations were detected in KIT exon 11 (88.9%), PDGFRα exon 18 (8.3%) and KIT exon 9 (2.8%). Mutations varied from simple substitutions and duplications to large deletions (some with nucleotide insertions) resulting in missense mutations. In addition, seven single nucleotide polymorphisms were detected in 17 patients, one of which appears novel. The incidence of mutations in KIT exon 11 and PDGFRα exon 18 is consistent with the literature, however, the low incidence of KIT exon 9 mutations detected was unexpected. In contrast to previous western and Asian studies, this mutation appears to be rare in the South African population. The present study contributes to the molecular understanding of GISTs in the South African population.

Gastrointestinal stromal tumor: advances in diagnosis and management

CONTEXT

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal neoplasms of the gastrointestinal tract and should be differentiated from other mesenchymal tumors. They harbor specific activating mutations in the KIT or platelet-derived growth factor receptor α ( PDGFRA ) receptor tyrosine kinases, which makes them responsive to pharmacologic inhibitors, such as imatinib mesylate and sunitinib malate.

OBJECTIVES

To provide a comprehensive review of the pathogenesis of GIST and the underlying principles of targeted therapy, to review the salient histologic and immunohistochemical features that facilitate the distinction of GIST from other mesenchymal neoplasms of the gastrointestinal tract, and to present the prognostic parameters for risk stratification that guide clinical management.

DATA SOURCES

Review of the English literature through PubMed as well as personal experience. Photographs were taken from cases encountered at the Cleveland Clinic.

CONCLUSIONS

The discovery of the KIT -GIST connection has not only improved the diagnostic accuracy of GISTs but also provided us with a better understanding of the histogenesis and molecular pathogenesis of these neoplasms.

Gastrointestinal stromal tumors: molecular mechanisms and targeted therapies

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal neoplasms of the gastrointestinal tract and are diverse not only in their clinical behavior but also in their histologic appearance. GISTs are insensitive to conventional sarcoma chemotherapy and radiation. However GISTs are sensitive to small-molecule tyrosine kinase inhibitors as 85–90% of GISTs have KIT or platelet-derived growth factor receptor alpha (PDGFRA) mutations, which drive tumorigenesis. This review will briefly touch on the clinicopathological features of GIST, while the majority of the review will focus on the clinical and treatment ramifications of KIT and PDGFRA mutations found in GIST.

Gastrointestinal stromal tumors

Gastrointestinal stromal tumors (GISTs) have emerged from being poorly defined, treatment-resistant tumors to a well-recognized, well-understood, and treatable tumor entity within only one decade. The understanding of GIST biology has made this tumor a paradigm for molecularly targeted therapy in solid tumors and provides informative insights into the advantages and limitations of so-called targeted therapeutics. Approximately 85% of GISTs harbor activating mutations in KIT or the homologous receptor tyrosine kinase PDGFRA gene. These mutations are an early event in GIST development and the oncoproteins serve as a target for the small molecule tyrosine kinase inhibitors imatinib and sunitinib. The existing and emerging treatment options demand exact morphologic classification and risk assessment. Although, KIT (CD117) immunohistochemistry is a reliable diagnostic tool in the diagnosis of GIST, KIT-negative GISTs, GISTs showing unusual morphology as well as GISTs which progress during or after treatment with imatinib/sunitinib can be a challenge for pathologists and clinicians. This review focuses on GIST pathogenesis, morphologic evaluation, promising new immunohistochemical markers, risk assessment, the role of molecular analysis, and the increasing problem of secondary imatinib resistance and its mechanisms.

Imatinib treatment for gastrointestinal stromal tumour (GIST)

Gastrointestinal stromal tumour (GIST) is the most common mesenchymal neoplasm of the gastrointestinal tract. GISTs are believed to originate from intersticial cells of Cajal (the pacemaker cells of the gastrointestinal tract) or related stem cells, and are characterized by KIT or platelet-derived growth factor receptor alpha (PDGFRA) activating mutations. The use of imatinib has revolutionized the management of GIST and altered its natural history, substantially improving survival time and delaying disease progression in many patients. The success of imatinib in controlling advanced GIST led to interest in the neoadjuvant and adjuvant use of the drug. The neoadjuvant (preoperative) use of imatinib is recommended to facilitate resection and avoid mutilating surgery by decreasing tumour size, and adjuvant therapy is indicated for patients at high risk of recurrence. The molecular characterization (genotyping) of GISTs has become an essential part of the routine management of the disease as KIT and PDGFRA mutation status predicts the likelihood of achieving response to imatinib. However, the vast majority of patients who initially responded to imatinib will develop tumour progression (secondary resistance). Secondary resistance is often related to secondary KIT or PDGFRA mutations that interfere with drug binding. Multiple novel tyrosine kinase inhibitors may be potentially useful for the treatment of imatinib-resistant GISTs as they interfere with KIT and PDGFRA receptors or with the downstream-signalling proteins.

