Julie and Paul lost one of their twins before he turned two. Now they're fighting for change.

When Julie Gravina found out she was having twins she thought she’d hit the baby jackpot.

Suddenly her family of three was about to become five and she found herself dreaming of the amazing life she was about to lead with two brand new squishy additions.

But Isaac and Charlize were born with a rare metabolic disorder called Propionic Acidemia, a condition that affects less than one in 100,000 people.

Here’s Julie speaking for Jeans for Genes. Post continues after video.

Suddenly Julie and Paul found themselves living in a nightmare and she describes the state of panic that followed as “fight or flight without an ending”.

“We knew there was a problem with Charlize when she was four days old. In the evening she was feeling cool, so we tried to warm her up with a bath.

“Through the night she wasn’t feeding as well as her brother, and during one of the feeds she just wouldn’t wake up and was cold and grey,” Julie told Mamamia.

By day seven Isaac was showing the same symptoms, and they were at a point with Charlize where they were being told she might not make it through the night.

"It's so hard because you know they're not going to have the life you wanted for them, and wished and hoped for them. It's one of those things we took for granted. I was so naive," Julie said.

Those living with Propionic Acidemia can't process certain parts of proteins properly. It's classified as an acid disorder and the ramifications are severe.

As Julie found out in a Google search, it leads to things like muscle weakness, heart problems, brain damage, coma and death.

Julie knew turning to Dr Google wasn't ideal and thought surely she'd just read a sensationalised worst case scenario. But her heart dropped when the doctor agreed with her findings; "I am sorry, that's the likely outcome," he said.

He then said the words that hit Julie the hardest: "There's no cure and there's no treatment".

"I was angry thinking why hadn't any more families that'd gone through this done more to find a treatment or pushed for a cure? How come my children were born at the wrong time in medical history? In 10 more years there might be something... why do we have to bear this burden?"

The Gravinas were told a liver transplant was the only thing that would help their twins, and so the family found themselves on a wait list that saw them confined within dashing distance of the hospital.

The kids spent hundreds of nights in hospital. Their little bodies had to be supported through every virus and cold and the ripple effects flowed onto the family. Amelia couldn't go to preschool in case she picked something up. Paul missed huge chunks of work. The family was forced to live in a bubble, to keep their littlest two alive.

"When they were unwell we had to feed them every two hours - day and night. The fear was if you missed a feed or if you weren't on top of your game and didn't pick up on the clues, the result is brain damage. It's a really harsh result. It's not something that'll happen over years, it's something that could happen at any time," explained Julie.

"With their disorder the biggest thing we've got to keep an eye on is energy. They can't run out of energy. If they do they start to break down muscle and fat which has protein in it which they can't process."

Isaac was sick more often than his sister and at nine months old, he developed brain damage. "He couldn't sit up, he was as floppy as a new born baby," Julie said.

"He stopped drinking, eating - we couldn't hold back the progression."

He was the first to undergo surgery at 19 months old and his new liver turned him into a new little man.

"He was eating and eating and eating," smiled Julie. "He was a different boy and on top of the world."

But as is the case with any major surgery there are risks, and one month after being discharged from hospital, Isaac went into septic shock and died.

"It took him very quickly," Julie told Mamamia.

As the family grieved they also had another sick little baby to concentrate on, but Julie says, "We didn't want to know about transplants for the next two months. It was our worst nightmare, which is why it was so hard to know what to do with Charlize".

Eventually, Julie and Paul knew they had to give Charlize the same chance they'd given Isaac.

"One day we looked at each other and said that was a tragic set of circumstances that you can't plan for. Each child has their own story to tell and every child's different and we have to do it for Charlize. What if we were too scared and we didn't and Charlize looks at her older sister and says, why didn't you give me a chance at a healthy future?"

They were given 10 minutes notice to rush Charlize to hospital for her transplant surgery, and it worked.

At four and a half, Charlize has had two transplants and is finally starting to feel well. "She's no longer foggy, that's the best way to put it," said Julie.

"Two months ago we went on our furthest trip (since the twins were born). We went an hour and a half away from the hospital," Julie told Mamamia.

Since the Gravina twins' surgeries, researchers have started working on a gene therapy for Propionic Acidemia.

"With gene therapy, they're targeting the liver. It will be like giving a child a transplant, without actually giving them one. It's an almost cure," explained Julie.

It's Julie's dream that children don't have to suffer like her kids have, and the Sydney mum is a passionate spokesperson for children's genetic diseases.

"The amazing thing about science is it's only getting better," she said.

"It's my dream that children will one day have a one time gene therapy injection and their kids will be fine. It's my dream that parents will be told their kids will be fine, when they get the diagnosis of Propionic Acidemia," she told Mamamia.

Tomorrow, August 2, is Jeans for Genes day. Wear your favourite jeans and raise money for the Children's Medical Research Institute. You can donate here.