Features of trinucleotide repeat instability in vivo

Unstable repeats are associated with various types of cancer and have been implicated in more than 40 neurode-generative disorders. Trinucleotide repeats are located in non-coding and coding regions of the genome. Studies of bacteria, yeast, mice and man have helped to unravel some features of the mechanism of trinucleotide expansion. Looped DNA structures comprising trinucleotide repeats are processed during replication and/or repair to generate deletions or expansions. Most in vivo data are consistent with a model in which expansion and deletion occur by different mechanisms. In mammals, microsatellite instability is complex and appears to be influenced by genetic, epigenetic and developmental factors.

DNA mismatch repair in trinucleotide repeat instability

Trinucleotide repeat expansions cause over 30 severe neuromuscular and neurodegenerative disorders, including Huntington's disease, myotonic dystrophy type 1, and fragile X syndrome. Although previous studies have substantially advanced the understanding of the disease biology, many key features remain unknown. DNA mismatch repair(MMR) plays a critical role in genome maintenance by removing DNA mismatches generated during DNA replication. However, MMR components,particularly mismatch recognition protein MutSβ and its interacting factors MutLα and MutLγ, have been implicated in trinucleotide repeat instability. In this review, we will discuss the roles of these key MMR proteins in promoting trinucleotide repeat instability.

Intergeneration CAG expansion in a Wuhan juvenile-onset Huntington disease family

Objective To make early diagnosis of IT15 gene mutation in a Wuhan juvenile-onset Huntington disease （HD） family, for providing them with genetic counseling, and making preparation 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 exon 1 of IT15 gene by modified touchdown PCR and DNA sequencing methods. Results Eight of twenty-five family members carried abnormal allele： Ⅲ10 Ⅲ12, IIIt4, Ⅳ3, and Ⅴ2 carded （CAG） 48, Ⅳ11 and Ⅳ12 carried （CAG） 67, and Ⅳ14 carried （CAG） 63, in contrast with the 8-25 CAG trinucleotides in the members of control group. Ⅳ14 carried 15 more CAG trinucleotides than her father Ⅲ10. Conclusion The results definitely confirm the diagnosis of HD and indicate the CAG trinucleotide repeat expansion of IT15 gene in this HD family. In addition, CAG expansion results in juvenile-onset and anticipation （characterized by earlier age of onset and increasing severity） of the patientⅣ12.

DNA analysis in a suspected individual with myotonic dystrophy family history and her abortus

OBJECTIVE: To observe trinucleotide repeat number, (CTG)n in the 3'-untranslated region of the myotonic protein kinase (MTPK) gene in a clinically suspected woman with myotonic dystrophy (DM) family history and her abortus, in order to confirm the necessity of exerting antenatal examination in patients or suspected individuals with DM family history. METHODS: Long Expand Template polymerase chain reaction (PCR) system was used to analyze CTG trinucleotide repeat numbers located in the 3' untranslated region of MTPK on chromosome 19q13.2-3 in both peripheral white cells and muscles of the suspected mother and the other two DM patients in the family. The tissues of her abortus and blood of a health woman were detected, too. RESULTS: CTG repeats in both peripheral white cells and muscles of the suspected mother and the tissue of abortus were higher than normal range of CTG repeat number. There is no significant difference between blood and muscle samples. High CTG repeats were detected in blood and muscles of the typical DM members in the family, but in the blood sample of control, CTG repeats is normal. CONCLUSION: CTG trinucleotide analyses and antenatal examination should be done in pregnant with a DM family history, in order to reduce the birth rate of DM offspring.

Clinical Characteristics, Radiological Features and Gene Mutation in 10 Chinese Families with Spinocerebellar Ataxias

Background： Spinocerebellar ata^ias （SCAs） are a group ofneurodegenerative disorders that primarily cause the degeneration in the cerebellum, spinal cord, and brainstem. We study the clinical characteristics, radiological features and gene mutation in Chinese families with SCAs. Methods： In this study, we investigated 10 SCAs Chinese families with SCAI, SCA3/Machado-Joseph disease （MJD}, SCA7, SCAB. There were 27 people who were genetically diagnosed as SCA, of which 21 people showed clinical symptoms, and 6 people had no clinical phenotype that we called them presymptomatic patients. In addition, 3 people with cerebellar ataxia and cataracts were diagnosed according to the Harding diagnostic criteria but tailed to be recognized as SCAs on genetic testing. Clinical characteristic analyses of each type of SCAs and radiological examinations were perlbrmed. Results： We found that SCA3/MJD was the most common subtype in Hart population in China, and the ratio of the pontine tegmentum and the posterior fossa area was negatively con＇elated with the number of cytosine-adenine-guanine （CAG） repeats： the disease duration was positively correlated with the International Cooperative Ataxia Rating Scale score; and the CAG repeats number of abnormal alleles was negatively correlated with the age of onset. Conchlsions： Collectively our study is a systematic research on SCAs in China, which may help for the clinical diagnosis and prenatal screening of this disease, and it may also aid toward better understanding of this disease.

Frequency analysis of autosomal dominant spinocerebellar ataxias in mainland Chinese patients and clinical and molecular characterization of spinocerebellar ataxia type 6

Background Dominantly inherited spinocerebellar ataxia (SCA) is a clinically and genetically heterogeneous group of neurodegenerative disorders. This study was to further assess the frequency of SCA1 (spinocerebellar ataxia type 1), SCA2, SCA3/MJD (spinocerebellar ataxia type 3/Machado-Joseph disease), SCA6, SCA7, SCA8, SCA10, SCA12, SCA14, SCA17 and DRPLA (dentatorubro-pallidoluysian atrophy) in mainland Chinese, and to specifically characterize mainland Chinese patients with SCA6 in terms of clinical and molecular features.Methods Using a molecular approach, we investigated SCA in 120 mainland Chinese families with dominantly inherited ataxias and in 60 mainland Chinese patients with sporadic ataxias. Clinical and molecular features of SCA6 were further characterized in 13 patients from 4 families. Results SCA3/MJD was the most common type of autosomal dominant SCA in mainland Chinese, accounting for 83 patients from 59 families (49.2%), followed by SCA2[8(6.7%)], SCA1[7(5.8%)], SCA6[4(3.3%)], SCA7[1(0.8%)], SCA8(0%), SCA10(0%), SCA12(0%), SCA14(0%), SCA17(0%) and DRPLA(0%). The genes responsible for 41 (34.2%) of dominantly inherited SCA families remain to be determined. Among the 60 patients with sporadic ataxias in the present series, 3 (5.0%) was found to harbor SCA3 mutations while none was found to harbor SCA6 mutations. In the 4 families with SCA6, significant anticipation was found in the absence of genetic instability on transmission.Conclusion A geographic cluster of families with SCA6 subtype was initially identified in a mainland Chinese population.