NeuroGenetics Curriculum·beginner·15 min

Central Dogma & Molecular Genetics

A focused review of molecular genetics for child neurology — emphasizing trinucleotide repeats, tissue-specific splicing, and the molecular consequences of variant types on gene function.

Tags: Basic Genetics

Learning Objectives

  1. 1.Explain how trinucleotide repeat expansions and de novo variants arise from replication errors
  2. 2.Describe pre-mRNA splicing and explain why splice-site variants are clinically significant in neurogenetics
  3. 3.Classify genetic variant types (missense, nonsense, frameshift, splice site, synonymous) and predict their likely functional impact
  4. 4.Explain nonsense-mediated decay (NMD) and its relevance to variant interpretation

01Genome Organization and the Genetic Code

The human genome contains approximately 3.2 billion base pairs organized into 46 chromosomes, but only ~1.5% encodes protein. The remaining ~98.5% includes regulatory elements, non-coding RNA genes (20,000–25,000), introns, and repetitive sequences — much of which is increasingly recognized as functionally relevant. The genetic code is a degenerate triplet code: 64 codons specify 20 amino acids plus 3 stop signals, with degeneracy partially buffering synonymous substitutions.

Key Points

  • Only ~1.5% of the genome is protein-coding (~20,000 genes); non-coding regulatory and RNA elements account for much of the remaining sequence and are increasingly linked to neurological disease
  • Degeneracy: synonymous codons partially buffer against nucleotide substitutions, but synonymous variants can still be pathogenic by disrupting splicing enhancers
  • GC-rich regions tend to be gene-dense and actively transcribed; CpG dinucleotides are mutation hotspots (~10× higher transition rate) due to spontaneous deamination of 5-methylcytosine

02Replication Fidelity, De Novo Variants, and Repeat Expansions

A three-tier fidelity system (base selection, proofreading, mismatch repair) reduces the replication error rate to ~1 in 10⁹–10¹⁰ per base per division. Despite this, the germline accumulates ~60–70 de novo SNVs per generation (~1–2 per genome per cell division), providing the substrate for both evolution and de novo genetic disease. Trinucleotide repeat expansions — a major class of neurological disease — arise from replication slippage at tandem repeat sequences, with expansion size often increasing across generations (anticipation).

Key Points

  • Mismatch repair (MMR) corrects post-replication errors; MMR deficiency causes microsatellite instability and Lynch syndrome
  • Trinucleotide repeat expansions: CAG in HTT (Huntington), CGG in FMR1 (Fragile X), GAA in FXN (Friedreich ataxia), CTG in DMPK (myotonic dystrophy) — arise from replication slippage; expansion size correlates with severity and age of onset
  • Germline de novo variant rate: ~60–70 SNVs per individual per generation; paternal age is the major contributor (~2 additional variants per year of paternal age), explaining the paternal age effect in de novo dominant conditions like achondroplasia and some epilepsy genes

03Transcription and Pre-mRNA Splicing

Splicing — the removal of introns and ligation of exons — is clinically the most important step in mRNA processing. Splicing is directed by conserved consensus sequences at the 5' splice donor (GT) and 3' splice acceptor (AG) sites flanking each intron. Approximately 10–15% of disease-causing variants affect splicing, making splice prediction a critical skill in variant interpretation.

Key Points

  • Canonical splice site rule (GT-AG): variants at the ±1 and ±2 positions almost always disrupt splicing and support PVS1 in ACMG classification
  • Exonic splicing enhancers (ESEs) are disrupted by some synonymous and deep-intronic variants, causing exon skipping — e.g., certain SCN1A synonymous variants cause Dravet syndrome through splicing disruption
  • Alternative splicing generates tissue-specific isoforms; brain-specific exons explain why variants in ubiquitously expressed genes (e.g., DYNC1H1, SCN1A) can cause purely neurological phenotypes
  • In silico splice predictors (SpliceAI, MaxEntScan) are essential tools for flagging cryptic splice variants; RNA studies (RT-PCR) provide definitive functional evidence

04Translation and Protein Function

Mature mRNA is exported to the cytoplasm where ribosomes translate it codon-by-codon into a polypeptide chain. The AUG start codon is recognized by the 43S pre-initiation complex and Met-tRNA. Elongation proceeds until a stop codon (UAA, UAG, or UGA) is encountered, triggering release of the completed polypeptide. Post-translational modifications — phosphorylation, glycosylation, ubiquitination — determine protein localization, activity, and stability.

Key Points

  • Ribosomes read mRNA in the 5'→3' direction, synthesizing protein N-terminus to C-terminus
  • Kozak sequence context around AUG affects translation efficiency; initiation codon variants (p.Met1?) abolish or reduce protein production
  • Signal peptides direct proteins to the endoplasmic reticulum for secretion or membrane targeting
  • Protein folding is assisted by chaperones (HSP70, HSP90); misfolded proteins are targeted for proteasomal degradation
  • Many neurological disorders result from loss-of-function (insufficient protein) or gain-of-function/dominant-negative protein mechanisms — the distinction critically determines therapeutic strategy

05Variant Types and Their Molecular Consequences

Genetic variants are classified by their molecular nature and predicted effect on gene function. Understanding variant type is the first step in variant interpretation: it determines which ACMG/AMP evidence criteria apply, whether nonsense-mediated decay is expected, and whether the variant is likely to cause loss of function or a gain-of-function effect. Not all variants of the same class have the same functional impact — context is everything.

