DNA mutation leads to Immunodeficiency:

DNA repair and Cancer:

Large distortion in the helical structure of DNA is repaired by nucleotide excision repair system. Genetic defects that inactivate nucleotide excision repair system leads to many genetic defects as Xeroderma pigmentosum. It is caused by pyrimidine dimer formation in humans which symptoms are extremely light sensitivity further leading to skin cancer. Nucleotide excision repair is the only repair mechanism for photoreactivation of pyrimidine dimers.

There are so many other diseases related to chromosomal defect as Severe Combined Immunodeficiency (SCID) where defect is in 11p13 locus causing defect in RAG-1/RAG-2 deficiency and transmitted as autosomal recessive disorder.

In Digeorge syndrome, 22q11chromosomal defect leads to impaired development of T and B lymphocytes and inherited as autosomal dominant disorder.

Autosomal 11q22 locus recessively inherited defect in cell cycle kinases leads to Ataxia Telangiectasia which results low immunoglobin level of IgA and IgE.

DNA in Cancer development:

Cancer by Gain of Function of Genes:

One type of cancer causing genes are proto-oncogenes which are basically involved in the secretion of growth factor or its product functions as transcription factor. They work in a regulated manner in normal cells. Mutation in proto-oncogene changes it into more active oncogenic form and now they behave abnormally by stimulating cell proliferation in an uncontrolled manner. Few examples are myc, jun and fos oncogenes which codes for transcription factors and their overactivity resulted unregulated proliferation leading to cancer.

Chromosomal translocation of c-myc proto oncogenes in Chronic Myelogenous Leukemia (chromosome 9→22 translocation) and Burkitt Lymphoma (chromosome 8 → 14 translocation) enhances the activity of c-myc and thus transcription factor production too, which leads to cancer.