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Romanian Biotechnological Letters
Copyright © 2014 University of Bucharest
Vol.19, No2, 2014
Printed in Romania. All rights reserved
ORIGINAL PAPER
Screening of RYR1 genotypes in swine population by a rapid and sensitive
method
Received for publication, July 15, 2013
Accepted, February 26, 2014
DANIELA ELENA ILIE1,2, VASILE BĂCILĂ3, ADA CEAN1,2, LUDOVIC TOMA
CZISZTER2, SIMINA NEO2
1
Research and Development Station for Bovine - Arad, Arad, Romania
Banat’s University of Agricultural Sciences and Veterinary Medicine, Faculty of Animal
Science and Biotechnologies, Timisoara, Romania
3
University of Agronomic Science and Veterinary Medicine – Bucureşti, Faculty of Animal
Science, Bucharest, Romania
*
Corresponding author: Vasile Băcilă,
Phone: +40213192932, Fax: +40213512000 E-mail: [email protected]
2
Abstract
Genetic polymorphism in the gene controlling the calcium release channel (ryanodine receptor,
RYR1) of sarcoplasmic reticulum in skeletal muscle have been shown to affect meat quality in pigs,
producing porcine stress syndrome (PSS) and malignant hyperthermia (MH). The MH condition
produces pale, soft and exudative (PSE) meat resulting in reduced commercial value of pork.
Therefore, monitoring swine population for RYR1 genetic variants is important in the subsequent
process of breeding and improving meat quality in pigs. The aim of the present study was to establish
and validate a rapid and sensitive molecular genotyping assay to detect C1843T mutations of the Sus
scrofa RYR1 gene using quantitative PCR (qPCR) followed by high-resolution melting (HRM) analysis.
The qPCR-HRM assay allowed the rapid and sensitive identification of RYR1 genotypes for
discrimination of wild type and mutant alleles, so the carriers can be detected. The method was
validated by analyzing a total of 195 DNA samples that were previously genotyped by PCR-RFLP
method. The results demonstrate that qPCR-HRM method is a fast, simple and reliable assay for
genotyping the C1843T (Arg615Cys) polymorphism of the RYR1 gene. The high frequency of RYR1
mutant allele suggested that implementing a routine testing system is necessary to gradually eradicate
the MH condition.
Key words: C1843T SNP, high - resolution melting, malignant hyperthermia, molecular
diagnostic, porcine stress syndrome, ryanodine receptor
Introduction
Ryanodine receptor (RYR1) is the calcium release channel from the sarcoplasmic
reticulum in skeletal muscle whose contraction, relaxation and energy metabolism are
regulated by the concentration of intracellular Ca2+ (D.H. MACLENNAN & al. [1]). An
abnormality in the ryanodine receptor facilitates opening and inhibits closing of Ca2+
channel resulting from an altered low-affinity Ca2+ binding site in the channel pore and thus
causes malignant hyperthermia (MH). Currently, this condition has worldwide economic
consequences and represents an economical major concern in the swine industry (D.H.
MACLENNAN & al. [1]; M. FILL & al. [2]; Y. KIM & al. [3]).
MH is a hypermetabolic syndrome involving the skeletal muscle characterized by
muscle rigidity, hyperthermia, tachycardia, tachypnea, increased oxygen consumption,
cyanosis, cardiac dysrhythmias, metabolic acidosis, respiratory acidosis, unstable arterial
blood pressure, and death (E. SUSAN & al. [4]). The primary features of MH are a direct
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Screening of RYR1 genotypes in swine population by a rapid and sensitive method
consequence of loss of skeletal muscle cell calcium homeostasis with a resulting increased
intracellular calcium ion concentration (P. J. HALSALL & , P.M. HOPKINS [5]).
