Analysis of uni and bi-parental markers in mixture samples: Lessons from the 22nd GHEP-ISFG Intercomparison Exercise

Since 1992, the Spanish and Portuguese-Speaking Working Group of the ISFG (GHEP-ISFG) has been organizing annual Intercomparison Exercises (IEs) coordinated by the Quality Service at the National Institute of Toxicology and Forensic Sciences (INTCF) from Madrid, aiming to provide proficiency tests for forensic DNA laboratories. Each annual exercise comprises a Basic (recently accredited under ISO/IEC 17043: 2010) and an Advanced Level, both including a kinship and a forensic module. Here, we show the results for both autosomal and sex-chromosomal STRs, and for mitochondrial DNA (mtDNA) in two samples included in the forensic modules, namely a mixture 2:1 (v/v) saliva/blood (M4) and a mixture 4:1 (v/v) saliva/semen (M8) out of the five items provided in the 2014 GHEP-ISFG IE. Discrepancies, other than typos or nomenclature errors (over the total allele calls), represented 6.5% (M4) and 4.7% (M8) for autosomal STRs, 15.4% (M4) and 7.8% (M8) for X-STRs, and 1.2% (M4) and 0.0% (M8) for Y-STRs. Drop-out and drop-in alleles were the main cause of errors, with laboratories using different criteria regarding inclusion of minor peaks and stutter bands. Commonly used commercial kits yielded different results for a micro-variant detected at locus D12S391. In addition, the analysis of electropherograms revealed that the proportions of the contributors detected in the mixtures varied among the participants. In regards to mtDNA analysis, besides important discrepancies in reporting heteroplasmies, there was no agreement for the results of sample M4. Thus, while some laboratories documented a single control region haplotype, a few reported unexpected profiles (suggesting contamination problems). For M8, most laboratories detected only the haplotype corresponding to the saliva. Although the GHEP-ISFG has already a large experience in IEs, the present multi-centric study revealed challenges that still exist related to DNA mixtures interpretation. Overall, the results emphasize the need for further research and training actions in order to improve the analysis of mixtures among the forensic practitioners.

Multiple nevoid basal cell carcinoma syndrome associated with congenital orbital teratoma, caused by a PTCH1 frameshift mutation

Gorlin-Goltz syndrome, or nevoid basal cell carcinoma syndrome (NBCCS), is a rare autosomal dominant disorder caused by mutations in the PTCH1 gene and shows a high level of penetrance and variable expressivity. The syndrome is characterized by developmental abnormalities or neoplasms and is diagnosed with 2 major criteria, or with 1 major and 2 minor criteria. Here, we report a new clinical manifestation associated with this syndrome in a boy affected by NBCCS who had congenital orbital teratoma at birth. Later, at the age of 15 years, he presented with 4 major and 4 minor criteria of NBCCS, including multiple basal cell carcinoma and 2 odontogenic keratocysts of the jaw, both confirmed by histology, more than 5 palmar pits, calcification of the cerebral falx, extensive meningeal calcifications, macrocephaly, hypertelorism, frontal bosses, and kyphoscoliosis. PTCH1 mutation analysis revealed the heterozygous germline mutation c.290dupA. This mutation generated a frameshift within exon 2 and an early premature stop codon (p.Asn97LysfsX43), predicting a truncated protein with complete loss of function. Identification of this mutation is useful for genetic counseling. Although the clinical symptoms are well-known, our case contributes to the understanding of phenotypic variability in NBCCS, highlighting that PTCH1 mutations cannot be used for predicting disease burden and reinforces the need of a multidisciplinary team in the diagnosis, treatment, and follow-up of NBCCS patients.