Intergeneration CAG Expansion In A Wuhan Juvenile-onset HD Family

Huntington disease(HD)is an autosomal dominant disorder characteristic of selective neurodegeneration caused by an expansion of CAG triplet repeats within coding region of the IT15 gene, producing a movement disease that includes the eponymous Huntington's chorea with cognitive and affective impairments. HD can occur at any age, but in most cases, the age at onset ranges from 35 to 50 years old. The disease is currently incurable and leads to death within 15-20 years after onset. In 1872, George Huntington systematically described the disease for the first time. In 1993, Huntington's Disease Collaborative Research Group identified and cloned the pathogenic HD gene called IT15, which located on chromosome 4p16.3. The mutation proved to be an expansion of CAG repeats in exon1 of IT15, making HD a member of a group of similar triplet repeats disorders.The pathologic change of HD is specifically localized in neural system characterized by brain atrophy and putrescence of nerve cells in some selective area. The most important atrophic areas contain striatum (including caudate nucleus and putamen) and cerebral cortex, which displays more obviously in terminal patients. Additionally, other regions, such as globus pallidus, substantia nigra, subthalanic nucleus and amygdala demonstrate putrescence of nerve cells of different severity. It is easy to give a clinical diagnosis for HD after onset, associated with representative symptoms and family history. However, there are other diseases, including DRPLA, HDL-2 and SCA17 exhibit choreic movement, which tend to be misdiagnosed as HD. For such an incurably inherited disorder, presymptomatic diagnosis and prenatal diagnosis play vital roles in effective and efficient HD prevention.With the discovery of HD pathogenic gene, PCR and DNA sequencing methods are widely employed in HD genetic diagnosis. Based on investigations of large samples in North America and Europe, the American College of Medical Genetics/American Society of Human Genetics ACMG/ASHG Huntington Disease Genetics Testing Group recommend the following criteria for diagnosis of HD: allele sizes of≤26 CAG repeats is normal and will not cause HD; allele sizes of 27-35 repeats will not cause HD but is unstable and may result in HD in the next generation; allele sizes of 36-39 displays incomplete penetrance of HD, it may cause HD or may not; allele sizes≥40 will cause HD eventually in all cases. Different populations share the same diagnostic criteria.Objective To conduct genetic diagnosis of IT15 gene for a Wuhan juvenile-onset HD family to provide them with genetic counseling and prepare for the further research on pathogenesis and experimental therapy of HD.Methods According to the principle of informed consent, we extracted genomic DNA from peripheral blood samples and carried genetic diagnosis of pathogenic exon1 of IT15 gene with modified touchdown PCR and DNA sequencing methods.Results 8 of 25 family members carry the abnormal allele, which include thatⅢ10,Ⅲ12,Ⅲ14,Ⅳ3,andⅤ2 share (CAG)48,Ⅳ11 andⅣ12 share (CAG)67,Ⅳ14 demonstrates (CAG)63, respectively. There are 8-25 CAG trinucleotides in members of normal control group, which definitely confirms the HD diagnosis of the family.Conclusions More 15 CAG trinucleotides ofⅣ14 than her fatherⅢ10 indicate that expansion occurs during the intergenerational transmission of IT15 gene in this HD family. In addition, expansion results in juvenile-onset HD and anticipation of the patientⅣ12, which is characterized by earlier age at onset and more severe conditions.