Fujii et al. (1991) reported that a point mutation in pig RYR1 gene is associated with
porcine stress syndrome (PSS), and is a major gene known to have a direct linkage with
pork quality (J. FUJII & al. [6]). The PSS-susceptibility is inherited as an autosomal
recessive dysfunction (J.E. ROJAS & al. [7]) and was first described by Topel et al. (1968),
who noted that physically stressed, susceptible pigs would collapse in a shock-like state and
die (D.G. TOPEL & al. [8]). PSS leads to carcasses containing pale, soft, exudative (PSE)
meat, with a low water retention capacity and a rapid decrease of pH, combined with a high
temperature of the meat which leads to a denatured sarcoplasmatic protein, making the meat
unfit for processing. Expression of PSS results on account of physical stressors such as
excessive movement, fighting, marketing, vaccination, castration, oestrus, mating,
parturition and hot weather (G. MITCHELL & J.J.A HEFFRON [9]). It has also been noted
that volatile anaesthetics such as halothane bring about the onset of PSS (A. J. WEBB & C.
H. C. JORDAN [10]). Pigs that are homozygous for the recessive RYR1 n allele are subject
to sudden death from stress. In addition, those surviving and those heterozygous for the
condition have many meat quality problems. This gene has multiple denominations, being
called the stress gene, halothane gene and PSS gene (A.TÄNAVOTS & al. [11]).
The porcine RYR1 locus was localized on chromosome 6p11-q21 where a substitution
at position 1843 (C1843T) lead to the substitution of Arg for Cyst at position 615 in amino
acid sequence (Arg615Cys), where the susceptible mutant allele is denominated T and the
wild-type – non-susceptible allele C (J. FUJII & al. [6]; I. HARBITZ & al. [12]; A. HOUDE
& al. [13]; P. VOGELI & al. [14]).
In pigs, MH have been reported for several breeds, however, the incidence is higher in
lean, heavily muscled breeds, such as Pietrain, Poland China, Landrace, Duroc, and Large
White (E. SUSAN & al. [4]). This incidence is due to a dual effect of the gene: negative and, in
the same time, beneficial. The beneficial effects of the MH gene are associated with leanness
and muscle hypertrophy (M.A. MANEA & al. [15]). It is this association that has allowed the
gene frequency to be increased initially through selection for increased muscularity.
Knowing the structure of this locus in pig populations is extremely important for
economic losses represented by recessive homozygous pigs. Homozygous recessive pigs, nn
(T/T) and the heterozygous pigs, Nn (C/T) have a very appealing conformation for the
specialists who make the selection. The unconscious promotion of these animals (nn and
Nn) in the herd, with the intention to produce a new generation, led to the automatic
increase of the mutant allele n frequency in pig populations.
In the recent years, several genetic tests were described to identify mutations and type
single-nucleotide polymorphisms (SNPs) including the RYR1. The majority of these
techniques require processing, separation steps or allele-specific primers or probes, which
make them less favourable for high-throughput assays (H. GRIEVINK & K. M STOWELL
[16]). The new approach reported is based on a High-resolution melting (HRM) assay that
was introduced as a homogeneous closed-tube system that allows post-PCR analysis of
genetic mutations or variance without the need of separation steps. This method is based on
the properties of dissociation (melting) of double-stranded DNA, which is subsequently
examined by melting curve analysis. Different melting profiles are obtained from the
transition of double-stranded DNA to single-stranded DNA due to a gradual increase in
temperature after the PCR (G.H. REED, & C.T. WITTWER [17]).
Melting curve analysis has advantages over other mutation detection methods that
derive information from the amplification process itself (M. HOFFMANN & al. [18]).
HRM is considered the simplest method for genotyping and mutation detection because it is
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DANIELA ELENA ILIE, VASILE BĂCILĂ, ADA CEAN, LUDOVIC TOMA CZISZTER, SIMINA NEO
done in the same tube and immediately after the PCR (J. MONTGOMERY & al. [19]).
We consider that the HRM is a reliable assay for fast, high- throughput post-PCR
analysis of genetic mutations or variance in nucleic acid sequences that can facilitate marker
assisted selection. Different sequence variants can be identified based on differences in
melting curves using the LightCycler Nano Software. Heterozygous samples are distinguished
from homozygous samples by an altered shape in the melting curve. These differences are
best visualized using difference plots because slight differences in curve shape and melting
temperature (Tm) become obvious (H. GRIEVINK & K. M STOWELL [16]).
The aim of the current study was to develop and test the feasibility of a inexpensive and
high-throughput HRM assay using the LightCycler Nano Real-Time PCR System in order to
allow genotyping of C1843T (Arg615Cys) polymorphism of the RYR1 gene in swine.