Gain-of-function PDGFRA mutations, earlier reported in gastrointestinal stromal tumors, are common in small intestinal inflammatory fibroid polyps. A study of 60 cases

The inflammatory fibroid polyp is a rare benign lesion occurring throughout the digestive tract. It usually forms a solitary mass, characterized by a proliferation of fibrovascular tissue infiltrated by a variable number of inflammatory cells. The etiology of this lesion is unknown and conflicting histogenetic theories have been proposed. Recently, mutations in platelet-derived growth factor receptor (PDGFRA) and PDGFRA expression were reported in gastric inflammatory fibroid polyps. In this study, PDGFRA exons 12, 14, and 18 were screened for activating mutations in 60 small intestinal inflammatory fibroid polyps. In addition, the PDGFRA expression was evaluated immunohistochemically. Mutations in PDGFRA were identified in 33 of 60 (55%) cases, whereas 95% expressed PDGFRA. There were 26 deletions, three deletion-insertions, duplication, and single nucleotide substitution in exon 12, and a single nucleotide substitution and deletion in exon 18. The majority (n=23) of exon 12 deletions were 1837_1851del leading to S566_E571delinsR. However, 1835_1852delinsCGC leading to the same S566_E571delinsR, were found in two tumors. Three inflammatory fibroid polyps had 1836_1850del leading to S566_E571delinsK. A complex deletion-insertion affecting a similar region (1837_1856delinsGATTGATGATC) and leading to S566_I573delinsRIDDL was identified once. In addition, duplication and single nucleotide substitution were found 5' to the common inflammatory fibroid polyp mutational 'hot spot'. These mutations consist of 1808_1828dup leading to I557_E563dup, and 1821T>A resulting in 561V>D substitution. A 2664A>T and 2663_2674del leading to 842D>V and D842_H845del, respectively, were identified in exon 18. Similar gain-of-function PDGFRA mutations reported in gastrointestinal stromal tumors have been considered to be a driving pathogenetic force. This study showed consistent expression and common mutational activation of PDGFRA in small intestinal inflammatory fibroid polyps as in their gastric counterparts, and these lesions should be considered PDGFRA-driven benign neoplasms. We also suggest that these polyps may develop from earlier described PDGFRA-positive mesenchymal cells distributed along the villus membrane after oncogenic PDGFRA activation.

Clinical significance of oncogenic KIT and PDGFRA mutations in gastrointestinal stromal tumours

Gastrointestinal stromal tumours (GISTs) are the most common mesenchymal neoplasms of the gastrointestinal tract. Despite clinicopathological differences, GISTs share oncogenic KIT or platelet-derived growth factor-alpha (PDGFRA) mutations. Imatinib, KIT and PDGFRA inhibitor, has been successfully used in the treatment of metastatic GISTs. There are primary KIT or PDGFRA mutations diagnosed before imatinib treatment, linked to GIST pathogenesis, and secondary mutations detected during treatment, causing drug resistance. KIT exon 11 mutations are the most common. Gastric GISTs with exon 11 deletions are more aggressive than those with substitutions. KIT exon 11 mutants respond well to imatinib. Less common KIT exon 9 Ala502_Tyr503dup mutants occur predominantly in intestinal GISTs and are less sensitive to imatinib. An Asp842Val substitution in exon 18 is the most common PDGFRA mutation. GISTs with such mutation are resistant to imatinib. PDGFRA mutations are associated with gastric GISTs, epithelioid morphology and a less malignant course of disease. GISTs in neurofibromatosis 1, Carney triad and paediatric tumours generally lack KIT and PDGFRA mutations. Secondary KIT mutations affect exons 13-17. GISTs with secondary mutations in exon 13 and 14 are sensitive to sunitinib, another tyrosine kinase inhibitor. KIT and PDGFRA genotyping is important for GIST diagnosis and assessment of sensitivity to tyrosine kinase inhibitors.

