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.]]