Homozygosity for extremely rare mutations in a human gene...

Homozygosity for extremely rare mutations in a human gene called $S C N 9 A$ cause complete insensitivity to pain (congenital pain insensitivity or $C P A$ ) and a total lack of the sense of smell (anosmia). The SCN9A gene encodes a sodium channel protein required for transmission of electrical signals from particular nerves in the body to the brain. The failure to feel pain is a dangerous condition as people cannot sense injuries. The $S C N 9 A$ gene has 26 exons and encodes a 1977-amino acid polypeptide. Consanguineous matings in three different families have resulted in individuals with CPA/anosmia. In Family $1,$ a G-to-A transition in exon 15 results in a truncated protein that is 898 amino acids long; in Family $2,$ deletion of a single base results in a 766 -amino acid polypeptide; and in Family $3,$ a C-to-G transversion in exon 10 yields a 458-amino acid protein. a. Hypothesize as to how each of the three $S C N 9 A$ mutations affects gene structure: Why are truncated proteins made in each case? b. How would you classify the mutant alleles? Do these cause loss of function or gain of function? Are they amorphs, hypomorphs, hypermorphs, neomorphs, or antimorphs? c. Explain in molecular terms why CPA/anosmia is a recessive condition.

Homozygosity for extremely rare mutations in a human gene called $S C N 9 A$ cause complete insensitivity to pain (congenital pain insensitivity or $C P A$ ) and a total lack of the sense of smell (anosmia). The SCN9A gene encodes a sodium channel protein required for transmission of electrical signals from particular nerves in the body to the brain. The failure to feel pain is a dangerous condition as people cannot sense injuries. The $S C N 9 A$ gene has 26 exons and encodes a 1977-amino acid polypeptide. Consanguineous matings in three different families have resulted in individuals with CPA/anosmia. In Family $1,$ a G-to-A transition in exon 15 results in a truncated protein that is 898 amino acids long; in Family $2,$ deletion of a single base results in a 766 -amino acid polypeptide; and in Family $3,$ a C-to-G transversion in exon 10 yields a 458-amino acid protein.
a. Hypothesize as to how each of the three $S C N 9 A$ mutations affects gene structure: Why are truncated proteins made in each case?
b. How would you classify the mutant alleles? Do these cause loss of function or gain of function? Are they amorphs, hypomorphs, hypermorphs, neomorphs, or antimorphs?
c. Explain in molecular terms why CPA/anosmia is a recessive condition.

Key Concepts

Recessive Inheritance in the Context of Loss?of?Function Mutations

In cases of loss?of?function mutations, the phenotype often follows a recessive pattern because a single functional copy of the gene is usually sufficient to maintain normal protein function. Only when both alleles are nonfunctional (in the homozygous state) do the clinical symptoms, such as insensitivity to pain and anosmia, become manifest. This is why even severe loss?of?function mutations can be recessive in effect.

Allelic Classification and Amorphic Mutations

Allelic classification helps in understanding how mutations affect gene function. An amorph, or null allele, completely abolishes the function of the gene. The mutations in the gene described lead to truncated proteins that are nonfunctional, fitting the definition of amorphs. This classification is important to predict the genetic outcome when such alleles are present in homozygous form.

Premature Termination Codon and Protein Truncation

Mutations such as nonsense mutations (base substitutions that create a stop codon) or frameshift mutations (insertions/deletions that disrupt the reading frame) can lead to premature termination of translation. This results in the production of a truncated polypeptide that is typically nonfunctional because it is missing essential domains required for normal activity.

Loss?of?Function Mutations

Loss?of?function mutations result in a protein product that lacks normal activity or is completely absent. When critical proteins, such as ion channels, are rendered nonfunctional, important biological processes are disrupted. In the context of gene mutations, the defective alleles produce proteins that are unable to perform their normal roles in cellular physiology.

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