KIT and PDGFRA mutations in gastrointestinal stromal tumors (GISTs)

Mutually exclusive KIT and PDGFRA mutations are central events in GIST pathogenesis, and their understanding is becoming increasingly important, because specific treatment targeting oncogenic KIT and PDGFRA activation (especially imatinib mesylate) has become available. KIT mutations in GIST are clustered in four exons. Most common are exon 11 (juxtamembrane domain) mutations that include deletions, point mutations (affecting a few codons), and duplications (mostly in the 3' region). The latter mutations most often occur in gastric GISTs. Among gastric GISTs, tumors with deletions are more aggressive than those with point mutations; this does not seem to hold true in small intestinal GISTs. Exon 9 mutations (5-10%) usually are 2-codon 502-503 duplications, and these occur predominantly in intestinal versus gastric GISTs. Lesser imatinib sensitivity of these tumors has been noted. Kinase domain mutations are very rare; GISTs with such mutations are variably sensitive to imatinib. PDGFRA mutations usually occur in gastric GISTs, especially in the epithelioid variants; their overall frequency is approximately 30% to 40% of KIT mutation negative GISTs. Most common is exon 18 mutation leading Asp842Val at the protein level. This mutation causes imatinib resistance. Exon 12 and 14 mutations are rare. Most mutations are somatic (in tumor tissue only), but patients with familial GIST syndrome have consitutitonal KIT/PDGFRA mutations; >10 families have been reported worldwide with mutations generally similar to those in sporadic GISTs. GISTs in neurofibromatosis 1 patients, children, and Carney triad seem to lack GIST-specific KIT and PDGFRA mutations and may have a different disease mechanism. Secondary mutations usually occur in KIT kinase domains in patients after imatinib treatment resulting in resistance to this drug. Mutation genotyping is a tool in GIST diagnosis and in assessment of sensitivity to kinase inhibitors. This is a US government work. There are no restrictions on its use.

Gastrointestinal stromal tumors: review on morphology, molecular pathology, prognosis, and differential diagnosis

CONTEXT

Gastrointestinal stromal tumors (GISTs) are specific, generally Kit (CD117)-positive, mesenchymal tumors of the gastrointestinal tract encompassing a majority of tumors previously considered gastrointestinal smooth muscle tumors. They are believed to originate from interstitial cells of Cajal or related stem cells.

OBJECTIVE

To review current clinicopathologically relevant information on GIST.

DATA SOURCES

Literature in Medline and authors' own experience.

CONCLUSIONS

GISTs usually occur in older adults (median age 55-60 years) and rarely in children in the second decade (<1%) throughout the gastrointestinal tract: 60% in stomach, 35% in small intestine, and less than 5% in rectum, esophagus, omentum, and mesentery; most GISTs in the latter 2 sites are metastatic. Five percent of GISTs occur in patients with neurofibromatosis type 1 syndrome (multiple small intestinal tumors) and in Carney triad (gastric epithelioid GISTs in young females). Familial GISTs occur in patients with inheritable germline Kit or platelet-derived growth factor receptor alpha (PDGFRA) mutations. Histologically GISTs vary from spindle cell tumors to epithelioid and pleomorphic tumors. Most GISTs (95%) express Kit (CD117), CD34 (70%), and heavy caldesmon (80%), whereas 25% are positive for smooth muscle actin and less than 5% for desmin. Tumor size and mitotic activity are best predictive prognostic features; small intestinal tumors behave more aggressively than gastric tumors with similar parameters. Mutually exclusive gain-of-function Kit or PDGFRA mutations occur in a majority of GISTs representing in-frame deletions, point mutations, duplications and insertions. Mutations in Kit juxtamembrane domain (exon 11) are the most common in GISTs of all sites, whereas rare Kit extracellular domain (exon 9) Ala502-Tyr503 duplication is specific for intestinal GISTs. Mutations in PDGFRA have been identified in juxtamembrane (exon 12) and tyrosine kinase domains (exons 14 and 18), nearly exclusively in gastric GISTs, mostly in epithelioid variants. Some Kit and PDGFRA mutations have a prognostic value. Kit/PDGFRA tyrosine kinase inhibitor imatinib has been successfully used in the treatment of metastatic GISTs for more than 5 years. However, primary and acquired secondary resistance linked to certain types of Kit and PDGFRA mutations is limiting long-term success necessitating the use of alternative treatments.

