Genetic Variation Nomenclature, Mutation Types, and Mendelian Inheritance: A Study Guide

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Genetic Variation Nomenclature

Standardized Nomenclature for Sequence Variants

Accurate and standardized nomenclature is essential for describing genetic variants at the DNA, RNA, and protein levels. The Human Genome Variation Society (HGVS) provides internationally accepted guidelines to avoid misinterpretation and ensure clear communication in clinical and research settings.

DNA Level: Variants are described using the 'c.' prefix for coding DNA (e.g., c.67A>T), 'g.' for genomic DNA, 'm.' for mitochondrial DNA, and 'r.' for RNA.
Protein Level: Variants are described using the 'p.' prefix (e.g., p.Ala67Thr).
Reference Sequences: Each gene may have multiple transcripts and reference sequences, each with a unique accession number and version (e.g., NM_000143.3).

Distinguishing nucleotide and protein level nomenclature

Example: The variant A67T could refer to a nucleotide change (c.67A>T) or a protein change (p.Ala67Thr). It is crucial to specify the level for clarity.

Reference Sequence Selection

Reference sequences are curated by organizations such as the HUGO Gene Nomenclature Committee and the MANE project, which harmonize transcript annotations for clinical reporting.

Nucleotide Numbering and Variant Description

Nucleotide 1 is the A of the ATG start codon.
Positions 5' to the start codon are numbered as -1, -2, etc.
Positions 3' to the stop codon are numbered as *1, *2, etc.
Intronic positions are described relative to the nearest exon (e.g., c.77+1G for the first nucleotide in the intron after exon 77).

Nucleotide positions within a gene

Types of Sequence Variants

Variants are classified by their effect and described using specific terms:

Substitution: One nucleotide is replaced by another (e.g., c.76A>C).
Deletion: One or more nucleotides are removed (e.g., c.76_78del).
Insertion: One or more nucleotides are added (e.g., c.76_77insT).
Duplication: A nucleotide or sequence is duplicated (e.g., c.76dupT).

Table of variant types and nomenclature examples

Variant Effects at the Protein Level

Variants can have different consequences at the protein level:

Missense: Changes one amino acid to another (e.g., p.Gly144Val).
Nonsense: Introduces a premature stop codon (e.g., p.Arg196*).
Frameshift: Alters the reading frame, often leading to a premature stop (e.g., p.Ile162Thrfs*7).
Silent: Does not change the amino acid (e.g., p.Gln192=).

Table of nucleotide changes, protein changes, and variant types Diagram of silent, nonsense, and missense mutations at DNA, mRNA, and protein levels

Splicing Variants

Variants can affect RNA splicing, leading to exon skipping or activation of cryptic splice sites, which may result in abnormal proteins.

Diagram of normal splicing, exon skipping, and cryptic splice site use Splicing consensus sequences

Frameshift and In-Frame Variants

Insertions or deletions not divisible by three cause frameshifts, while those divisible by three are in-frame and may have milder effects.

Frameshift and in-frame small deletions and duplications

Classification of Variants

Variants are classified based on their predicted effect, frequency, segregation, evolutionary conservation, and functional studies.

Diagram of variant classification criteria

Mendelian Inheritance and Pedigree Analysis

Mendel's Laws

Mendelian inheritance describes how traits are transmitted from parents to offspring through genes. Mendel's laws include:

Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation.
Law of Independent Assortment: Alleles of different genes assort independently during gamete formation, except for linked genes.

Diagram of Mendel's laws of segregation and independent assortment Punnett square for Mendelian inheritance

Pedigree Analysis

Pedigrees are graphical representations of family relationships and are used to determine inheritance patterns of genetic traits.

Pedigree chart example Pedigree symbols legend

Modes of Inheritance

Autosomal Dominant: Trait appears in every generation; affected individuals have at least one affected parent.
Autosomal Recessive: Trait can skip generations; affected individuals often have unaffected carrier parents.
X-linked Recessive: Primarily affects males; females are usually carriers.
X-linked Dominant: Rare; affected males pass the trait to all daughters but no sons.
Y-linked: Only males are affected; passed from father to all sons.

Pedigree showing autosomal recessive inheritance Punnett square for autosomal recessive inheritance Pedigree showing X-linked recessive inheritance Diagram of unstable repeat expansion diseases

Special Inheritance Patterns

Imprinting: Expression depends on the parent of origin.
Unstable Repeat Expansions: Diseases caused by expansion of nucleotide repeats (e.g., Huntington disease, Fragile X).
Mitochondrial Inheritance: Passed from mother to all offspring; affects high-energy tissues.

Table of repeat expansion diseases Pedigree and clinical features of mitochondrial inheritance

Penetrance and Expressivity

Penetrance is the proportion of individuals with a genotype who express the phenotype. Expressivity is the degree to which a trait is expressed. Both can be influenced by genetic, environmental, and stochastic factors.

Diagram of variable expressivity Pedigree showing reduced penetrance

Summary Table: Variant Types and Effects

Nucleotide Change	Protein Change	Variant Type
c.586C>T	p.Arg196*	Nonsense
c.485_488del	p.Ile162Thrfs*7	Frameshift
c.586_588dup	p.Arg196dup	In-frame duplication
c.701A>C	p.Tyr234Cys	Missense / Nonsynonymous
c.576G>A	p.Gln192=	Silent / Synonymous

Key Takeaways

Use standardized nomenclature to describe genetic variants at the DNA, RNA, and protein levels.
Understand the types and consequences of sequence variants, including their effects on gene function and inheritance patterns.
Apply Mendelian principles and pedigree analysis to determine inheritance patterns and assess genetic risk.