- Compare and contrast the phenotypes of the SRS vs. BWS patients (you will need to look at Table 1). Do you notice any trends? Knowing that all the patients studied in this article have mutations in the same imprinted cluster, what could explain the differences in phenotypes?
The SRS patient had low birth weight, low birth length, had post and pre natal growth failure, facial defects, digit deformities, and had nervous muscle deficiencies. The BWS patients were generally large at birth, maintain large stature, and do not have facial or digit deformities. The SWS patient has a duplication of all of the genes between ICR1 and 2, but the methylation pattern is the same meaning there is no inhibition by the lnRNA in ICR2. This means that there would be an increased amount of CDKN1C which is a cell growth inhibition, leading to growth deficiencies. The BWS patients have a mircoduplication that results in a truncated lnRNA that inhibits expression of the growth inhibitor, leading to an increase in growth and large size.
- The authors report using OMIM to obtain some information for their research. Take a few minutes to look up Kcnq1ot1 on OMIM and see what information you get.
Maps to chrom 11p.15.5, lnRNA, overlaps KCN1 in antisense
3. Look at the pedigree in Figure 1.
a) What can immediately be concluded about BWS (even without knowing who inherits the mutant allele from whom)?
The syndrome is probably not recessive autosomal and probably not X linked. Individuals with the syndrome are fertile, and the syndrome is not extremely lethal.
b) II-2 and II-4 both have BWS, and both have one child with BWS and one child without BWS. Briefly explain how this is possible.
If the affected mother passes on the allele that she inherited from her father (with the correct maternal methylation) then the allele will be normal resulting in a normal phenotype.
- Briefly describe the mutation detected in the BWS patients and the mutation detected in the SRS patient, and their respective effects at the molecular, cellular, and organismal levels (use figures 2, 3, and 8, as well as your answer to Question 1).
NOTE: molecular level = DNA sequence, DNA methylation, gene expression; cellular = proteins present in the cell, potential effects on the cell; organismal = effects on the entire organism.
SRS – 1.2 Mb duplication and inversion of the entire 11p15.5 region, methylation is consistent with maternal methylation (ICR1 is not methylated and ICR2 is methylated), the lnRNA is not expressed on the maternal chromosome, there is a double dosage effect of the genes in the 11p15.5 region on the maternal copy, increased cell growth inhibitor decreases cell growth, has the syndrome phenotype (low birth weight, low birth length, had post and pre natal growth failure, facial defects, digit deformities, and had nervous muscle deficiencies)
BWS – 160 kb duplication and inversion of the ICR2 and part of the KCNQ10TI sequence, methylation on the original copy is normally methylated but the partial RNA sequence is un-methylated, there is expression of the partial lnRNA leading to the down regulation of the cell growth inhibitor, cells grow larger, the individual is larger
- Explain what the data in Figure 7 show, and how you interpret them.
BWS patients have increased expression of the lnRNA and this is due to increased expression on the maternal allele. Paternal lnRNA expression is unchanged among individuals. The control individual only shows expression of one allele, while the DNA sequence shows that there are 2 different alleles present. The BWS patients show Chirp sequences from both alleles, meaning that there is both maternal and paternal expression of the lnRNA.
- What valuable fundamental information was gained about imprinting control regions through the study of these patients?
Having a sequence in cis to the ICRs does not necessarily mean that it will be properly imprinted. This is shown by the improper imprinting of the duplicated region in the BWS patients.