Targeting c-Kit mutations: basic science to novel therapies

The Kit receptor tyrosine kinase is a transmembrane receptor that is expressed in a variety of different tissues and mediates pleiotropic biological effects through its ligand stem cell factor (SCF). Sporadic mutations of Kit as well as autocrine/paracrine activation mechanisms of the SCF/Kit pathway have been implicated in a variety of malignancies, where its primary contribution to metastases is in enhancing tumor growth and reducing apoptosis. For example, Kit is frequently mutated and activated in gastrointestinal stromal tumors (GISTs) and there is ligand-mediated activation of Kit in some lung cancers. Kit is a convenient target in Kit-induced tumors and inhibition of this receptor with the small molecule drug Gleevec (imatinib mesylate, STI571) in GIST has shown dramatic efficacy. Unfortunately, past experience has demonstrated that chemotherapy of cancers with a single drug often leads to resistance of the cancer. Further understanding of the molecular mechanisms underlying Kit-mediated transformation is therefore important and may lead to the identification of further novel drug targets. These Kit-specific signaling pathways may then be targeted to overcome potential drug resistance. This review will focus on our understanding of the molecular mechanisms involved in transformation by Kit. The potential mechanisms by which Kit induces cellular transformation are described. We will also discuss the role and expression of Kit in various malignancies. Ultimately, the understanding of c-Kit biology, biochemistry, and mutational analysis will lead to better therapeutics.

Myb and Ets Proteins Are Candidate Regulators of c-kit Expression in Human Hematopoietic Cells

Kit is a tyrosine kinase receptor that plays an important role in human hematopoietic cell growth. The promoter elements that modulate the gene's expression have not been extensively studied. Because of c-kit's acknowledged importance in hematopoiesis, we sought to address this issue in more detail. To perform these studies we analyzed a human c-kit 5′ flanking fragment ∼1 kilobase in length. Deletion constructs showed a region ∼139 nucleotides upstream from the translation initiation site that was critical for promoter activity. A region containing a potential silencing element was also identified. Sequence analysis indicated several potential Myb- and Ets-binding sites. The functional significance of these sites was explored by showing that both wild-type Myb and Ets-2 protein, but not a DNA binding-deficient Myb mutant protein, bound to distinct 5′ flanking fragments that included these sites. Furthermore, binding of recombinant Myb and Ets-2 protein to these fragments could be competed with an excess of double stranded oligodeoxynucleotides containing canonical, but not mutated,Myb- or Ets-binding sites. We also showed that the 5′ flanking region of c-kit exhibited promoter activity in nonhematopoietic cells only when the cells were transfected with c-myb or ets-2 expression vectors. Moreover,Myb and Ets-2 coexpression in such cells augmented transactivation of c-kit promoter constructs in comparison to that observed in cells transfected with either construct alone. Promoter constructs lacking various Myb and Ets sites deleted were much less effective in this same system. Finally,Myb and Ets-2 mRNA expression was detected in CD34+, Kitlow as well as CD34+, Kitbright cells. In aggregate, these data further define the human c-kit promoter's functional anatomy and suggest that Myb and Etsproteins play an important, perhaps cooperative, role in regulating expression of this critical hematopoietic cell receptor.

Myb and Ets Proteins Are Candidate Regulators of c-kit Expression in Human Hematopoietic Cells

Kit is a tyrosine kinase receptor that plays an important role in human hematopoietic cell growth. The promoter elements that modulate the gene's expression have not been extensively studied. Because of c-kit's acknowledged importance in hematopoiesis, we sought to address this issue in more detail. To perform these studies we analyzed a human c-kit 5′ flanking fragment ∼1 kilobase in length. Deletion constructs showed a region ∼139 nucleotides upstream from the translation initiation site that was critical for promoter activity. A region containing a potential silencing element was also identified. Sequence analysis indicated several potential Myb- and Ets-binding sites. The functional significance of these sites was explored by showing that both wild-type Myb and Ets-2 protein, but not a DNA binding-deficient Myb mutant protein, bound to distinct 5′ flanking fragments that included these sites. Furthermore, binding of recombinant Myb and Ets-2 protein to these fragments could be competed with an excess of double stranded oligodeoxynucleotides containing canonical, but not mutated,Myb- or Ets-binding sites. We also showed that the 5′ flanking region of c-kit exhibited promoter activity in nonhematopoietic cells only when the cells were transfected with c-myb or ets-2 expression vectors. Moreover,Myb and Ets-2 coexpression in such cells augmented transactivation of c-kit promoter constructs in comparison to that observed in cells transfected with either construct alone. Promoter constructs lacking various Myb and Ets sites deleted were much less effective in this same system. Finally,Myb and Ets-2 mRNA expression was detected in CD34+, Kitlow as well as CD34+, Kitbright cells. In aggregate, these data further define the human c-kit promoter's functional anatomy and suggest that Myb and Etsproteins play an important, perhaps cooperative, role in regulating expression of this critical hematopoietic cell receptor.

