Mother–child exclusion due to paternal uniparental disomy 6

Maternal uniparental disomy of chromosome 21 as a cause of pseudo-exclusion from paternity

Uniparental disomy (UPD) is a rare chromosomal condition, which apart from its importance in medical genetics can affect an outcome of parentage DNA testing, often causing pseudo exclusions. We describe a case of trio paternity test using 24 informative STR loci with potential exclusion at 2 systems located on chromosome 21. Consequent genotyping of an additional 25 autosomal and 27 Y-specific STRs revealed one other inconsistency, also located on this chromosome. All three inconsistent markers had the same heteroallelic state between the child and the biological mother providing evidence for maternal heterodisomy of chromosome 21. The case highlights the importance of considering UPD as a cause of genetic inconsistencies, especially when the inconsistent marker systems are located on the same chromosome.

Uniparental disomy of chromosome 21: A statistical approach and application in paternity tests

Considering the overall frequency of paternity investigation cases including mutational events, there is a real possibility that at least a fraction of all inconsistencies reported in paternity cases are caused not by polymerase slippage mutations, but to chromosomic abnormalities, as Uniparental Disomy (UPD). We report here the investigation of a trio paternity case (mother, child and alleged father), with observed inconsistencies that can alternatively be explained by occurrence of maternal uniparental isodisomy of chromosome 21 (miUPD21). A total of 350 short tandem repeat (STR) and single nucleotide polymorphism (SNP) markers were tested, statistically suggesting true biological linkage within the trio. Additionally, we propose miUPD21 explains, with significantly greater probability, the occurrence of detected inconsistencies, when compared to alternative hypothesis of multiple and simultaneous slippage mutations. Similar cases could have their statistical conclusions improved or even altered by including unusual chromosomal segregation patterns in the hypothesis formulation, as well as in mathematical calculations. Such reports of allelic inconsistencies being explained by chromosomal alterations are common in clinical genetics, and such situations might have impact on forensic investigation.

Complete Paternal Uniparental Disomy of Chromosome 2 in an Asian Female Identified by Short Tandem Repeats and Whole Genome Sequencing

Uniparental disomy (UPD) is a rare type of chromosomal aberration that has sometimes been detected in paternity testing. We examined a 3-person family (father, mother, daughter) first by using short tandem repeat markers, which revealed 4 markers, TPOX, D2S1338, D2S1772, and D2S441, on chromosome 2 that were not transmitted in a Mendelian style. We then performed whole genome sequencing (WGS) to determine the range of the UPD. Chromosome 2 in the daughter showed a complete paternal UPD. To the best of our knowledge, this is the 4th case of complete paternal UPD of chromosome 2 with no clinical phenotype. Our study suggests that WGS, when performed to enhance the accuracy and reliability of parentage testing, can provide a powerful method to detect an UPD.

Maternal uniparental isodisomy for chromosome 6 discovered by paternity testing: a case report

Background

Uniparental disomy (UPD) is a rare condition in which a child inherits both copies of a chromosome or chromosome segment from one parent. Medical consequences of UPD may include abnormal imprinting, unmasking of genetic disease, and somatic mosaicism; alternatively, the condition may be clinically silent. We present a case of maternal UPD for chromosome 6, a rare condition previously reported less than 20 times. In our patient with a normal phenotype, the condition was discovered through abnormal paternity testing results. Uniparental isodisomy is a rare cause of discordant parentage testing results, but it is an important phenomenon to recognize.

Case presentation

We present a female born at 32 weeks gestational age with birth weight 10–25%ile when corrected for prematurity. Paternity testing was obtained for legal reasons, and initial results appeared to exclude the alleged father. However, the lab performed additional testing which indicated that the patient was homozygous for maternal alleles for all three tested loci located on chromosome 6. Based on these results, the patient was referred for a medical genetics evaluation for possible maternal uniparental disomy. She presented for her consultation at 10 months of age and appeared to be developing appropriately. Her age-adjusted weight, length, and head circumference were <3%ile, 10%ile, and 25%ile respectively. Chromosomal microarray testing confirmed maternal UPD6. The patient was seen again at 14 months of age, and her weight and length were 10–25%ile. She had not developed concerning symptoms or physical exam findings.

Conclusions

The presence of UPD, especially in asymptomatic patients, has implications for paternity testing, as standard methods may miss cases of both isodisomy and heterodisomy. This rare inheritance pattern should be considered when discordant paternity results come under suspicion. It is unusual for a parentage testing lab to perform the amount of testing done for this case, but the initial inconsistencies necessitated further investigation. UPD6 has uncertain effects and variable phenotypes, so this patient’s genetic abnormality likely would have gone undiscovered if not for the non-medical indication for the laboratory analysis. Her asymptomatic presentation raises the possibility that UPD may be more common than previously estimated.

