Schematics of the proposed multi-allele genotyping platform are
shown in Fig. 1. Non purified PCR amplified (1087 bp) fragment of
either HBA1 and HBA2 gene is subjected to few (10) cycles of multiplex
PEXT reaction in the presence of allele-specific primers (two primers
per mutation, one for the wild type and the second for the mutant
allele) and biotin-modified nucleotide that is incorporated in the
extended product. Allele-specific primers consist of two parts, the 3′
end that is complementary to the particular HBA1 or HBA2 allele and
the 5′ end containing a unique segment that enables subsequent capture
of the primer on the test zone of the dipstick through hybridization
with complementary immobilized capture oligonucleotides. Extension
by DNA polymerase, lacking 5′ → 3′ exonuclease activity occurs only
if primer is perfectly complementary to the target sequence. Finally,
the extension products are detected within 15 min by means of dry
reagent dipstick assay. The PEXT reaction mixture is applied to the dipstick
and the latter is immersed in the developing solution. As the solution
flows along the strip, the products are captured by immobilized
complementary oligonucleotides at the appropriate test spots and
detected by antibiotin-functionilized gold nanoparticles. Extended
products carrying biotin are bound to the nanoparticles through
biotin-antibiotin interactions and the hybrids produce characteristic
red spots denoting the presence of the corresponding allele in the sample.
The excess of gold nanoparticles is captured at the control zone of
the dipstick by immobilized biotinylated oligonucleotides forming
another red spot that indicates the proper performance of the system