Molecular Counting

Book excerpt: The concept of a personal genome stems from the fact that every human genome is unique. Measuring the unique features of a personal genome would help uncover the genetic basis of diseases and traits, and would be increasingly important in clinical diagnosis especially with the growing emphasis on personalized medicine. This thesis focuses on exploring the power of molecular counting to develop novel strategies that address the inadequacy of existing technologies in measuring the unique features of a human genome. The first focus of the thesis is aneuploidy detection, which has major application in prenatal diagnosis. While karyotyping of fetal cells is well-established for detecting aneuploidy, invasive sampling of fetal materials impose a small but significant risk to the health of both the mother and the fetus. A major research focus in the field of prenatal diagnosis has been to develop a noninvasive test for detecting fetal aneuploidy. Here, the concept of single molecule counting was applied to the problem of aneuploidy detection. The concept was first tested with digital PCR on invasively collected fetal materials, and subsequently extended to the noninvasive setting by shotgun sequencing maternal plasma DNA, which contains a small amount of fetal DNA. The former work led to the development of a polymorphism-independent method for rapid invasive diagnosis of aneuploidy, while the later work marked the development of the first polymorphism-independent method for the noninvasive diagnosis of fetal aneuploidy documented in the literature. The second focus of the thesis is molecular haplotyping. Present sequencing and other molecular techniques concentrate at identifying variants at isolated locations throughout a genome but largely ignore the haplotypes formed by these variants. Direct experimental determination of the haplotypes of an individual is challenging because of the lack of techniques to separate the two highly similar homologous copies of a chromosome. Here, a whole-genome haplotyping method was devised by analyzing amplified materials from single intact chromosomes within single cells, made possible by microfluidics. Such strategy enabled, for the first time, completely deterministic measurement of personal whole-genome haplotypes. It sets the stage for the direct sequencing of the two unique haploid genomes of any individual human, which has not been achieved by any personal genomes sequenced to date, and can potentially facilitate noninvasive fetal genome sequencing.