A.L. is a healthy toddler with a normal perinatal history. Her parents notice a

Question

A.L. is a healthy toddler with a normal perinatal history. Her parents notice a white spot in the pupil of one eye and request a referral to a pediatric ophthalmologist. Examination of the eye suggests that A.L. has developed a retinoblastoma, a cancer in her eye. This cancer occurs in approximately 1 in every 1500 live births and approximately 250 children are diagnosed each year in the United States. The family is referred for genetic testing and counseling. Retinoblastoma can occur because of a change in chromosome 13 or a mutation in the RB 1 gene, which in its normal state prevents abnormal mitosis. A.L.'s tumor results from mutation of the RB 1 gene, but neither parent shows this mutation. What is the difference in the cause and inheritance of chromosomal changes compared with the inheritance of a mutation in a specific gene? How did the mutation likely occur in the genes in A.L.'s tumor cells? Testing of A.L.'s blood shows no evidence of the RBI mutation: how likely is it that A.L. could transmit the mutated gene to her children? How has the mutation caused growth of the tumor in A.L.'s eye? What is the prognosis if the tumor is limited to the intraocular tissues?

Explanation / Answer

1. Difference in the cause and inheritance of chromosomal changes compared with the inheritance of a mutation in a specific gene:Inheritance of chromosomal changes occurs when one of the parents is having the same disorder. Such inheritance involves deletion in chromosome 13 which changes some other genes also leading to associated problems such as intellectual problem, abnormal growth etc. This condition is called germinal retinoblastoma.

Whereas, if the parents are not having any such problem and the children have, this is caused due to change in a single gene which is BB1. Thus, there is only retinoblastoma symptom in the child. This condition is called non-germinal retinoblastoma.

2. Mutation is likely to occur in the genes in A.L.’s tumor cell as there is no other disorder in the body associated with change in chromosome. The mutation has occurred specifically in the gene, so, there is not an evidence of this in blood too. If mutation had occurred on chromosome, it had affected all body cells and not only eye.

3. There will not be any transmission of mutated gene to children as this is a condition of non-germinal retinoblastoma where change has occurred in only RB1 gene in eye. There is no chromosomal change so it is not associated with other body cells and blood.

4. RB1 gene is responsible for suppression of tumor by controlling cell growth. When there is a mutation in RB1 gene, it hampers the functioning of RB1 gene. Therefore, it is unable to control cell growth and it leads to tumor formation.

5. The non-germinal retinoblastoma generally occur in single eye of child. If it is diagnosed it can be treated by various therapies or removal of that eye. This results in preserving the life of child even if vision is lost.

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