Using High Resolution Melting Method for Detecting Nucleotide Polymorphisms of STK11 Gene Germline in Patients with Digestive System Cancers

Following PCR proliferation, high resolution melting (HRM) is a new and homogenous method, which is performed in a closed tube. This method is feasible in PCR products for analyzing genetic changes (SNPs, mutation, and methylation). HRM differentiates nucleic acid samples based on sequence, length, and GC volume. It is featured with proliferation of the gene in 80-250bp pieces in a reaction in which florescent dyes are bound to two-stranded DNA. Following PCR, and as HRM process goes on, the double-stranded DNA melts and the dye fluoresces at a lower level. The process is plotted in a melt curve with high resolution that is highly variable Abstract

STK11 or LKB1 gene was discovered in 1998 through linkage analysis in Peutz-Jeghers syndrome (PJS) as suppressor of tumors [4]. PJS is a rare autosomal disorder with decreased prevalence that was first diagnosed in a Danish family by Peutz in 1921 and later was explained by Jeghers et al. in 1949. In addition, to mucus and cutaneous pigments and gastric-intestinal polypose hamartomateuse there is a risk of development of benign and malignant tumors in digestive limbs and other areas like chest, intestine, testicle, cervix, lungs, pancreases, and skin. Deactivation of STK11 gene in PJS patients reaches to 91%, out of which 78% is due to point mutation and 13% is due to deletion [4][5].
STK11 gene demonstrates serine/threonine kinase activity in 19p13.3 chromosome. It expanses in 23kbase region with 9 encoder exons and one non-encoding exon. mRNA of the gene is 2.4kbase in length and transcribed in telomere -centromere. This gene is translated into a protein with 433 amino acid with molecular weight of 48kd. The protein is featured with peptide signal at the end of amino acid 338-43 and a kinase domain (amino acid 49-309) [4][5][6].
STK11 gene is expressed in all body tissues both in fetus and adults and liver, pancreases, testicle, and skeletal muscles in particular. The product of STK11 is the first upstream kinase for adenine monophosphate-activated protein kinase (AMPK) that is activated allosterically through attaching to a pseudokinase (STRAD) and an adaptor protein (MO25). Hetromic set of STK11-STRAD-MO25, as an active biological unit is capable of phosphorylation and activation of AMPK and 12 other kinases at least [4][5][6][7]8]. In fact, AMPK is the essential part of the cellular metabolism that is needed to preserve cellular hemostasis. AMPK activation by STK11 leads to suppression of growth and proliferation when nutrient level decreases. Moreover, activation of AMPK-like kinases by STK11 plays a key role in preserving cellular polarity, which results in suppression of tumor cells.
In short, STK11 gene is a key element in cellular polarity, metabolism, p53 depended apoptosis, inhibition of cellular cycle in G1 and cell proliferation. Thereby, in addition to PJS, any disorder in the gene leads to prevalent disorders like diabetes and cancer [9].

Materials and Methods
To examine mutation of STK11 gene germline, complete blood samples of 56 patients (different ethnic groups) with cancer of digestive system and age range of 45-80 were procured, with observance of moral codes, from Imam Khomeni and Javaheri Hospitals in Tehran and BuAli Hospital in Hamedan. The samples were stored at -20ºC and then used for extracting genome DNA.
Genome DNA was extracted from 200µl complete blood using High pure PCR Template Preparation kit (Roche) according to manufacturer's recommended process.
Proliferation of 10 exons of STK11 gene was done using the primers that have been studied for this gene [10]. Twenty pairs of primers were synthesized by TAG Copenhagen. Primer sequence and size of each proliferated piece in PCR reaction are listed in Table 1.
All the samples and exons that showed nucleotide

Discussion
High resolution DNA melting is powerful and new method for screening diversity of nucleotides. Although sensitivity and specificity of the method depends on variety of factors such as length of PCR product, type of dye, density of Mg2+, it still outperforms other techniques for detecting unknown mutation. Advantages of HRM are ease of implementation, accuracy, and time efficiency so that there is no need for Post-PCR processes and the risk of secondary contamination is eliminated [1,11]. To examine sequence of STK11 gene in exons and exon/ intron border, HRM technique was used. The participants were anonymous and all diagnosed with digestive system cancer. Among 56 participants, only 10 (four with intron 6 and six with intron 7) demonstrated two types of nucleotide changes and no correlation between these two have been reported thus far. HRM results were consistent with PCR sequencing.
A key feature of this technique is the possibility of distinguishing homozygous and heterozygous cases using melting graph. As illustrated in Figures 2 and  4, heterozygoutes are distinguished by two peaks and homozygoutes by one peak. Additionally and in presence of DMSO (10%), the reaction creates more sensitivity in diagnosing single nucleotide differences in a thermal change step of 0.2ºC/s. At any rate, mutation of germline in STK11 encoding regions was not examined, which does not mean there was no mutation in STK11 gene. Given the complicated and variant role of STK11 gene in function of cells, future studies can examine the gene germline in