Genomic DNA samples of known RYR1 genotypes were used to validate the HRM assays.
Materials and Methods
Samples and DNA extraction
In order to develop a HRM method for screening of the RYR1 C/T SNP (C1843T), we
analyzed a total of 195 animals belonging to four distinct breeds (Landrace, Duroc, Pietrain
and Large White) from commercial farms in which a low frequency of halothanesusceptible animals was known to exist.
Blood samples for DNA genotyping were collected in 2 mL vacutainers containing
K3EDTA. Genomic DNA was isolated from blood samples (300 μl) using the Wizard
Genomic DNA Purification Kit (Promega, Madison, USA), according to manufacturers’
instructions. DNA concentration was evaluated spectrophotometrically, with NanoDrop2000 (Thermo Fisher Scientific Inc., USA), and visually by standard agarose gel
electrophoresis [1% agarose (w/v) in TBE] (J.M.S. BARTLETT & D. STIRLING [20]).
PCR and HRM conditions
Initially, the 195 DNA samples were processed for screening the RYR1 C/T SNP
(C1843T), using PCR-RFLP analysis with the enzyme HhaI (Fermentas, Thermo Fisher
Scientific Inc., USA), as described elsewhere (B. BRENIG & G. BREM [21];
A.TÄNAVOTS & al. [11]). The digested DNA fragments were visualized on a 3.5%
agarose gel, with a standard DNA ladder (pUC19/Sau3A) as a molecular weight marker,
and stained with ethidium bromide (10 mg/mL). Images of the gels were taken using Vilber
Loumart Print II Systems.
Primers RYR1-F (5’- GTG CTG GAT GTC CTG TGT TCC CT-3’) and RYR1-R (5’CTG GTG ACA TAG TTG ATG AGG TTT G-3’) for RYR1 gene used for both normal
PCR and qPCR - HRM assay amplified a 134 bp fragment (B. BRENIG & G. BREM [21]).
Starting from our normal PCR conditions using C1000 Thermal Cycler (Bio-Rad
Laboratories, Inc.), we first determined the optimal conditions to perform the reaction in the
LightCycler® 480 High Resolution Melting master mix containing a saturating DNA dye.
Real-Time PCR was then performed on a LightCycler Nano Real-Time PCR System
(Roche Diagnostics GmbH, Mannheim, Germany), in 8 tube strips, in 18 µl volume
containing 20 ng of genomic DNA as template, 10 pmoles of each primer, 2.5 mM of
MgCl2 and 1x LightCycler 480 High Resolution Melting Master (Roche Diagnostics GmbH,
Mannheim, Germany) consisting of FastStart Taq DNA Polymerase, dNTP mix, reaction
buffer and high resolution melting dye. Cycling conditions were: one cycle at 95°C for 10
minutes, followed by 40 cycles at 95°C for 10 seconds, 60°C for 25 seconds and 72°C for
15 seconds. After amplification, the PCR amplicons were denatured at 95°C for 1 minute
and cooled down to 40°C for 1 minute to allow heteroduplex formation. The final HRM
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Screening of RYR1 genotypes in swine population by a rapid and sensitive method
step was performed from 65°C to 95°C with 0.05°C increase/ second. Each assay performed
included reference samples of the 2 genotypes of RYR1 (NN and Nn) and a negative control
containing distilled water instead of template DNA. One negative control (i.e. DNA from
the normal animal) was used to normalize melting profiles of the other samples against this
pre-defined horizontal baseline.
Data analysis
Data were analyzed using automated quantification, Tm calling (melt temperature
analysis) and HRM (high resolution melt analysis and genotyping) provided by the
LightCycler Nano Software 1.0. For sample analysis, melting curves were normalized,
temperature – adjusted and, finally, a difference plot was generated. Samples analyzed by
HRM were automatically grouped according to their melting profiles. The melting curves
were normalized by calculation of two normalization regions before and after the major
fluorescence decrease representing the melting of the amplicons. This algorithm allows the
direct comparison of the samples that have different starting fluorescence levels (M. BALIC
& al. [22]). Also, afterwards, agarose gels were run to confirm that clean products of the
expected length had been obtained.
