Clinical and cellular phenotype of two Italian sibs with ATLD

We studied two sibs with a slowly progressive neurological syndrome mimicking ataxia telangiectasia. The neurological features appeared during infancy with unsteadiness and progressed to ataxic gait, oculomotor apraxia, nystagmus and cerebellar dysarthria. The two sibs, now 34 and 33, are still able to walk for few steps unaided. In both patients laboratory investigations, including immunoglobulins, alphafetoprotein, lysosomial enzymes, vitamin E and lipoproteins, resulted normal. EMG revealed a slight motor sensory neuropathy. Brain CT scan and MRI showed cerebellar atrophy with a moderate increase in the size of the fourth ventricle. Tests performed on X-irradiated peripheral blood lymphocytes at G2 phase showed mild radiosensitivity, intermediate between classical Ataxia Telangiectasia patients and controls. Western analysis revealed normal levels of ATM but markedly reduced amounts of NBS1, Mre11 and Rad50. No mutations in ATM and NBS1 genes were found. By cDNA sequence of Mre11 gene the mutated maternal allele (1442C>A) was identified, while the second allele was actually null, possibly due to nonsense-mediated mRNA decay. The sequencing of genomic DNA revealed a single base change C>T in exon 15, i.e. nucleotide 1714 in the cDNA sequence, that changes the normal codon CGA (Arg) into the stop codon TGA and reduces the Mre11 protein size from 81 to 65KDa. The patients were classified as Ataxia Telangiectasia Like Disorder (ATLD). To better understand the effects of their mutations on the function of the NMR complex we analysed the radiation induced Mre11 nuclear foci. While before irradiation ATLD cells showed a more diffuse Mre11 staining than wild-type cells, after irradiation they failed to form Mre11 nuclear foci. Moreover, the radiation induced Chk2 phosphorylation in the ATLD fibroblasts appeared defective. These results contribute to clarify the pathway of the cellular response to IR-induced DNA damage.