Case A

Case B

Case C

Background: Sickle cell is a disease of red blood cells, caused by a mutation in the hemoglobin gene.  A single base change results in a single amino acid substitution.  This mutation causes the hemoglobin to change its conformation to a more elongated form under certain conditions, distorting the red blood cells and impairing their ability to carry oxygen.  Sickle cell disease is considered a recessive trait since both chromosomes have to carry the mutation in order for the full-blown disease symptoms to appear.

Click here to explore the nature of the sickle cell mutation using NCBI tools.

The sickle cell mutation also eliminates a restriction enzyme site – the recognition site for the enzyme MstII.  To detect the sickle cell mutation, a patient’s DNA is digested with MstII and a Southern blot is performed using a probe corresponding to this region of the hemoglobin gene.  The presence or absence of the sickle cell mutation can be determined based on the size of the fragment identified by the probe.

Procedure: Digest each of the DNA samples with MstII.  Then run a Southern blot, using the probe corresponding to the region of the hemoglobin gene mutated in sickle cell disease, to determine the genotype of each individual.

Case A:   Steve and Martha are expecting their second child.  They know that sickle cell disease runs in both of their families.  They want to know whether this child could be affected.  Neither they nor their 10-year-old daughter, Sarah, have shown any symptoms of the disease.   They decide to have DNA tests to determine the status of the fetus, as well as to find out whether they in fact are carriers of the disease gene.

DNA samples:

• Steve (father)
• Martha (mother)
• Sarah (daughter)
• Fetus
• Control DNA, homozygous for sickle cell mutation
• Control DNA, homozygous normal, without sickle cell mutation

Results of Southern Blot analysis for Case A

Explore nature of the sickle cell mutation

1. What conclusions can you draw from the results? 
2. What is the molecular basis of this disease, and why does this result in the observed gel patterns?
3. What options are available to the family?
4. What issues are raised by this type of testing?

Case B:  Mattie has just returned form the hospital after visiting KC, her favorite nephew.  She and her family are already grieving the loss they know is coming.  She has watched her only brother, Josiah, and his wife, Emma, deal with KC’s illness over the years.  She feels as helpless for them as she does for KC.  Josiah shook her up when he blurted out, during a period of overwhelming stress, that if they had known ahead of time, perhaps they would have chosen a different route, and that she should get tested to avoid the same suffering.  Mattie knew it was the stress talking, and that Josiah would not trade any of his moments with KC, but maybe he was right about her.  Maybe she should go into parenting with her eyes open.  Maybe she should find out if she could bear a child with sickle cell disease.

DNA samples:

• Mattie (sister)
• Josiah (brother)
• KC (nephew)
• Emma (wife)
• Control DNA, homozygous for sickle cell mutation
• Control DNA, homozygous normal, without sickle cell mutation

Results of Southern Blot analysis for Case B

Explore nature of the mutation – see Case A

1. What chance does Mattie have to bear a child with sickle cell disease?
2. What other conclusions can you draw from the results?
3. What is the molecular basis of this disease, and why does this result in the observed  gel patterns?
4. What issues are raised by this type of testing?

Case C:  Claudine and Andre Kasonga live in a small community in sub-Sahara Africa, surrounded by family and friends whose children frequently suffer from malaria or sickle cell disease.  They themselves have both had siblings succumb to each of these diseases.  While they both appear to be fine, they are expecting their first child and wish to know how to prepare themselves.  Should they move away from the malaria-carrying mosquitoes, or wouldn’t it matter?  They decide to get tested.

DNA samples:

• Claudine (mother)
• Andre (father)
• Fetus
• Control DNA, homozygous for sickle cell mutation
• Control DNA, homozygous normal, without sickle cell mutation

Results of Southern Blot analysis for Case C

Explore nature of the mutation – see Case A

1. What is the connection between the malaria-carrying parasite and sickle cell disease?
2. Under what fetal genetic conditions would it make sense to move out of the area where malaria is endemic?
3. What conclusions can you draw from the results?
4. What is the molecular basis of this disease, and why does this result in the observed   gel patterns?
5. What options are available to the family?
6. What issues are raised by this type of testing?