Identification of ATM Mutations in Korean Siblings with Ataxia-Telangiectasia
An 8-yr-old boy and his 6-yr-old sister presented to the neurology outpatient clinic with a chief complaint of progressive ataxia. The patients displayed progressive limb and truncal ataxia, as well as involuntary movements of the extremities. There was no history of perinatal complications, cognitive dysfunction, or seizures. The proband had developed recurrent respiratory infections. The parents were healthy, non-consanguineous ethnic Koreans. The proband was the first child in his family, and no family history other than the two patients was known.\nThe proband first presented to the pediatrics clinic four years prior to the neurology visit. A blood test at the initial visit showed mild anemia (11.6 g/dL), thrombocytosis (441×109/L), and an elevated phosphate level (5.2 mg/dL). Serum levels of lactate, pyruvate, and cholesterol were within normal limits. There were no detectable abnormalities in the urine, and the levels of serum amino acids and urine organic amino acids were normal. Left undiagnosed for three years, the patient had only received conservative care and rehabilitation.\nWhen both siblings presented to the department of neurology, the proband exhibited more severe symptoms. A neurological examination revealed the presence of limb and truncal ataxia, oculomotor apraxia with gaze-evoked nystagmus, and choreoathetosis of the bilateral extremities. There was no cranial nerve dysfunction, motor weakness, or sensory dysfunction. Deep tendon reflexes were decreased. Ophthalmologic evaluation did not reveal telangiectasia in either of the children. The AFP level was increased (119.68 IU/mL) in the proband's serum. No chromosomal abnormalities were found. The number of CAG repeats was not increased in the genes for spinocerebellar ataxia type 1, 2, 3, 6, 7, 8, or 17. Cerebellar atrophy on brain magnetic resonance imaging (MRI) images taken two years prior to the visit was more prominent than that in the initial image taken 4 yr ago.\nAfter obtaining informed consent from the parents, the genomic DNA was isolated from peripheral blood leukocytes of the proband and his siblings by using a Wizard genomic DNA purification kit (Promega, Madison, WI, USA). All the coding exons and flanking intronic regions of the ATM gene were amplified using primer sets designed by the authors (sequences available upon request). PCR was performed using a thermal cycler (Model 9700; Applied Biosystems, Foster City, CA, USA). Direct sequencing was performed using an ABI Prism 3130xl Genetic Analyzer (Applied Biosystems) with a BigDye Terminator Cycle Sequencing-Ready Reaction Kit (Applied Biosystems).\nRNA was extracted to perform reverse-transcription PCR (RT-PCR). cDNA was generated using an RNA PCR kit (Applied Biosystems). RT-PCR.based sequencing was performed using a set of primers (RT-F, 5'-CCAATTCTTCACAGTAATTTTCCTC-3'; RT-R, 5'-CGATACAAAGAACACACATTGGA-3'). The PCR product obtained was subjected to direct sequencing as described above.\nThe cDNA nucleotide sequences of the gene examined were numbered according to the GenBank accession number NM_000051.3 for ATM. The mutation nomenclature used here follows the recommendations of the Human Genome Variation Society ().\nIn the proband, a previously reported heterozygous mutation at position 8,546 (c.8546G>C; p.Arg2849Pro) [] and a 13-base deletion in intron 17 (c.2639-19_2639-7del13) were identified (). RT-PCR analysis revealed that the intronic variant resulted in a marked increase of an aberrant transcript missing exon 18 (954 bp) that was only faintly observed in normal controls (). These two variants were also present in the affected sister, but they were absent in the proband's healthy brother. In addition, the c.2639-19_2639-7del13 variant was not found in 100 control chromosomes.