Niemann-Pick C1-like 1 (NPC1L1) protein, a newly identified sterol influx transporter, located at the apical membrane of the enterocyte, which may actively facilitate the uptake of cholesterol by promoting the passage of sterols across the brush border membrane of the enterocyte.[12] The protein has been characterized by the presence of a signal peptide, 13 putative transmembrane regions, a conserved NPC1 domain and a sterol sensing domain (SSD). Although expressed within several tissues in the body, the expression is predominant in liver and small intestine.[3–5] In rodents, NPC1L1 is highly expressed on the surface of jejunal absorptive cells whereas in humans the expression is more in the hepatoma cells in the liver.[6] The protein plays a key role in cholesterol uptake and intracellular cholesterol trafficking from the plasma membrane to the endoplasmic reticulum.[7] A fact further strengthened by the observation of Gracio-Calvo et al.[8] that the protein was the direct molecular target of ezetimibe, a drug that inhibits cholesterol absorption. Recently, Temel et al.[9] have suggested the presence of NPC1L1 on the canalicular membrane of hepatocytes which may modulate biliary cholesterol excretion. Thus, this protein is actively involved in the cholesterol homeostasis pathway.[1011]

Single nucleotide polymorphisms (SNPs), together with copy number variation, are the primary source of variability in the human genome. As amino acid substitutions currently account for approximately half of the known gene lesions responsible for human inherited disease, study of non-synonymous single nucleotide polymorphisms (nsSNPs) are important in delineating the etiology of many such disorders.[1213] These SNPs may lead to changes in protein confirmation and may be associated with altered response to drug treatment, susceptibility to disease, and other phenotypic variations.[14] The study of population data for the distribution of NPC1L1 has lead to the identification of six nsSNPs. Among the 6 identified nsSNPs, M510I (rs1468384) shows decrease in the stability of the protein as analyzed in silico by MuPro[15] and StructureSNP[16] softwares. The M510I polymorphism is the result of a nucleotide change G to A at position 2993 of the cDNA sequence in exon 2, and it results in the substitution of isoleucine for methionine at amino acid 510 of the NPC1L1 protein. The SNP has already been studied in Caucasian, Asian, and African American populations by sequencing as given in NCBI database. Till date, no data is available on Indian population. Thus, a reproducible and cost-effective PCR-RFLP based assay was developed to study the distribution of this SNP as well as its frequency distribution comparison with other world populations.

High molecular weight genomic DNA was extracted from 3.0 ml of the blood samples collected from 150 normal healthy individuals in the age group 20-50 years with informed consent. The DNA was isolated by methodology as given by Lahiri et al.[17]

An efficient PCR reaction not only generates product of requisite size but also should utilize the primers completely. Thus, a minimal difference in their melting temperatures (Tm) is favorable[20] and in the case of all primer pairs selected as per Table 1 the Tm difference is of 1°C. Both primers and target sequence affect this efficiency.

At room temperatures nucleic acids fold into conformations (secondary structures) which have high negative free energy. The stability of these template secondary structures depends largely on their free energy and melting temperatures and is extremely important for designing primers.[21–23] Keeping these guidelines in mind, the four primer pairs were selected. The primer pair finally selected, had least number of secondary structures, with no hair pin loop, no bulge loops and just 1 internal loop in sense strand at the temperature range during PCR. Further, the 2 dimers in sense strand and 1 in antisense strand were observed to be formed at temperatures far below the experimental temperature range. Moreover the primers so designed were unique i.e., they targeted and amplified only the specific gene in the genomic DNA as given by the Blast[24] [Table 1]. The second primer pair gave minimum and unique hits and was thus selected.

The PCR reaction was set at three different annealing temperatures i.e., 59.5°C, 60.5°C, and 61.5°C. The results are shown in the Figure 2. The annealing temperature of 61.5°C proved to be the most stringent, giving optimum results at which no spurious amplification were observed in the PCR products as compared to other temperatures [Figure 2].

The optimization of amount of Taq polymerase and number of PCR cycles was then carried out. The amount of Taq polymerase was tested in the range 0.25 to 2 U per reaction. As little as 0.5 U enzymes could be used without a decrease in the yield of PCR product (data not shown). PCR was performed for 25, 30, and 35, cycles, including the initial cycle. The PCR products could be detected after 25 cycles, maximal PCR product was obtained with 30 cycles of PCR amplification without the production of nonspecific amplification and complete utilization of primers (data not shown). The product was purified and sequenced to confirm the region amplified. The Figure 3 depicts the sequence of the PCR product as obtained from Bangalore Genei and viewed in CHROMAS.

The amplified product was subjected to digestion by Bcc1 restriction enzyme. After digestion of the 437 bp fragment obtained by PCR, the three possible genotypes were distinguishable: homozygous GG (437 bp), heterozygous GA (437, 278 and 159 bp), and homozygous AA (278 and 159 bp). The Table 2 and the gel picture Figure 4 below depicts the band pattern for different genotypes.

The genotype frequency distribution for the M510I polymorphism is shown in Table 3. The difference in allele frequency distribution between different population groups was observed. The “G” allele present in Indian Population showed the least frequency of 0.46 (95% CI) and Caucasian Population samples the maximum at 1.000. For “A” allele it is 0.54 (95% CI) in Indian Population [Table 4]. The data was then analyzed for χ² value to study difference in allele distribution amongst the Indian population and other populations of the world studied so far [Table 5]. Statistically, highly significant differences were observed on comparison with these populations. Even the population labeled as Asian on closer scrutiny was found to be a mixed group constituted of populations of mongoloid origin like the Chinese, Malaysians, etc. This group also showed the allele distribution to be highly significantly different from the North Western Indian Population under study.

The above analysis makes it very clear that the PCR-RFLP methodology optimized in current study is robust enough allowing screening of populations for NPC1L1 SNP rs1468384. The earlier studies using sequencing are not only expensive but also not amenable to high throughput population screening.

The PCR-RFLP technique described here is a proficient NPC1L1 genotyping assay starting with 50 ng genomic DNA extracted from 0.3 mL of blood that makes it cost effective as well as time and labor saving. The PCR product enhancement and specificity were improved by using 10% glycerol. The use of less Taq polymerase, single-step PCR cycling, and polyacrylamide gel electrophoresis not only improved efficiency but also lowered the cost per test. The protocol is thus useful for clinical diagnostic laboratory as well as a research laboratory performing population screening. The importance of study of this SNP lies in the fact that it is predicted to lead to localized structural instability that may have functional implications in disorders involving cholesterol influx, drug binding and susceptibility to complex disorders.