Available Assays

When UPD is suspected, confirmatory testing is based on detection of polymorphic DNA markers in both parents and the child (Figure 6-2). The markers of choice typically are single nucleotide polymorphisms (SNPs), di-, tri-, or tetranucleotide repeat polymorphisms (short tandem repeats [STRs], microsatellites), or variable number of tandem repeats (VNTRs, minisatellites). These natural variants occur throughout the human genome, and characterized DNA markers are available for all chromosomes.

Figure 6-2. DNA marker analysis for isodisomy and heterodisomy. Polyacrylamide gel electrophoresis of a DNA marker indicating typical patterns found when maternal isodisomy (a) or heterodisomy (b) occurs. Parental patterns are opposite if paternal UPD is present (not shown). Autoradiographic signals were produced using PCR incorporating a radioactive label. Maternal DNA is designated as "M," paternal DNA as "P," and DNA from their child with UPD as "C." (a) Maternal isodisomy is indicated by the absence of paternal alleles and the presence of a single maternal allele in the child. (b) Maternal heterodisomy is indicated by the absence of paternal alleles and the presence of both maternal alleles in the child.

Figure 6-2. DNA marker analysis for isodisomy and heterodisomy. Polyacrylamide gel electrophoresis of a DNA marker indicating typical patterns found when maternal isodisomy (a) or heterodisomy (b) occurs. Parental patterns are opposite if paternal UPD is present (not shown). Autoradiographic signals were produced using PCR incorporating a radioactive label. Maternal DNA is designated as "M," paternal DNA as "P," and DNA from their child with UPD as "C." (a) Maternal isodisomy is indicated by the absence of paternal alleles and the presence of a single maternal allele in the child. (b) Maternal heterodisomy is indicated by the absence of paternal alleles and the presence of both maternal alleles in the child.

While DNA markers can be detected by several laboratory techniques, almost all use PCR amplification to produce sufficient material for analysis. Normal biparental inheritance will result in one of each parental allele for each DNA marker being present in the offspring. Uniparental disomy is present when no alleles are detected from one parent. When no paternal markers are detected, resulting from maternal UPD, paternity can be confirmed by using DNA markers from other chromosomes. Once a marker identifies UPD, additional markers in the chromosomal region can be used to fully explore potential segmental hetero- or isodisomy.

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