"When the day comes that we get married, the most important person to us will not be there, but don't worry, he'll be up there watching, hopefully with a big smile on his face. We love you Charlie and your legacy will carry on"
The parents, who are aged in their 30s, have launched a foundation to provide a "brighter future" for other sick children.
The couple, from Bedfont, west London, raised more than £1.3m to pay for therapy in America and said they wanted to establish a foundation with the donations.
They also said they want to raise awareness of mitochondrial depletion syndrome.
"Our vision is to enhance the quality of life for mitochondrial sufferers through innovative research, family support, and raise much-needed awareness for this devastating condition," they said on the Charlie Gard Foundation website.
Doctors at Great Ormond Street Hospital for Children in London said the therapy would not help and said life-support treatment should stop..
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"Although extensive reviews are conducted on each unsuccessful transplant, the recent patient outcomes deserve an in-depth review before we move forward with the program," Doug Lawson, CEO of Catholic Health Initiatives Texas Division, which owns St. Luke's, said in a statement. "Our prayers are with the families, as well as all those on the waiting list."