Human sex characteristics develop in the womb. About one in 100 babies are born with variations, some of which are linked to health problems, such as increased cancer risk, heart disease, osteoporosis, intellectual disability, or infertility.

Reproductive biologist Associate Professor Katie Ayers is working to identify the genes involved in these variations. She and her team are growing 3D models of developing gonads engineered from stem cells in the lab to study variations in sex development and the role of genetics.

The Foundation is backing her research through its Matched Funding Program in collaboration with the Cybec Foundation.

About one in 4,500 children are born with genitals that are of neither typical female nor typical male appearance. Uncertainty about gender can carry profound physical and mental health consequences for the child and family. Failure of testes development is another issue, with a prevalence of one in 20,000.

A lot remains unknown about the genes that control development of the gonads (the testis or ovaries) in humans. Recently genetic sequencing in people born with a variation or difference of sex development has revealed some of the genes involved.

In many cases, having a correct genetic diagnosis can help the young person and their family to understand differences in their gonadal and genital development and inform decisions during their clinical care. Currently, however, a genetic cause can only confidently be found in around 40 per cent of cases, leaving many affected individuals and families facing uncertainty and possibly invasive testing.

Katie’s research group at the Murdoch Children’s Research Institute (MCRI) aims to improve this, by working with families and clinicians at the Royal Children’s Hospital and other centres internationally to find underlying genetic causes.

“Our goal is to provide doctors, affected individuals and families with the clearest possible picture, so they can make informed decisions,” says Katie.

Human embryos begin developing the main reproductive glands – the gonads – very early in gestation, at around 5 weeks from conception. At this stage, there is no difference between gonads that will eventually become ovaries and testes.

From 5 weeks, the presence of the male Y-chromosome kickstarts genetic signalling that drives development of testicular tissue in genetic males, whereas embryos without the Y-chromosome will develop ovaries. The fetal testis begins producing testosterone between 9- and 12-weeks gestation, leading to other tissues developing differently between genetic males and females, including the genitals.

“Scientists don’t really have a very clear idea of how human gonads go through that initial stage of development before the production of testosterone,” says Katie.

“This is because we don’t have a good model of the human fetal testis in the lab – animal models are not perfect, there aren’t any cell lines that represent the human fetal testis and getting human fetal tissue is not feasible. Therefore, we are using stem cell technologies to build a model of the human gonad in the lab.”

Katie and her team are building an innovative 3D arrangement of gonadal supporting cells (the cells that support the germ cells which become eggs or sperm), mimicking the physical structure of developing gonads. This gonadal model also holds promise as a disease model, to allow researchers to understand how gonad development might be disrupted in differences of sex development.

Katie and her collaborators recently used this model to identify a new rare genetic syndrome that causes intellectual disability and differences of sex development in children. Published in Nature Communications, the work describes 9 children from 6 families who have intellectual disability, delays in reaching developmental milestone and neurodevelopmental (brain development) changes.

The children with male sex chromosomes (XY) also had atypical development or appearance of the gonads and external genitals. Disruption to testis development meant reduced production of the masculinising hormone, testosterone, leading to female or atypical male genital appearance. All affected children carry recessive variants in a gene called SART3, inherited from both parents.

In this study, the researchers used stem cell models of the brain and the gonads to show that the genetic changes they found are the underlying cause of this new condition, providing a diagnosis to the families, some who had been waiting more than 12 years.

The discovery paves the way for more diagnoses in affected individuals, providing families searching for answers with some certainty around this extremely rare disease. It could also lead to the development of new treatments.

Support from the National Stem Cell Foundation of Australia, which has been matched by philanthropic partner Cybec Foundation, means that Katie and her team are now able to test how other genetic variants may cause differences of sex development and affect gonad development and signalling.

Katie hopes to build on this work over the next several years, applying stem cell models to all testicular and ovarian cell types and testing a range of different genetic changes with the goal of improving diagnoses and testing potential therapies.