Results and Discussion
From a total of 195 animals used in the present study belonging to four distinct breeds
(Landrace, Duroc, Pietrain and Large White), 7.00% had the allele n associated with
susceptibility to porcine stress syndrome, while 93.00% had the allele N normal or resistant
to PSS. Genotypic frequencies observed were 86.00% homozygous normal or resistant
individuals (NN), 14.00% of individuals heterozygous (Nn), while for homozygous
susceptible (nn) we did not find any animal. The obtained results suggest that in Romanian
swine population it still a potential impact of susceptible allele (n) of RYR1 gene (Table 1).
Table 1. Genotypic and allelic frequencies of RYR1 C/T SNP (C1843T)
Gene
Breed
No.
Genotypic frequency
NN (C/C)
Nn (C/T)
nn (T/T)
Landrace
69
0.826 (57)
0.174 (12)
0.0 (0)
Duroc
48
0.812 (39)
0.188 (9)
0.0 (0)
RYR1
Pietrain
35
0.942 (33)
0.058 (2)
0.0 (0)
Large White
43
0.860 (37)
0.140 (6)
0.0 (0)
Allelic frequency
N
n
0.913
0.087
0.906
0.094
0.971
0.029
0.930
0.070
For establishment and optimization of the HRM assay, DNA samples with known
genotype were firstly tested by PCR-RFLP technique by using the HhaI restriction enzyme.
The genotype analysis for the C1843T SNP obtained trough PCR-RFLP assay is shown in
Figure 1, where genotypes found in investigated animals (NN and Nn) were clearly
distinguishable.
Figure 1. The PCR-RFLP analysis for the swine RYR1 C1843T SNP, determined by digestion with HhaI on
3.5% agarose gel electrophoresis stained with ethidium bromide. Lane 1 standard markers pUC19/Sau3A;
Lanes 2-4, 6, 8-10 DNA from homozygous normal individuals NN (C/C): Lanes 5 and 7 DNA from
heterozygous individuals Nn (C/T)
Amplicon melting analyses in the presence of the LightCycler 480 HRM dye binding
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the dsDNA heteroduplexes were used to detect RYR1 polymorphisms employing the
LightCycler Nano Real-Time PCR System. The analyzed C1843T SNP located in RYR1
gene affect the melting behaviour of PCR products and therefore generates different melting
curves. Amplicons were 134 bp in length and allow discrimination and identification of
RYR1 genotypes by high - resolution melting analysis.
We started our study with PCR mix optimization. The concentration of Mg+2 supplied
in the PCR buffers is normally not enough to ensure efficient amplification of template. An
increase in the concentration of Mg+2 to 2-3 mM significantly enhances the amplification.
Increased Mg+2 concentration enhance nonspecific amplification and therefore has to be
optimized for each primer set (K.T. WOJDACZ & al. [23]). In the current study 2.5 mM
MgCl2 was found to be optimal (Figure 2).
Figure 2: Effect of increasing MgCl2 on melting behavior. The four different curves show the dependence of
amplicon melting on MgCl2 concentration.
The fluorescence-based data were normalized with the following pre- and post-melt
normalization regions: 84.12–84.61°C and 88.23–88.76°C. Figure 3 shows the normalized,
temperature-shifted melting curves produced by the HRM analysis, which followed the realtime PCR amplification of 134 bp amplicons from genomic DNA using the LightCycler 480
HRM dye. HRM analysis allows clear discrimination between the normal homozygous and
heterozygous genomic DNA samples, based on shape of the melting curves and difference
in Tm. All samples were of known genotypes and were grouped correctly by the
LightCycler Nano Software. The Tm values ware: 86.45 – 86.58°C for heterozygotes
variants (C/T) and 86.84 – 86.96°C for homozygotes variants (C/C). The Tm difference
between the heterozygous (C/T) and homozygous (C/C) variants was 0.26°C and proved to
be sufficient for discrimination between the two variants.
