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Hong Kong's First Fragile X Carrier Screening

From left: Prof. Kong Wing-shan Grace, Department of Obstetrics and Gynaecology; Prof. Hon Kam-lum Ellis, Department of Paediatrics; Prof. Leung Tak-yeung and Prof. Choy Kwong-wai Richard, Department of Obstetrics and Gynaecology, explaining the Fragile X PCR method

Fragile X syndrome, affecting one in 4,000 to 10,000 males, is one of the most common causes of congenital mental retardation. Males inherit the abnormal gene from their carrier-mothers. To help doctors identify women with this hidden genetic problem, the Department of Obstetrics and Gynaecology has launched the territory’s first Fragile X carrier screening by introducing the new Fragile X PCR technology.

About one in 250–1,000 females are asymptomatic carriers of the defected (pre-mutation) FMR1 gene located in the X chromosome, in which there is an increased number of unstable CGG trinucleotide repeats. Once this pre-mutation is transmitted to a male baby, there is a significant risk that the repeats will further expand to beyond 200 times (full-mutation), leading to mental disability and autism. Female carriers are also at risk of pre-mature menopause. Carrier screening may assist prenatal diagnosis of abnormal fetuses as well as assessment of women’s health.

Although the traditional diagnostic method of Fragile X syndrome is accurate in counting the number of CGG repeats, it is labour intensive and time-consuming.The Fragile X PCR is not only capable of counting the CGG repeats up to 900 times, it is also a high throughput method that can produce results after one day. The department has recently validated the Fragile X PCR method on 14 full-mutation cases, 20 pre-mutation cases, and 75 normal controls with 100% accuracy.

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