WBR0105

Author	[[PageAuthor::William J Gibson (Reviewed by Yazan Daaboul, M.D.)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Genetics
Sub Category	SubCategory::Cardiology, SubCategory::Musculoskeletal/Rheumatology, SubCategory::General Principles
Prompt	[[Prompt::A 16-year-old male adolescent presents to his pediatrician for an annual check-up. The patient complains only of pain in his joints and muscles that he has attributed to growing pains. Otherwise, a 14-point review of systems is unrevealing. During physical examination, the physician notices several unusual features. The patient is unusually tall, with long slender limbs, fingers, and toes. On ophthalmic exam, the patient displays a slight superotemporal subluxation of the lens in the right eye. The physician orders molecular genetic testing of the FBN1 gene and confirms the diagnosis. Which of the following molecular defects underlie the syndrome that phenotypically resembles that of the patient's condition?]]
Answer A	AnswerA::Mutation of ''COL1A1''
Answer A Explanation	[[AnswerAExp::Mutations of the COL1A1 and COL1A2 genes, which code for collagen type I, are responsible for osteogenesis imperfecta.]]
Answer B	AnswerB::Mutation of ''TGFBR2''
Answer B Explanation	[[AnswerBExp::Mutations of TGFBR1 and TGFBR2 genes cause Loeys-Dietz Syndrome, a rare disorder that closely resembles Marfan syndrome.]]
Answer C	AnswerC::Mutation of ''COL3A1''
Answer C Explanation	[[AnswerCExp::Mutation of COL3A1, the gene coding for collagen type III, is responsible for the vascular subtype of Ehlers Danlos syndrome.]]
Answer D	AnswerD::Mutation of ''HFE''
Answer D Explanation	[[AnswerDExp::Mutation of the HFE gene is responsible for hereditary hemochromatosis.]]
Answer E	AnswerE::Mutation of ''FGFR3''
Answer E Explanation	[[AnswerEExp::Mutation of the FGFR3 gene is responsible for achondroplasia, the most common cause of dwarfism.]]
Right Answer	RightAnswer::B
Explanation	[[Explanation::The patient is most likely diagnosed with Marfan syndrome. Marfan syndrome is a clinically variable autosomal dominant genetic disorder that primarily affects connective tissue. It has a wide range of expressivity, with clinical symptoms ranging from mild to severe. Patients with the disease have a unique habitus, characterized by musculoskeletal abnormalities and joint laxity. Patients typically have tall, slender extremities with long, thin fingers. Other abnormalities include scoliosis, pectus excavatum, or pectus carinatum. The most serious complications are typically cardiovascular, such as dilation of the aortic valves at the level of Valsalva sinuses, or mitral valve prolapse with or without regurgitation. Ocular manifestations of Marfan syndrome include myopia (most common), glaucoma, cataract, retinal detachment. Sublaxation (zonules still in place) or dislocation (zonules out of place) of the lens is a rare but a classical ocular manifestation of Marfan syndrome whose symptoms may range from an asymptomatic course to decreased visual acuity and vision loss. Unlike homocystinuria where lens sublaxation is typically inferolateral, lens sublaxation among patients with Marfan syndrome is usually superotemporal. Marfan syndrome is caused by a mutation of the gene FBN1, which encodes the connective protein fibrillin-1. Fibrillin-1 protein is essential for the proper formation of the extracellular matrix, including the biogenesis and maintenance of elastin fibers. In addition to being a connective protein that forms the structural support for tissues outside the cell, the normal fibrillin-1 protein binds to the cytokine transforming growth factor beta (TGF-β). TGF-β signaling has deleterious effects on vascular smooth muscle development and the integrity of the extracellular matrix. Fibrillin-1 directly binds a latent form of TGF-β, keeping it sequestered and unable to exert its biological activity. It is now believed, excessive TGF-β accumulation in the lungs, heart valves, and aorta secondary to mutated fibrillin weakens the tissues and causes the features of Marfan syndrome. Loeys-Dietz Syndrome (LDS) is also an autosomal dominant connective tissue disorder caused by a heterozygous missense mutation of either TGFBR1 or TGFBR2 genes that encode TGF-β receptor. LDS is a recently discovered disease that phenotypically resembles Marfan syndrome and is also characterized by the risk of formation of vascular aneurysms and dissection, along with hypertelorism, cleft palate, or bifid/broad uvula. Educational Objective: Loeys-Dietz syndrome (LDS) phenotypically resembles Marfan Syndrome and is caused by defects in the TGF-β receptor encoded by the TGFBR1 or TGFBR2 genes. References: Canadas V, Vilacosta I, Bruna I, et al. Marfan syndrome: pathophysiology and diagnosis. Nat Rev Cardiol. 2010;7:256-65. Loeys BL, Chen J, Neptune ER, et al. A syndrome of altered cardiovascular, craniofacial, neurocognitive and skeletal development caused by mutations in TGFBR1 or TGFBR2. Nat Genet. 2005;37(3):275-81. MacCarrick G, Black JH, Bowdin S, et al. Loeys-Dietz syndrome: a primer for diagnosis and management. Genet Med. 2014;16:576-87. Neptune ER, Frischmeyer PA, Arking DE, et al. Dysregulation of TGF-beta activation contributes to pathogenesis in Marfan syndrome. Nat Genet. 2003;33(3):407-11. First Aid 2014 page 87]]
Approved	Approved::Yes
Keyword	WBRKeyword::Connective tissue, WBRKeyword::Marfan, WBRKeyword::Loeys-Dietz, WBRKeyword::Genetics, WBRKeyword::Autosomal dominant
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::