Bio 363 Human Genetics Spring 2013
Bloom syndrome study sheet
Back to syllabus

Background web reading and Chu & Hickson review paper
Guidance for what to read and what to skip:
Read through the Bloom's syndrome section. Other than looking up the types of mutations that cause Werner syndrome, skip the Werner section and the RECQ4 section. Read the Biochemical features section but then skip the multienzyme complex subsection. Focus on Roles of RecQ helicases in recombination.

What is the difference between mitotic recombination and sister chromatid exchange?
How can you detect each kind of event?
What are the symptoms of Bloom syndrome at the level of the whole organism?
What type of protein is encoded by the gene associated with Bloom syndrome?
What other disorders are caused by mutations in similar genes? Are the symptoms the same?
What are the structural features of members of the RecQ family?
What types of mutations lead to Bloom syndrome as opposed to those leading to Werner syndrome?
Read enough of the text so that the concepts in Figs 3-5 make sense. Don't get bogged down in all the gene/protein names and abbreviations in the text. What does the Bloom's syndrome RecQ helicase do as its main function?

Ellis et al., 1995
This is obviously an old paper, but it's one of my favorites because of the ingenuity of the experiments. As you read, keep in mind that both mitotic recombination and sister chromatid exchange are at work and that both are used in different ways by the researchers. The approach in this paper is a highly unusual way to do linkage mapping, possible only because of the very nature of the disease itself.
The introduction says that the gene had already been regionally mapped-- in general, how was that done?
How do the researchers assess sister chromatid exchange (SCE) in lymphoblastoid cell lines from Bloom patients?
In most patients, all lymphoblastoid cell lines have high SCE. But some patients (the interesting ones) give rise to a subset of lymphoblastoid lines with low SCE. What is the difference in these patients in terms of their specific Bloom genotypes? How is it possible for them to develop some lymphoblastoid cell lines that are back to normal recombination levels?
What does "reduction to homozygosity" have to do with the gene mapping in this context? How did looking for this phenomenon in low SCE cells lead to the identification of the Bloom gene? Look at Fig 1A, keep in mind that the centromere is on the left, and think about mitotic recombination.
Keep looking at Fig 1. What kinds of polymorphisms are used? Why are there so many bands? Why did it help to find that all low SCE lines were still heterozygous for the D15S1108 polymorphism? And why did it help to show that all low SCE lines were now homozygous for D15S127?
How was the correct gene identified from this region? (Remember that this was before the genome was completed.)
They isolated cDNAs; why weren't all of them full length? Why is it important to make sure you have a full length cDNA? How did they confirm the identity of the 5' end of the gene? (see Fig 3)
Fig 5: How does the transcript size compare with the size of the gene in Fig 2?
Fig 5: What were the sources of all the RNA samples?
Fig 5: Compare with the nature of the mutations as listed in Table 1. Does any aspect of the Fig 5 results fail to match your predictions based on the nature of the mutations? e.g. look at cell line HG1584
Fig 5: In HG1926, how can a deletion of 3 base pairs lead to a stop codon?
How did the researchers confirm that they had identified the right gene?
What do the researchers speculate about possible molecular roles for the BLM gene product?