Highly polymorphic short tandem repeat analyses clarify complex molecular test results.

Judicious application of highly polymorphic short tandem repeat (STR) analyses and modification of assay conditions readily distinguished nonparentage from true parentage, with occasional failure to transmit one parentally derived allele. These categories were resolved with a reliability of >99.9%, the standard applied to most DNA evidence presented in a U.S. court of law. While completing a single prenatal diagnosis submitted at 19 weeks gestation, the authors found that one polymorphic grandpaternal allele did not amplify, a duplicate control propositus' DNA sample had been switched by an outside laboratory, and recombination occurred in maternal meiosis within the mutant gene region. In two parentage cases with one available parent, a Y-linked STR or an autosomal STR was modified when transmitted to the offspring. In contrast, some apparently inconsistent results between parental DNA and offspring were resolved by purifying or diluting the original extracted DNA samples. Thus, the source of each complex molecular test result was characterized unambiguously by testing a sufficient number of highly polymorphic STR loci and by purifying or diluting troublesome DNA samples to diminish polymerase chain reaction amplification artifact.

Tandem duplication/deletion in a maternally derived chromosome 9 supernumerary derivative resulting in 9p trisomy and partial 9q tetrasomy.

A 19-week stillborn female fetus with bilateral cleft palate, horseshoe kidney, bicornuate uterus, low-set ears, and intrauterine growth retardation (IUGR) was found to have a supernumerary derivative chromosome 9 (der(9)) with an apparent tandem duplication in the long arm. PCR analysis at five polymorphic loci confirmed the duplication and showed an adjacent deletion, while whole chromosome FISH demonstrated only chromosome 9 to be involved. Further FISH studies of der(9) found the 9qh region to be duplicated, telomeric sequences to be intact, and subtelomeric sequences to be absent. Thus, the fetus was determined to be trisomic for 9pter-->9q12 and 9q34.3-->9qter, tetrasomic for 9q12--> 9q33, and disomic for 9q33-->9q34.3. These results are consistent with a tandem duplication of 9q12-->9q33 and adjacent distal deletion as designated by the karyotype, 47,XX,+der(9)dup(9) (q12q33)del(9) (q33q34.3).ish der(9)(WCP9+,D9Z1x2,STP9q-, AHT+) de novo. In addition to characterizing der(9), the combined PCR and cytogenetic studies refined the Genome Database Map of three loci (D9S907, D9S155, and D9S302) approximately to the distal 9q33 region. Without the attempt to refine breakpoints by PCR analysis, the deletion in distal 9q would not have been detected. Tandem direct duplication/deletion chromosomes have been reported in fewer cases than inverted duplication/deletions. We propose mechanisms of origin, consistent with those for recurrent inter stitial microdeletion and microduplication syndromes, shown to arise by recombination at homologous, flanking DNA sequences.

Rare etiology of autosomal recessive disease in a child with noncarrier parents.

A child with maple syrup urine disease type 2 (MSUD2) was found to be homozygous for a 10-bp MSUD2-gene deletion on chromosome 1. Both purported parents were tested, and neither carries the gene deletion. Polymorphic simple-sequence repeat analyses at 15 loci on chromosome 1 and at 16 loci on other chromosomes confirmed parentage and revealed that a de novo mutation prior to maternal meiosis I, followed by nondisjunction in maternal meiosis II, resulted in an oocyte with two copies of the de novo mutant allele. Fertilization by a sperm that did not carry a paternal chromosome 1 or subsequent mitotic loss of the paternal chromosome 1 resulted in the propositus inheriting two mutant MSUD2 alleles on two maternal number 1 chromosomes.