Genetically engineered monkeys hold promise for sufferers of common genetic kidney disease

Polycystic kidney disease is passed genetically from parents to children and has no cure. In patients, thousands of cysts, or sacs of fluid, grow in the kidneys and eventually cause kidney failure. One form of the illness, autosomal dominant polycystic kidney disease (ADPKD), affects 1 in every 400–1,000 people and is the most common genetic kidney disease and, in fact, one of the most common human genetic disorders.

Only one copy of a mutated PKD1 or PKD2 gene is enough to result in type 1 or type 2 ADPKD, respectively. If one parent is affected, half of the children will carry a mutated version of the gene. Unfortunately, ADPKD research that uses small animals, such as mice, is of limited help because their ADPKD development does not mimic that in humans. However, a breakthrough from Kyoto University now offers hope for the many current and future patients with this condition.

Writing in the journal Nature Communications, a team at Kyoto University’s Institute for the Advanced Study of Human Biology reports having developed a suitable ADPKD animal model. Through genetic targeting, the researchers successfully simulated the disease in cynomolgus monkeys, also known as long-tailed or crab-eating macaques.

“In humans, we often see early signs of kidney cysts in children of ADPKD patients. Mice with induced ADPKD do not show this presymptomatic stage, so we need a more reliable and relevant animal model to understand ADPKD. Cynomolgus monkeys have a very similar physiology and genetic profile to humans and live longer than mice,” explains lead author Tomoyuki Tsukiyama.

The team sought to recreate the genetic origin and symptoms of type 1 ADPKD in the monkeys, by using CRISPR/Cas9 gene-editing technology to generate PKD1 gene mutations in embryos. From 29 embryos that had successfully implanted in surrogate mothers, 10 live monkeys carrying the mutant gene were born and survived.

In all the surviving monkeys, ultrasound scans revealed the presence of kidney cysts, starting as early as the first few months after birth. Genetic analysis of external tissue such as the umbilical cord revealed that the number of cysts was associated with the extent of PKD1 mutation. Because blood tests indicated normal kidney function in all monkeys, this animal model may mimic presymptomatic-stage ADPKD in children, the researchers suggest.

In the monkeys that did not survive, genetic analysis and tissue microscopy revealed that the degree of kidney cyst formation was again generally associated with the PKD1 mutation rate. Five monkeys were heterozygous, meaning that only one of the two PKD1 genes was mutated, thereby mimicking the major genetic profile in human ADPKD. Three monkeys, one of which initially survived for 216 days, were analyzed in detail and the kidneys showed only a few small cysts. This situation mirrors that in humans with early-stage heterozygous ADPKD, the researchers note.

By contrast, the most extensive mutations in which both copies of the PKD1 gene had been completely disrupted, or “knocked out”, led to abortion of the fetus. The scarred kidneys of three animals were especially massive owing to the presence of many cysts. These features, along with cysts in the liver and pancreas, resembled late-stage ADPKD in humans in whom the second PKD1 gene has also mutated.

To further examine the effect of heterozygous PKD1 mutations, the team specifically mutated only one PKD1 gene in single-cell embryos, leaving one copy of the gene intact. In two heterozygous monkeys that survived, a few kidney cysts were present at or shortly after birth and they enlarged by age 6 months. The results prompted the team to propose heterozygous monkeys as a useful model of investigating heterozygous-type ADPKD in humans, including children. Furthermore, the slow aging of the monkeys will allow for long-term monitoring of disease progression and its mechanism.

Tsukiyama says, “We have opened a new path to research intractable diseases that cannot be reproduced in small animals. In particular, our model will be a basic resource for elucidating the key pathological processes underlying cyst development in presymptomatic ADPKD. We hope this approach will help establish therapeutic strategies, including novel drug treatments.”