​Pathology - Ataxia Telangiectasia Syndrome 
Ataxia-telangiectasia syndrome is an autosomal recessive disorder stemming from a deficiency in DNA damage repair. Specifically, the ataxia-telangiectasia gene (ATM) is mutated. This gene encodes a protein, which normally serves as a regulator of cell-cycle checkpoints to give time for repair of double-stranded DNA breaks.
This abnormality results in deficient cellular immunity and humoral immunodeficiency with thymic hypoplasia due to the fragile chromosomes that are created within the cells. Instability of the chromosomes also puts patients at increased risk for lymphomas and leukemias. Patients will typically present after the first year of life, when they will be delayed in walking secondary to ataxia. The ataxia occurs subsequent to atrophy of the cerebellum, which normally functions to coordinate balance (in conjunction with the vestibular system) and fine motor control. Oculomotor apraxia, which is difficulties with coordinated head and eye movements, can appear as an inability to track objects in young children. Speech will likely also fail to develop adequately. Patients will exhibit characteristic ocular and cutaneous telangiectasias, which are dilated blood vessels. In addition to decreased IgE and IgA, they also interestingly have raised serum alpha-fetoprotein. Patients with Friedrich’s ataxia, which can present similarly, can be separated from ataxia-telangiectasia syndrome because those patients will not have signs of oculomotor apraxia or the immunologic abnormalities.