• Children had a gene mutation that blocked a protein needed for hearing
• The treatment injected a version of the gene in the inner ear
• Children began hearing just six weeks after the single injection 

A breakthrough treatment has allowed six deaf children, ages one through 11, to hear the world for the first time.

The children were part of two experimental groups in China and the US, who were all born with a gene mutation that blocked production of a protein needed for hearing.

Scientists injected a version of the gene, called otoferlin (OTOF), into the inner ear, and the cells began producing the missing protein.

The children's hearing levels are now up to 70 percent normal after 26 weeks of the treatment, with progress starting at just six weeks.

Progress videos show a one-year-old responding to his name called for the first time and another little girl repeating father, mother, grandmother, sister, and 'I love you' - when before she could not speak.

Aissam Dam, 11, heard for the first time this week after receiving treatment at the Children's Hospital of Philadelphia (CHOP) - marking a first in the US.

Gene therapy restores hearing in children born deaf in Shanghai

Zheng-Yi Chen, a professor at Harvard Medical School and study author for China's experiments, said: 'If children are unable to hear, their brains can develop abnormally without intervention.

'The results from this study are truly remarkable. We saw the hearing ability of children improve dramatically week by week, as well as the regaining of their speech.'

Hereditary deafness is the latest condition scientists are targeting with gene therapy, which is already approved to treat illnesses such as sickle cell disease and severe hemophilia. 

About 34 million children worldwide suffer from deafness or hearing loss, and genes are responsible for up to 60 percent of cases.

Dam was born 'profoundly deaf' because of a highly rare abnormality in his OTOF gene, which was also the case for the five children in China.

A defective gene prevents the production of otoferlin, a protein necessary for the 'hair cells' of the inner ear, which convert sound vibrations into chemical signals sent to the brain.

Otoferlin gene defects are rare, accounting for one to eight percent of hearing loss at birth.

Dam underwent a surgical procedure in October that involved partly lifting his eardrum and then injecting a harmless virus, which was modified to transport working copies of the otoferlin gene, into the internal fluid of his cochlea.

As a result, the hair cells began making the missing protein and functioning properly.

Almost four months after receiving the treatment in one ear, Aissam's hearing has improved- he only has mild-to-moderate hearing loss.

He is 'literally hearing sound for the first time in his life,' CHOP shared in a statement.

John A. Germiller, who works at CHOP, said: 'Gene therapy for hearing loss is something that we physicians and scientists in the world of hearing loss have been working toward for over 20 years, and it is finally here.

'While the gene therapy we performed in our patient was to correct an abnormality in one, very rare gene, these studies may open the door for future use for some of the over 150 other genes that cause childhood hearing loss.'

Dam's success story came just days after five children in China heard after receiving the same treatment.

A six-year-old girl, nicknamed YiYi, did not develop the ability to speak due to being deaf since birth.

One month after the injection, YiYi's mother, Quin Lixue, said her daughter was hearing with the treated ear for the first time and repeated what she heard, MIT Technology Review reports.

A video shows Lixue covering her mouth so YiYi couldn't read her lips and asking her daughter to repeat what she was saying.

Daily Mail