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nbme24/Block 2/Question#42

A 3-week-old female newborn is brought to the ...

The patient has another mutation that was not included in the previous analysis

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The analysis only showed a mutation in one allele. CF is an autosomal recessive disease: the disease only manifests if there are mutations in both alleles of the CFTR gene.

If you still have 1 functional copy of the CFTR gene, you can still make the CFTR protein (the chloride channel/transporter), hence your body won’t have any issues.

This is analogous to tumor suppressor genes like Rb: so long as one of the alleles you have is functional, you can make enough of the protein to “make up” for the defective allele. If both get knocked out (Rb-/-), you lose the protection provided by the gene because now you make no protein at all.

The only thing that made sense for this question was the fact that the other allele was not included in the analysis.

charcot_bouchard  OR another allele has a diff type of mutation because CF is done by like hundreds of diff type of mutation. SO the 70 types that we screened covered one type from one parent but not another that was inherited from other parent. +10  
soph  I put D thinking there was a mutation in another protein that interacts with CFTR....thus u dont have CF but some disease with similar phenotype. Is this wrong bc its simply not the case ?? +1  
nbmehelp  @charcot_bouchard I think that makes more sense if I understand what you're saying- Probably had a mutation only in 1 of 2 of the same alleles in the analysis but had another mutation in 2 of 2 alleles at a different location not included in the analysis, right? +  
fallot4logy  CF is a rare disease , and the possibility to have a mutated gene plus a gene that its not belong to 70 most common cf mutations is extremely rare +1  
gubernaculum  @soph i picked D too but now looking back, the panel had 70 of the most common CFTR gene mutations so it is unlikely that they didn't already check a gene that codes for a protein that interacts with CFTR? that's the only way i can rationalize it. its bad writing ultimately +  


Theres multiple allele variants for the CFTR gene in a single locus, so you could get cystic fibrosis from a mutation in any one of those allele variants(theres over 1500 different mutations described) the question stem mentioned they tested for the most common types, so we can assume they probably just missed testing for mutations in other alleles.

IRT is measured in routine heel-prick blood taken for biochemical screening of all newborn infants born in the UK. This test is one of a number of completed in newborn screening (the "Guthrie Test"). In Australia it is known 94% of those with eventual diagnosis of CF have a positive IRT on newborn screen. Samples with a raised IRT (defined as highest 1% of values) are then screened for common CF gene mutations. Each centre has a slightly different gene panel; currently 40-50 of the most common genes are sequenced. However, there are more than 2000 known mutations, so gene panel testing does miss occasional CF patients

If gene testing finds one mutation they will then have a sweat test to help confirm the diagnosis. Sweat testing is more likely to be equivocal in infants and typically not attempted in those under 5kg. If sweat test is positive more expansive gene testing is considered. If two mutations are found they are diagnosed with CF.

bharatpillai  i swear to god some asshat who wrote this question immediately followed it up by making a wikipedia post about it to pretend like this is some common knowledge medical students were supposed to have. +  

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submitted by sweetmed(83),

you need a mutation in 2 alleles to get CF [since it is autosomal recessive]/