الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
Acute lymphoblastic leukemia is a malignant neoplasm of the lymphocyte precursor cells - lymphoblasts- that occurs annually in nearly 4000 people in the US. In Egypt, according to the National Cancer Registry (NCR), leukemia is the leading cause of malignancy in children, constituting 36.7% of cases of childhood cancer diagnosed annually. ALL is the main subtype of childhood leukemia (approximately 80%) and represents about 20% of acute leukemias in adults. It shows a bimodal age distribution with a peak incidence between 2 to 5 years and a smaller secondary peak after 50 years of age.
Studies throughout the past decades have shown that leukemogenesis occurs due to an interplay between genetic abnormalities and exposure to environmental, chemical, infectious or physical factors. Established risk factors for ALL include exposure to ionizing radiation in utero, postnatal high-dose radiation, chemotherapeutic agents, and several genetic syndromes, but these together account for only a small proportion of childhood ALL cases diagnosed. Increasing evidence indicates that these, and other proposed risk factors may have differential effects on childhood leukemia risk depending on the timing of exposure and individual genetic susceptibility.
Outcome of ALL has been influenced by modest changes in drug dose or exposure. It is well known that most medications exhibit wide inter-patient variability in their efficacy and toxicity. These inter-individual differences result in part from genetic polymorphisms and mutations in drug metabolizing enzymes, drug transporters, receptors and other drug targets (e.g. toxicity targets). Although, ALL is cured in approximately 94% of children with multi-agent chemotherapeutic regimens, treatment-related toxicity can not only be life threatening but is also one of the main reasons for interruption or discontinuation of chemotherapy, which may increase relapse risk. Germline polymorphisms in genes encoding drug-metabolizing enzymes, drug transporters, and drug targets can significantly influence the pharmacokinetics and pharmacological effects of medications and can be significant determinants of the efficacy and toxicity of antileukemic therapy.
Indeed, numerous candidate genes may be of importance in response to treatment in childhood ALL. With regard to treatment, most of the research on genetic variability in patient populations conducted to date has focused on drug-metabolizing enzymes. The most extensively studied of the drug-metabolizing enzymes is thiopurine methyltransferase (TPMT), which catalyzes the S-methylation of thiopurines (e.g., 6-MP, 6-thioguanine). The thiopurines are the backbone of current therapy for childhood ALL. Since their introduction to leukemia treatment in the 1950s, they have played an essential role in treatment protocols for ALL. Several contemporary treatment protocols for childhood ALL apply consecutive cycles of either 6-mercaptopurine or thioguanine starting as early as during induction consolidation treatment and continue administration during maintenance therapy for up to 36 months after diagnosis.
The TPMT gene exhibits significant genetic polymorphism in all large ethnic groups studied to date, including Caucasians, Africans, African- Americans, and Asians. This results predominantly from single nucleotide polymorphisms (SNPs). The wild-type allele is designated as TPMT*1 and to date, at least 28 variant alleles of the TPMT gene have been identified. The prevalent four polymorphic alleles that have been associated with impaired activity of the enzyme, are TPMT*2 (238G>C), TPMT*3A (460G>A, 719A>G), TPMT*3B (460G>A) and TPMT*3C (719A>G). These four alleles account for 80–95% of inherited TPMT deficiency and low enzyme activity. Approximately 90-95% of the population has no TPMT variant allele. Among these four most common TPMT variants, TPMT*3C is the most prevalent mutant allele in African and Asian populations. In Egyptians, this mutant allele represents 86% of the TPMT variant allele.
TPMT polymorphisms have been associated with the therapeutic efficacy and toxicity of thiopurine drugs. Numerous studies have shown that patients with inherited low levels of TPMT activity are at greatly increased risk for thiopurine-induced toxicity, when treated with standard doses of these drugs. These patients can be treated without major side effects (myelosuppression) when the dose is adjusted properly.
Thiopurines are associated with two categories of adverse events. The first is the allergic type and the second is the non-allergic type. The first type usually occurs within the first 3-4 weeks of treatment and is not dose-dependent. It includes pancreatitis, fever, skin rash, general malaise and digestive intolerance. The second type seems to occur later; it is dose-dependent and includes hepatic toxicity, bone marrow toxicity, infections and malignancy. Myelosuppression is the most common serious complication.
Since TPMT polymorphism represents a determinant of 6-MP response and ALL outcome, this study investigates the relevance to introduce the prospective analysis of TPMT prior to any treatment, in order to individually optimize 6-MP therapy and avoid adverse reactions to this drug.
The aim of our study was to investigate and determine the frequency of the genetic polymorphisms in the thiopurine S-methyltransferase (TPMT) gene in children with acute lymphoblastic leukemia (ALL) and its relation to drug toxicity.
This study was carried out on a total of 80 subjects divided into two groups. The first group included 40 pediatric patients with standard risk acute lymphoblastic leukemia (ALL) who are subjected to 6-Mercatopurine therapy for consolidation while the second group included 40 age and sex matched controls.
All patients in the study were subjected to the following:
1. Full history taking including: (Age, gender, drug history, medical history and Family history of leukaemia or other malignancies)
2. Complete clinical examination including:
• Constitutional manifestations, lymphadenopathy and organomegaly.
3. Routine Laboratory investigations including:
• Complete blood picture, Serum alanine aminotransferase and serum aspartate aminotransferase at diagnosis and after continuation of consolidation chemotherapy to evaluate treatment outcome and thiopurine intolerance.
4. Bone marrow aspiration (at diagnosis and after continuation of consolidation chemotherapy to evaluate treatment outcome and thiopurine intolerance) and immunophenotyping by flowcytometry. (patients’ only)
5. TPMT genotype detection by PXG - TPMT StripAssay based on Polymerase Chain Reaction (PCR) and reverse hybridization.
The results of the study showed that:
• Egyptian children have a relatively low frequency of TPMT mutant alleles (3.1%).
• In the study population, the overall frequency for homozygous wild-type TPMT*1/*1 genotype was 93.8% and the overall frequency for TPMT polymorphism was 6.2% (5% for heterozygous TPMT*1/*3A genotype and 1.2% for heterozygous TPMT*1/*3C genotype).
• No homozygous mutant TPMT genotypes (e.g. TPMT*3A/*3A, TPMT*2/*2, TPMT*3A/3C, etc.) were detected in the study population.
• Also neither the cases nor the controls in the study population had TPMT*1/*2 and TPMT*1/*3B genotypes.
• In ALL patients, 39 patients (97.5%) were of the homozygous wild-type TPMT*1/*1 genotype and 1 patient (2.5%) only was of the heterozygous TPMT*1/*3A genotype.
• In the control group, 36 subjects (90%) had homozygous wild-type TPMT*1/*1 genotype, 3 (7.5%) with heterozygous TPMT*1/*3A genotype and 1 (2.5%) heterozygous TPMT*1/*3C genotype.
• The only patient with variant TPMT*1/*3A genotype did not show any evidence of thiopurine intolerance (hematotoxicity and hepatotoxicity) after continuation of 4 weeks of consolidation chemotherapy.
• 13 ALL patients with wild-type TPMT*1/*1 genotype developed thiopurine intolerance in the form of hematotoxicity with no hepatotoxicity.