Correct Answer: D. Follows autosomal dominant mode of inheritance
Ataxia-telangiectasia (A-T) is a rare autosomal recessive disorder, not autosomal dominant. This is the critical discriminating fact. A-T is caused by mutations in the ATM (Ataxia Telangiectasia Mutated) gene located on chromosome 11q22-23, which encodes a protein kinase essential for DNA damage response and cell cycle checkpoint control. The disease manifests only when both alleles are mutated (homozygous or compound heterozygous state), requiring inheritance of defective genes from both parents. Heterozygous carriers are typically asymptomatic but have increased cancer risk. The autosomal recessive inheritance pattern is well-established in Indian pediatric literature and international registries. This inheritance mode is fundamental to genetic counseling, family screening, and recurrence risk calculation (25% for affected parents). The question tests whether students can distinguish A-T's true inheritance pattern from the other well-documented features of the syndrome.
Why the other options are wrong
A. Linked to adenocarcinomas — This is TRUE. A-T patients have a 100-fold increased risk of malignancies, particularly lymphomas and adenocarcinomas (gastric, breast, lung). The defective ATM gene impairs DNA repair, leading to genomic instability and cancer predisposition. This is a well-recognized clinical feature and a major cause of morbidity/mortality in A-T patients. NBE includes this to test knowledge of A-T's oncologic complications. B. Mutations in 11q gene is implicated — This is TRUE. The ATM gene is located at chromosome 11q22-23. Mutations in this gene are the molecular basis of A-T. Over 700 different mutations have been identified. This is the genetic hallmark of the disease and is tested frequently in genetics-focused questions. Including this option tests whether students know the chromosomal location. C. Humoral and cellular immunodeficiency — This is TRUE. A-T patients have combined immunodeficiency with low IgA (most common), variable IgG, and IgE levels, plus T-cell lymphopenia and impaired cell-mediated immunity. This leads to recurrent sinopulmonary infections. The immunodeficiency is a cardinal clinical feature alongside ataxia and telangiectasia. NBE pairs this with the inheritance question to test comprehensive A-T knowledge.
High-Yield Facts
- Ataxia-telangiectasia inheritance: Autosomal recessive (not dominant) — both alleles must be mutated for disease manifestation.
- ATM gene location: Chromosome 11q22-23 — encodes a DNA damage response protein kinase.
- Cancer risk in A-T: 100-fold increased risk of lymphomas and adenocarcinomas (gastric, breast, lung); major cause of death.
- Immunodeficiency in A-T: Combined defect — low IgA (most common), variable IgG/IgE, T-cell lymphopenia; recurrent infections.
- Clinical triad of A-T: Progressive cerebellar ataxia, oculocutaneous telangiectasia, and immunodeficiency (onset typically 1–3 years).
- Heterozygous carriers: Asymptomatic but have increased cancer risk (breast cancer in females); important for family counseling in India.
Mnemonics
A-T Inheritance Rule Ataxia-Telangiectasia = Autosomal Recessive (NOT dominant). Remember: 'A-T = AR' (both start with A). A-T Clinical Triad AAI: Ataxia, Angiectasia (telangiectasia), Immunodeficiency. Covers the three cardinal systems affected.
NBE Trap
NBE exploits the fact that students often confuse A-T with other genetic syndromes (e.g., Marfan syndrome is autosomal dominant). By listing "autosomal dominant" as an option, NBE tests whether students have memorized A-T's true recessive inheritance versus relying on vague syndrome knowledge. The other three options are all TRUE, making this a "not true" question that requires precise recall.
Clinical Pearl
In Indian pediatric practice, A-T diagnosis is often delayed because cerebellar ataxia is initially attributed to other causes (nutritional, infectious). The key clinical clue is the appearance of oculocutaneous telangiectasia (typically by age 3–5 years) combined with recurrent infections and progressive neurologic decline. Genetic counseling for Indian families is critical: affected parents have 25% recurrence risk per child, and heterozygous carriers (especially mothers) need breast cancer surveillance.
_Reference: OP Ghai Pediatrics Ch. 10 (Genetic Disorders); Robbins Pathology Ch. 5 (Genetic Disorders)_