Heterozygote PCR Product Melting Curve Prediction

Pitfalls of commercially available HPV tests in HPV68a detection

Background

Human papillomavirus 68 (HPV68) is a probable carcinogenic HPV genotype which is included in almost all HPV screening assays and exists as two genetically variable subtypes (HPV68a and HPV68b). Routine HPV sample testing has shown that the cobas 4800 HPV Test (Roche) provides higher false-negative rates for HPV68 status than PapilloCheck HPV-Screening (Greiner Bio-One). The aim of our study was to evaluate the efficacy of cobas 4800 in HPV68 detection.

Methods

A total of 2,145 cervical/cervicovaginal samples from women aged 17–88 were tested for HPV68 status using the cobas 4800 and PapilloCheck HPV tests. Viral load was assessed by quantitative PCR in all of the HPV68-positive cases. HPV68a/b subtyping was performed with real-time PCR followed by high resolution melting curve analysis, and was subsequently confirmed by Sanger sequencing.

Results

Cobas 4800 detected HPV positivity in only 13/33 HPV68 single-genotype infection cases. Viral load was comparable across both tested subgroups. HRM analysis and Sanger sequencing identified the HPV68a subtype in all of the 20 instances of cobas 4800 false negatives. HPV68a and HPV68b were detected in 3/13 and 10/13 cases identified as other HPV-positive by cobas 4800.

Conclusion

The HPV68a subtype was missed by cobas 4800 in more than 85% of all HPV68a-positive cases. Therefore, commercially available assays may underestimate HPV68 prevalence.

Identification and discrimination of Toxoplasma gondii, Sarcocystis spp., Neospora spp., and Cryptosporidium spp. by righ-resolution melting analysis

The objective of this study was to standardize the high-resolution melting method for identification and discrimination of Toxoplasma gondii, Sarcocystis spp., Neospora spp., and Cryptosporidium spp. by amplification of 18S ribosomal DNA (rDNA) using a single primer pair. The analyses were performed on individual reactions (containing DNA from a single species of a protozoan), on duplex reactions (containing DNA from two species of protozoa in each reaction), and on a multiplex reaction (containing DNA of four parasites in a single reaction). The proposed method allowed us to identify and discriminate the four species by analyzing the derivative, normalized, and difference melting curves, with high reproducibility among and within the experiments, as demonstrated by low coefficients of variation (less than 2.2% and 2.0%, respectively). This is the first study where this method is used for discrimination of these four species of protozoa in a single reaction.

Assessment of high resolution melting analysis as a potential SNP genotyping technique in forensic casework

High resolution melting (HRM) analysis is a simple, cost effective, closed tube SNP genotyping technique with high throughput potential. The effectiveness of HRM for forensic SNP genotyping was assessed with five commercially available HRM kits evaluated on the ViiA™ 7 Real Time PCR instrument. Four kits performed satisfactorily against forensically relevant criteria. One was further assessed to determine the sensitivity, reproducibility, and accuracy of HRM SNP genotyping. The manufacturer's protocol using 0.5 ng input DNA and 45 PCR cycles produced accurate and reproducible results for 17 of the 19 SNPs examined. Problematic SNPs had GC rich flanking regions which introduced additional melting domains into the melting curve (rs1800407) or included homozygotes that were difficult to distinguish reliably (rs16891982; a G to C SNP). A proof of concept multiplexing experiment revealed that multiplexing a small number of SNPs may be possible after further investigation. HRM enables genotyping of a number of SNPs in a large number of samples without extensive optimization. However, it requires more genomic DNA as template in comparison to SNaPshot®. Furthermore, suitably modifying pre-existing forensic intelligence SNP panels for HRM analysis may pose difficulties due to the properties of some SNPs.