Hydrocortisone-induced increase of PDGF beta-receptor expression in a human malignant mesothelioma cell line

The effect of hydrocortisone (HC) on PDGF beta-receptor expression was studied in the human malignant mesothelioma cell line Mero-14. HC was found to induce a time- and dose-dependent increase of PDGF beta-receptor mRNA. Nuclear run off analysis revealed that HC induced increased transcription of the PDGF beta-receptor gene. The expression of PDGF beta-receptor protein was also elevated by HC as demonstrated with an immunoblotting assay. However, the number of PDGF-BB binding sites on the cell surface of Mero-14 remained unchanged upon HC treatment. These results suggest that steroid hormones can regulate PDGF receptor expression in vivo.

Deletion of the c-kit protooncogene in the human developmental defect piebald trait

The protooncogene c-kit is critical for development of hematopoietic stem cells, germ cells, and melanoblasts in the mouse. Homozygous mutations of this gene in the mouse cause anemia, infertility, and albinism, whereas heterozygous mutant mice usually exhibit only a white forehead blaze and depigmentation of the ventral body, tail, and feet. The heterozygous mouse phenotype is very similar to human piebald trait, which is characterized by a congenital white hair forelock and ventral and extremity depigmentation. To investigate the possibility that alterations in the human c-kit gene may be a cause of piebald trait, DNA from seven unrelated affected individuals was examined by Southern blot analysis. One subject, although cytogenetically normal, has a heterozygous deletion of the c-kit protooncogene. This deletion encompasses the entire coding region for c-kit and also involves the closely linked gene for platelet-derived growth factor receptor alpha. Fluorescence in situ hybridization of genomic c-kit probes to metaphase chromosomes independently confirmed the deletion in this case. These findings provide molecular evidence mapping piebald trait to the c-kit locus on chromosome 4. Although we cannot exclude the involvement of other closely linked genes, the demonstration of a genomic c-kit deletion in one subject with piebald trait and the marked concordance of the human and mouse phenotypes provide strong evidence for the role of c-kit in the development of human melanocytes and in the pathogenesis of piebald trait.

Proliferative pattern of head and neck cancer

Tumor growth is primarily dependent on the fraction size of growing cells, their growth and proliferative rate, and the fractional cell death. In the present study, we focused specifically on the proliferative characteristics of squamous cell carcinomas of the head and neck region by using monoclonal antibodies and immunohistochemical methods. We studied two normally occurring antigens representative of cell proliferation: (1) ribonucleotide reductase, which is an independent cytoplasmatic enzyme and is intimately integrated in DNA synthesis, and (2) Ki-67, which is a nuclear antigen being expressed only in replicative cells. We also used intravenously injected bromodeoxyuridine (BRDU) for specific detection of tumor cells in the S phase of the cell cycle. In addition, in vivo injections of BRDU were also given to tumor-bearing mice to illustrate tumor cell kinetics by means of flow cytometry. The main observation was morphologic heterogeneity, with a high frequency of proliferative cell clusters in the cancer specimens interspersed among quiescent cells, which was demonstrated by the three monoclonal antibodies independent of each other. The experimental studies clearly visualized the transfer of BRDU throughout the tumor cell cycle. We conclude that immunohistochemical analysis provides valuable qualitative information on the proliferative pattern of tumor growth and, together with dynamic flow cytometry, may improve the clinical basis for individualized management of the cancer patient.

Isolation and chromosomal localization of a novel FMS-like tyrosine kinase gene

We have isolated and sequenced part of a new gene of the tyrosine kinase family. This gene, called FLT3, has strong sequence similarities with members of a group of genes encoding growth factor receptors: FMS, KIT, and PDGFR. We have localized the human FLT3 gene to chromosome 13, band q12, and its mouse homolog to chromosome 5, region G.

The human PDGF receptor alpha-subunit gene maps to chromosome 4 in close proximity to c-kit

The gene encoding the alpha-subunit of the human platelet-derived growth factor receptor (PDGFRA) maps to band q11-q12 of chromosome 4 by in situ hybridization, which was confirmed by Southern analysis of a Chinese hamster x human cell hybrid that retains only human chromosome 4.