Identification of a rare tri-allelic inheritance at the D8S1179 locus in a case of paternity testing

Abstract

Short Tandem Repeat (STR) typing is routinely used in forensic casework, paternity disputes and in the detection of aneuploidies. Anomalous peak patterns are occasionally encountered in STR typing due to genetic variations, mutations and, other abnormalities. We report here a rare tri-allelic variant at the D8S1179 locus in a 6-month-old female child, observed during routine paternity testing in the Indian Tamil population.DNA eluted from the blood samples collected on FTA elute microcards were subjected to STR typing for autosomal and sex STR loci using the commercial AmpFlSTR Identifiler Plus kit and the Promega PowerPlex® Fusion kit. Capillary electrophoresis was carried out in ABI 3130xl Genetic Analyzer and genotyped using GeneMapper ID-X Software v1.5. Copy number variations and aneuploidy status of chromosome 8 were analyzed using Multiplex ligation-dependent probe amplification (MLPA) and Prenatal BoBs assays. In this case of paternity exclusion, the child displayed a Type II tri-allelic pattern (10,13,14) at the D8S1179 locus. In addition, the mother displayed an abnormal 10,14 genotype (with heterozygous peak imbalance - 2:1) at the D8S1179 locus suggesting 10,10,14 pattern. MLPA and prenatal BoBs analyses did not detect any abnormalities on chromosome 8. These results suggest an intrachromosomal segmental duplication event at the D8S1179 locus (8q24.13) as the reason behind tri-allelic pattern and its inheritance. Understanding anomalous cases such as these can help in appropriate interpretation of the results in STR typing.