Non-pathological complete paternal uniparental isodisomy of chromosome 2 revealed in a maternity testing case

We present a duo paternity test case to assess the biological relationship between a woman and her female child. After analyzing 57 autosomal and 19 X-chromosomal short tandem repeat loci, mother-daughter exclusions were discovered at four loci, which were all located on chromosome 2. Further testing of whole-genome single nucleotide polymorphisms confirmed that the daughter had complete uniparental disomy (UPD) of chromosome 2. This study presents a cautionary case demonstrating that hasty decisions of parentage exclusion should not be made when genetic markers on the same chromosome do not conform to Mendel's laws due to UPD.

Technical report: Ethical and policy issues in genetic testing and screening of children

The genetic testing and genetic screening of children are commonplace. Decisions about whether to offer genetic testing and screening should be driven by the best interest of the child. The growing literature on the psychosocial and clinical effects of such testing and screening can help inform best practices. This technical report provides ethical justification and empirical data in support of the proposed policy recommendations regarding such practices in a myriad of settings.

Complete paternal uniparental isodisomy for Chromosome 2 revealed in a parentage testing case

BACKGROUND

Uniparental disomy (UPD) is a rare cytogenetic event that has previously been reported mostly via genetic analysis of patients with phenotypes of recessive diseases. The incidence of UPD of any chromosome is estimated to be approximately1:3500 live births.

CASE REPORT

In a case of disputed paternity involving a phenotypically normal male child, mother-child exclusions were observed at five short tandem repeat markers, which were all located on Chromosome 2. Ten additional dinucleotide repeat markers spanning both arms of Chromosome 2 were investigated. The results revealed that the child was homozygous for all markers tested with all alleles originating from a single paternal Chromosome 2, which was consistent with paternal UPD for Chromosome 2.

CONCLUSION

This case and other previous reports demonstrate that UPD poses a high risk for false exclusion and incorrect expert opinion. Furthermore, this case highlights that a conclusion of exclusion of paternity or maternity should not be postulated if multiple genetic incompatibilities are located on the same chromosome because of the occurrence of UPD.

Good ethics requires good science: why transplant programs should not disclose misattributed parentage

In 1996, I argued that the recommendation by the Institute of Medicine (IOM) to inform women when tests reveal misattributed paternity and not to disclose this information to the women's partners was morally wrong. I argued in favor of disclosure to both parties. It is a position that I still hold. But claims of misattributed paternity are not 'incidental findings' as it was called in the old genetics literature, but a rather serious indictment of biological infidelity. In this paper I argue that the tests used by transplant programs for living donor-recipient compatibility are inadequate to accurately determine misattributed paternity. Further I argue that it is not the responsibility of the transplant community to undertake such serious forensic evaluations. Genetic inconsistencies in ABO and HLA inheritance should be reported as variations. Families who want further clarification should be referred to a genetic professional.

A paternity case with three genetic incompatibilities between father and child due to maternal uniparental disomy 21 and a mutation at the Y chromosome

A parentage case is described that revealed a potentially erroneous exclusion from paternity in three systems, two on chromosome 21 and one on chromosome Y. Follow-up tests, especially of chromosome 21, were subsequently performed. Actually, the child's chromosome 21 showed alleles of maternal but not of paternal origin being consistent with a maternal uniparental disomy of chromosome 21. The third genetic incompatibility was observed at the Y chromosome and attributed to a usual one-step de novo mutation. This case is emphasizing the (generally adopted) requirement that an exclusion from paternity must not be based on the absence of paternal alleles at genetic systems all located on the same chromosome. In fact, the need for extended typing programmes is demonstrated.

New alleles and mutational events at 14 STR loci from different German populations

The molecular origin of DNA mutations and the mutation rates were analyzed at 14 short tandem repeat (STR) loci with samples from trio cases derived from 10 different German population samples. STR loci comprised of D2S1360, D3S1744, D4S2366, D5S2500, D6S474, D7S1517, D8S1132, D10S2325, D12S391, D18S51, D19S246, D20S480, D21S226, and D22S689. In a total of 488 meioses, 16 isolated genetic inconsistencies in 8 different STRs were observed, whereas no mutations were found at the other loci. The data of five mutations suggested the presence of silent or null alleles due to sequence variation in primer binding site. This could be confirmed for four suspected cases by the use of alternative primer sets and by DNA sequence analyses. Furthermore, this study revealed nine new allelic variants at five different loci.

X-chromosomal markers: past, present and future

Experience gained in clinical genetics led to the fundamental idea of using X-chromosomal markers in a wide range of forensic applications. To date more than 30 STRs have been established as forensic markers. Joint typing of very tightly linked STRs yields stable haplotypes, and can be used for establishing the relationship between distant relatives, such as aunt-niece pairs and cousins. For such applications the new ChrX typing kit Argus X-8 which is commercially available now is a powerful tool. This paper is aimed at presenting a brief survey of historical developments and discussing present and future aspects of forensic X-chomosomal testing.