Preventing the transmission of mitochondrial DNA disorders using prenatal or preimplantation genetic diagnosis

Mitochondrial disorders are among the most common inborn errors of metabolism; at least 15% are caused by mitochondrial DNA(mtDNA) mutations, which occur de novo or are maternally inherited. For familial heteroplasmic mtDNA mutations, the mitochondrial bottleneck defines the mtDNA mutation load in offspring, with an often high or unpredictable recurrence risk. Oocyte donation is a safe option to prevent the transmission of mtDNA disease, but the offspring resulting from oocyte donation are genetically related only to the father. Prenatal diagnosis (PND) is technically possible but usually not applicable because of limitations in predicting the phenotype. For de novo mtDNA point mutations, recurrence risks are low and PND can be offered to provide reassurance regarding fetal health. PND is also the best option for female carriers with low-level mutations demonstrating skewing to 0% or 100%. A fairly new option for preventing the transmission of mtDNA diseases is preimplantation genetic diagnosis (PGD), in which embryos with a mutant load below a mutation-specific or general expression threshold of 18% can be transferred. PGD is currently the best reproductive option for familial heteroplasmic mtDNA point mutations. Nuclear genome transfer and genome editing techniques are currently being investigated and might offer additional reproductive options for specific mtDNA disease cases

Mutations in the vasopressin type 2 receptor gene (AVPR2) are associated with nephrogenic diabetes insipidus

Nephrogenic diabetes insipidus (DIR) is an X-linked disorder characterized by insensitivity of the distal nephron for the pituitary hormone, vasopressin. The genetic map location of the DIR gene on chromosome Xq28 coincides with the physical map location of the functional vasopressin renal V2-type receptor. Recently, the human and rat cDNAs for the vasopressin V2 receptor (AVPR2) have been identified. We show here that the structural AVPR2 gene is localized between DXS52 and G6PD, which is within the genetic map location of DIR. We also tested eight X-linked DIR probands and their families for mutations in one of the most conserved extracellular regions of AVPR2: in three of them, we have identified point mutations resulting in non-conservative amino acid substitutions which cosegregated with DIR in all families

Preimplantation genetic diagnosis of spinal muscular atrophy

After Duchenne muscular dystrophy, spinal muscular atrophy (SMA) is the most common severe neuromuscular disease in childhood. Since 1995, homozygous deletions in exon 7 of the survival motor neuron (SMN) gene have been described in >90-95% of SMA patients. However, the presence of a highly homologous SMN copy gene complicates the detection of exon 7 deletions. This paper describes the adjustment and evaluation of an established SMN exon 7 polymerase chain reaction (PCR) protocol at the single cell level, and the first preimplantation genetic diagnosis (PGD) of SMA with this PCR protocol. To determine PCR efficiency and allelic loss, 200 leukocytes of normal individuals, SMA carriers and patients, and 25 blastomeres were tested. The PCR efficiency of the SMN exon 7 and the adjacent copy gene sequence, tested in the leukocytes, were 90% and 91% respectively. No allelic loss was detected. One out of 25 blastomeres tested revealed a negative PCR signal for the SMN exon 7 sequence. All 25 showed the copy gene sequence. PGD of SMA was offered to a couple with an affected child homozygous for the SMN exon 7 deletion. After intracytoplasmic sperm injection, four and five embryos could be genotyped for the SMN exon 7 in two cycles respectively. After embryo transfer in the second PGD cycle an ongoing gemelli pregnancy was achieved. This study demonstrates that PGD for SMA is feasible when a previous child is homozygous for the SMN exon 7 deletion