EVIDENCE FROM HUMAN OOCYTES FOR A GENETIC BOTTLENECK IN AN MTDNA DISEASE

We have examined oocytes from a patient with Kearn-Sayre syndrome caus ed by mtDNA rearrangements. In mtDNA diseases, mutant and wild-type mt DNA frequently coexist in affected individuals (the condition of heter oplasmy). The proportion of mutant mtDNA transmitted from mother to of fspring is variable because of a genetic bottleneck, and the ''dose'' of mutant mtDNA received influences the severity of the phenotype. The feasibility of prenatal diagnosis is critically dependent on the natu re and timing of this bottleneck. Significant levels of rearranged mtD NA were detectable in the majority of the patient's oocytes, by use of multiplex PCR, with wide variation, in the levels of mutant and wildt ype molecules, between individual oocytes. We also used length variati on in a homopolymeric C tract, which is often heteroplasmic in normal controls, to identify founder subpopulations of mtDNAs in this patient 's oocytes. We present direct evidence that the number of segregating units (n) is three to five orders of magnitude less than the number of mitochondria in the human female oocyte. In some cases, the best esti mate of it may correspond to a single mitochondrion, if it is assumed that intergenerational transmission of mtDNA can be treated as a singl e sampling event. The bottleneck appears to contribute a major compone nt of the variable transmission from mother to oocyte, in this patient and in a control. That this bottleneck had occurred by the time that oocytes were mature advances the prospects for prenatal diagnosis of m tDNA diseases.