Mosaicism for mitochondrial DNA polymorphic variants in placenta has implications for the feasibility of prenatal diagnosis in mtDNA diseases

Abstract

Women who have had a child with mitochondrial DNA (mtDNA) disease need to know the risk of recurrence, but this risk is difficult to estimate because mutant and wild-type (normal) mtDNA coexist in the same person (heteroplasmy). The possibility that a single sample may not reflect the whole organism both impedes prenatal diagnosis of most mtDNA diseases, and suggests radical alternative strategies such as nuclear transfer. We used naturally occurring mtDNA variants to investigate mtDNA segregation in placenta. Using large samples of control placenta, we demonstrated that the level of polymorphic heteroplasmic mtDNA variants is very similar in mother, cord blood and placenta. However, where placental samples were very small (o10 mg) there was clear evidence of variation in the distribution of mtDNA polymorphic variants. We present the first evidence for variation in mutant load, that is, mosaicism for mtDNA polymorphic variants in placenta. This suggests that mtDNA mutants may segregate in placenta and that a single chorionic villous sample (CVS) may be unrepresentative of the whole placenta. Duplicates may be necessary where CVS are small. However, the close correlation of mutant load in maternal, fetal blood and placental mtDNA suggests that the average load in placenta does reflect the load of mutant mtDNA in the baby. Provided that segregation of neutral and pathogenic mtDNA mutants is similar in utero, our results are generally encouraging for developing prenatal diagnosis for mtDNA diseases. Identifying mtDNA segregation in human placenta suggests studies of relevance to placental evolution and to developmental biology.

Information for genetic management of mtDNA disease: sampling pathogenic mtDNA mutants in the human germline and in placenta

“…In addition, mtDNA copy number varied approximately fivefold (see online figure 3). Similarly, we saw a statistically significant variation of similar magnitude in the distribution of one of two polymorphic variants, consistent with previous data 37. We previously demonstrated that the average level of polymorphic heteroplasmic mtDNA variants is very similar in mother, cord blood and placenta.…”

“…In preliminary data from human blastomeres41 42 (supplementary information 2), we provide some support for earlier findings from mouse43 and human44 that heteroplasmy in blastomeres varies more between embryos than within embryos. Nevertheless, in the absence of extensive data, it may be necessary to take two samples at CVS37 because of the considerable variation in the level of mtDNA mutant that we documented in placental patches. This also argues for repeated sampling at PGD, say two separate blastomeres.…”

Quantitative and qualitative profiling of mitochondrial DNA length heteroplasmy

“…In the case of HV1 length heteroplasmy, all of these samples contained a T-to-C transition at position 16189, which resulted in an unbroken series of 10Cs. This long C-stretch was considered to lead to poor replication fidelity and consequently length heteroplasmy [21,22]. According to a previous report [23], length heteroplasmy in the HV1 region can be easily identified by the dramatic decrease in sequencing quality that occurs beyond the heteroplasmic region.…”

“…On the other hand, length heteroplasmies in the HV2 region do not contain a T-to-C transition and do not show significant interference of sequencing analysis in the C-stretch region. In sequencing electropherograms, the heteroplasmic ratios in HV2 vary greatly from sample to sample (ranging from no detectable heteroplasmy, through barely discernible, to pronounced heteroplasmy) [22]. Therefore, length heteroplasmies in the HV2 region cannot be identified only from sequencing electropherograms.…”

“…As compared with point heteroplasmy, length heteroplasmy has been considered to be somewhat general and to have less population specificity. Nonetheless, the genetic characteristics of length heteroplasmy have been the subject of investigation, primarily to gain insight into the factors that influence the transmission of the mtDNA, particularly in relation to the germ-line bottle neck [13,14]. One of the polymorphic tracts, associated with an mtDNA 16189C variation which is located within and interrupts a homopolymeric tract of cytosines (Cs) between nucleotide (nt) 16184 and nt 16193 in the HV1 region, shows length variants of between 8 and 14 Cs within a single individual [12].…”