Question

A 5-year-old boy presents with short stature, facial dysmorphism, pulmonary valve stenosis, and features suggestive of Noonan syndrome. His parents ask about the genetic basis of the disease. What should they be counseled regarding gene mutation and inheritance?

• A. Noonan syndrome is X-linked recessive and always inherited from the mother
• B. Noonan syndrome is caused by mutations in multiple genes, most commonly PTPN11, with autosomal dominant inheritance
• C. Noonan syndrome is due to a single gene defect and inherited in an autosomal recessive manner
• D. Noonan syndrome is sporadic, and there is no risk of recurrence in siblings

Hint:

Noonan syndrome is often referred to as the "male counterpart" to Turner syndrome due to similar clinical features like a webbed neck and short stature.
However, unlike Turner syndrome (45,X0), Noonan syndrome affects both sexes, has normal karyotypes, and typically presents with right-sided heart lesions (pulmonary stenosis) rather than left-sided lesions (coarctation of the aorta).

Answer 1

The Correct Option is B

Concept:
The question asks for the genetic etiology and inheritance pattern of Noonan syndrome in a boy presenting with classic clinical features.
Noonan syndrome is a clinically and genetically heterogeneous disorder belonging to the RASopathy family.
It is characterized by distinct facial features, short stature, congenital heart defects, and bleeding diathesis.
Explanation:
• Noonan syndrome is inherited in an autosomal dominant manner, meaning a single copy of the mutated gene is sufficient to cause the disorder.

• The syndrome is genetically heterogeneous, meaning mutations in several different genes can lead to the same clinical phenotype.

• These genes are primarily involved in the RAS/MAPK signaling pathway, which is crucial for cell cycle regulation, differentiation, and growth.

• The most common genetic cause, responsible for approximately 50% of Noonan syndrome cases, is a missense mutation in the PTPN11 gene.

• Other associated genes include SOS1, RAF1, KRAS, NRAS, and BRAF, among several others.

• Because it is autosomal dominant, an affected individual has a 50% chance of passing the mutated gene to each offspring.

• However, it is important to note that many cases arise from de novo (new) mutations, where neither parent is affected.

• Option (A) is incorrect because Noonan syndrome is not X-linked recessive; it can affect both males and females equally.

• Option (C) is incorrect because it is not typically autosomal recessive, and it is not caused by just a single gene defect but rather exhibits locus heterogeneity.

• Option (D) is incorrect because while de novo cases occur, inherited cases are common, and in inherited cases, the recurrence risk is 50%.
Final answer:
The correct counseling regarding the genetic basis is that it involves mutations in multiple genes, predominantly PTPN11, and follows an autosomal dominant inheritance pattern.