WBR0100

Author	[[PageAuthor::William J Gibson (Reviewed by Yazan Daaboul, M.D. and Rim Halaby, M.D. [1])]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Genetics
Sub Category	SubCategory::General Principles
Prompt	[[Prompt::A 4-year-old boy is brought by his mother to the physician's office for an immigration medical exam. He has recently immigrated with his family from Brazil to the United States. His mother reports that the boy has begun bumping into furniture very often; she has concerns that his vision may be compromised. She shows you an image in which the pupil of his left eye has a bright whitish reflection due to the camera flash. Upon further questioning, it is revealed that the boy’s father underwent surgery at a young age, leaving him blind in both eyes. Two days later, the offending lesion is surgically removed and the tissue is analyzed for molecular genetic features. Chromosome 13 displays a normal karyotype. A set of common single nucleotide polymorphisms (SNPs) on chromosome 13q is profiled and the resulting SNP identities are displayed below. Assume all of the SNPs have an allelic fraction in the general population of approximately 50%. The RB1 gene spans positions 48,934,153 to 48,951,170 on chromosome 13. Which of the following molecular events most likely occurred in the boy’s somatic tissues to cause his condition? SNP ID Chromosome Genomic Position Genotype rs4771054 13 27562342 CC rs17626553 13 28968770 CT rs2776961 13 30101280 GT rs144430040 13 31085367 CC rs4941731 13 33568060 GG rs9528533 13 34588335 CT rs9548644 13 39720610 AA rs9315803 13 41623184 GT rs9634767 13 42270305 CT rs2044732 13 42833400 AG rs17536002 13 43035209 TT rs11619528 13 44826355 TT rs12866473 13 46037501 CC rs1413077 13 47875303 AA rs7329865 13 49154165 AA rs895967 13 49347566 GG rs2191226 13 50940143 AA rs4943003 13 52090383 GG rs476136 13 53413498 GG rs10507581 13 54154463 GG rs9591632 13 55611511 CC rs4885962 13 56863954 AA rs2592866 13 58226942 GG rs1411645 13 59052343 CC rs6562010 13 59711493 TT ]]
Answer A	AnswerA::Mutation
Answer A Explanation	[[AnswerAExp::While a de novo mutation causing a “second hit” to the RB gene is possible, there is no evidence that this has occurred. The SNPs clearly show recombination on chromosome 13, making loss of heterozygosity more likely.]]
Answer B	AnswerB::Gene deletion
Answer B Explanation	AnswerBExp::While a deletion of the wild type allele of the ''RB'' gene could cause the “second hit” to the ''RB'' gene, the normal karyotype of chromosome 13 makes this possibility less likely.
Answer C	AnswerC::Mitotic recombination
Answer C Explanation	[[AnswerCExp::Note that all of the SNPs after rs2044732 are homozygous. Since there are two copies of chromosome 13 remaining from the karyotype, the only possibilities for the presence of homozygous rs2044732 are either the child is homozygous for all of these alleles in the germline or chromosome 13q underwent mitotic recombination. Possibility 1 is unlikely, because as was stated in the question prompt, all of these alleles have an allelic fraction of around 50%. Additionally, the probability of 15 consecutive homozygous alleles is (0.5)^15 = 0.00003, making this hypothesis very unlikely.]]
Answer D	AnswerD::Gene duplication
Answer D Explanation	[[AnswerDExp::There is no evidence for duplication of the RB gene, and such a duplication would not be expected to cause a retinoblastoma tumor.]]
Answer E	AnswerE::Methylation
Answer E Explanation	[[AnswerEExp::While methylation is responsible for silencing genes, methylation is most associated with Prader-Willi syndrome and Angelman syndrome. There is no evidence for gene methylation in this question, but there is clear evidence of mitotic recombination leading to loss of heterozygosity.]]
Right Answer	RightAnswer::C
Explanation	[[Explanation::Familial retinoblastoma is a highly penetrant autosomal dominant disorder that is considered the most common intraocular tumor of childhood. It is a rapidly developing cancer that originates from the immature cells of the retina, caused by defects in the retinoblastoma gene on chromosome 13. Most patients present with leukocoria (white pupillary reflex) before the age of 4 years, but other symptoms include strabismus (deviating eyes), unilateral/bilateral decreased visual acuity, retinal detachment, glaucoma, pain, or even hypopyon (cancer cells in the anterior chamber of the eye). Retinoblastoma is in fact inherited by an autosomal recessive trait at the cellular level. Nonetheless, it is classified as an autosomal dominant because unlike normal patients, patients with one mutant allele are considered to be predisposed to the development of retinoblastoma due to the decreased function of retinoblastom, a tumor suppressor that regulates cellular division. Thus, predisposition to the disease requires 1 mutant allele, but expressing the disease require 2 mutant alleles. Retinoblastoma may be classified as either somatic or germline. In somatic cases, the retinoblastoma locus of both alleles is mutated within one retinal cell leading to the development of a unilateral retinoblastoma (60% of cases). In contrast, germline mutations may occur and patients develop bilateral lesions because RB genes are mutated in every cell, as observed in cases of familial retinoblastoma or Li-Fraumeni syndrome. Patients with retinoblastoma due to germline mutations have an increased risk of other non-ocular tumors such as osteosarcoma, pinealoma, and melanoma. The patient in this vignette must have received one mutant allele of the RB1 gene from his father and one wild-type allele from his mother. The patient must have had a "loss of heterozygosity" event in his left retina. RB gene follows the classical Knudson's two-hit hypothesis. A sole germline mutation acts as a recessive trait at the cellular level. A “second hit”, which alters the remaining wild type allele in somatic tissues, is required for the initiation of tumorogenesis. In the case presented in the vignette, there is clear evidence of mitotic recombination in which the maternal chromosome 13 has been replaced by the paternal alleles after rs2044732. Thus, the cell which inherits both paternal alleles has no wild-type retinoblastoma protein and will go on to form a tumor. Educational Objective: Loss of heterozygosity is a common mechanism for the "second-hit" in tumor suppressor gene inactivation. References: Abramson DH, Mendelsohn ME, Servodidio CA, et al. Familial retinoblastoma: where and when? Acta Ophthalmol Scand. 2003;76(3):334-8 O'Brien JM. Retinoblastoma: clinical presentation and the role of neuroimaging. AJNR Am J Neuroradiol. 2001;22(3):426-8 First Aid 2014 page 84 Knudson AG. Mutation and cancer: statistical study of retinoblastoma. Proc Natl Acad Sci USA. 1971;68(4):820-3.]]
Approved	Approved::Yes
Keyword	WBRKeyword::Cancer, WBRKeyword::Genetics, WBRKeyword::Tumor suppressor, WBRKeyword::Retinoblastoma, WBRKeyword::Tumor, WBRKeyword::Cell division, WBRKeyword::Recombination, WBRKeyword::Cancer, WBRKeyword::Childhood cancer
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::