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Screening of RYR1 genotypes in swine population by a rapid and sensitive method
Figure 3. Normalized HRM plots related to the RYR1 amplicons. The two genotypes are clearly distinguished
in this assay. The C/C homozygote melts at the highest temperature (blue) while the C/T homozygote melts at
0.26°C lower (red).
The assay was validated by screening 166 homozygous wild-type and 29
heterozygous genomic DNA samples of known genotypes for the RYR1 C/T SNP (C1843T)
causing the Arg615Cys substitution in amino acid sequence.
The difference plots in the HRM assay for the 134 bp amplicons shown in Figure 4
clearly separate normal homozygous genomic DNA samples (C/C) from heterozygous ones
(C/T). Heterozygotes were easily distinguished from normal homozygotes based on the
shape of the melting curves. Discrimination of different genetic variants is based on
differences in Tm and amplicon melting data should be analyzed both with and without
temperature shifting.
Figure 4. The difference plots related to the RYR1 amplicons, using one of the normal samples (C/C) as the
baseline. Difference plot analysis allowed differentiation between wild-type (blue) and heterozygous (red)
samples.
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In HRM analysis, differences in Tm and normalized curve shape are used together to
discriminate between different genotypes. A good reaction optimization and an appropriate
assay design are crucial points that can increase the amplitude of the profile difference and
make sequence discrimination easier. However, the primers used for HRM must generate
short amplicons. According to the manufacturer's recommendation the best results can be
obtained with amplicons up to 300 bp (www. roche-applied-science.com) because reducing
amplicon size increases the difference in signal at a given temperature between two
sequences that differ at only one nucleotide position. After primers design, reaction
optimization can involves primer and DNA template concentrations and also the use of a
thermal gradient in order to determine the best annealing temperature that yields 100%
reaction efficiency and produces a single product. Thus, careful sample preparation and
assay design are crucial for robust and reproducible results.
In the present study, only RYR1 NN (C/C) and Nn (C/T) variants occurring in the
investigated population were genotyped. Our result showed that HRM is a close tube,
homogenous genotyping assay, that can be used successfully in genotyping of RYR1 and can
replace PCR-RFLP analysis, which is more labour intensive and expensive.
HRM analysis characterizes samples by their dissociation behaviour, which is based
on sequence length, GC content and DNA sequence complementarities, and can be used to
detect single base sequence variations (M. LIEW & al. [24]). When HRM genotyping
accuracy was compared to PCR-RFLP assay no discordant results were observed. The
commonly used PCR–RFLP technique was laborious and time-consuming comparing with
qPCR-HRM assay, where the results were obtained faster and with less labour. The qPCRHRM assay setup was fast, since it took less than 2 hours to prepare the PCR mix, to
perform PCR reaction, HRM analysis and genotypic data acquisition. We were able to
obtain results within 6 h for the whole procedure, including the genomic DNA extraction
and spectrophotometric measurements.
Our findings are in accordance with those of other researchers which have previously
stated that high-resolution melting analysis is a convenient technique, with no processing or
separation steps required (C.T. WITTWER & al. [25]). In addition to genotyping, highresolution melting analysis is an accurate mutation scanning tool (G.H. REED & al. [26]).
Conclusions
A more recent development in fluorescent analysis of PCR amplicons was
investigated, named high-resolution melting analysis, and this assay was applied to the
analysis of RYR1 gene in pigs. In summary, our results showed that the HRM technique was
a rapid, reliable and cost-effective assay that allowed us genotyping the C1843T SNP
located in RYR1 swine gene, based on post-PCR melting curve analysis under highresolution conditions.
The detection of RYR1 polymorphism using HRM technique can be widely used by
breeding companies and producers in order to reduce the frequency of susceptible allele (n)
in Romanian swine population trough proper selection. This could lead to an improvement
of the pork meat quality, with beneficial effects in overall profitability of farms.
Acknowledgements
This work was published during the projects: PN II PCCA 120/2012 and
“Postdoctoral School of Agriculture and Veterinary Medicine", POSDRU/89/1.5/S/62371,
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Romanian Biotechnological Letters, Vol. 19, No. 2, 2014
Screening of RYR1 genotypes in swine population by a rapid and sensitive method
co-financed by the European Social Fund through the Sectorial Operational Programme
Human Resources Development 2007-2013.
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