Key Points

  • Missense variant: single nucleotide substitution causing an amino acid change (e.g., p.Arg176Trp); effect ranges from benign to highly damaging depending on position and residue chemistry
  • Nonsense (stop-gain) variant: nucleotide change introducing a premature stop codon (e.g., p.Arg100Ter); typically causes NMD if the stop codon is >50–55 nt upstream of the final exon-exon junction
  • Frameshift variant: insertion or deletion of non-multiples of 3 nucleotides, shifting the reading frame; almost always introduces a premature stop → NMD
  • Splice-site variant: disrupts canonical ±1/2 donor or acceptor splice sites → exon skipping, intron retention, or cryptic splice site activation
  • Synonymous (silent) variant: nucleotide change that does not alter the amino acid but may affect splicing, mRNA stability, or translation efficiency — not always benign
  • Nonsense-mediated decay (NMD): surveillance pathway that degrades mRNAs with premature termination codons >50–55 nt upstream of the last exon-exon junction, preventing production of truncated, potentially dominant-negative proteins

Quiz Questions

1. A synonymous variant (c.300G>A, p.Thr100=) is identified in a patient with an unexplained genetic condition. Which of the following is the most accurate statement about this variant?

  1. A.Synonymous variants cannot cause disease because the amino acid sequence is unchanged
  2. B.This variant should be classified as Benign (B) under ACMG/AMP criteria without further analysis
  3. C.The variant may affect splicing by disrupting an exonic splicing enhancer (ESE) and warrants RNA-level functional assessment✓
  4. D.Synonymous variants only affect translation rate and have no impact on mRNA stability

Synonymous variants — those that do not change the encoded amino acid — are not inherently benign. Nucleotide changes within exons can disrupt exonic splicing enhancer (ESE) sequences, causing exon skipping even without altering the amino acid. Additionally, some synonymous changes affect mRNA stability or translation efficiency. If a synonymous variant in a disease gene is identified in an unsolved patient, RNA-level studies (RT-PCR, RNA sequencing) should be performed to rule out a splicing effect before applying a benign classification.

2. A boy with intellectual disability and a CGG repeat expansion of 650 repeats in the 5' UTR of FMR1 has absent FMRP on immunocytochemistry. His carrier mother (CGG repeat: 85) has a normal FMRP level. Which mechanism best explains the difference between full mutation and premutation alleles?

  1. A.Full mutations produce a toxic polyglutamine protein, while premutations do not reach the polyglutamine threshold
  2. B.Full mutations trigger promoter hypermethylation that silences transcription, while premutation alleles remain unmethylated and actively transcribed✓
  3. C.Full mutations cause chromosomal breaks at Xq27.3, while premutations are structurally stable
  4. D.Full mutations alter the FMR1 coding sequence, while premutations only affect the 3' UTR

Full mutation alleles (>200 CGG repeats) in FMR1 trigger hypermethylation of the repeat and the adjacent promoter CpG island, silencing transcription and abolishing FMRP production. Premutation alleles (55-200 repeats) remain unmethylated and are actively transcribed — in fact, they produce elevated FMR1 mRNA levels. This creates a paradox: premutation carriers have normal FMRP but excess mRNA, which causes distinct diseases (FXTAS, FXPOI) through an RNA gain-of-function mechanism. The CGG repeat is in the 5' UTR, not the coding sequence, so neither allele alters FMRP's amino acid sequence.

3. Alternative splicing of a gene produces both a 'short' isoform (expressed in muscle) and a 'long' isoform (expressed in brain). A patient has a splice site variant that disrupts exon inclusion in the brain isoform only. This variant most likely causes:

  1. A.A systemic multi-organ disorder because all tissues express the same gene
  2. B.A brain-restricted phenotype due to isoform-specific splicing in the nervous system✓
  3. C.No disease because the muscle isoform is unaffected
  4. D.Disease only if the variant also affects the muscle isoform

Tissue-specific alternative splicing can restrict the functional impact of a variant to a particular organ. If the pathogenic splice site variant only affects the brain-specific isoform, then only brain tissue lacks the normal protein product — explaining why some variants in ubiquitously expressed genes cause pure neurological phenotypes. This principle is clinically important: a 'splice-disrupting' variant identified in a brain-expressed isoform may be missed if only the reference (muscle) transcript is analyzed.

4. A pathogenic variant is identified as c.247C>T (p.Arg83Ter) in exon 3 of a gene with 10 exons. Which statement best predicts the molecular consequence?

  1. A.The variant will produce a stable truncated protein of 83 amino acids
  2. B.The mRNA will likely undergo nonsense-mediated decay, resulting in reduced or absent protein✓
  3. C.The variant is synonymous and will have no effect on protein sequence
  4. D.The variant will cause an in-frame deletion of exon 3

A nonsense (stop-gain) variant introducing a premature termination codon (PTC) in exon 3 of a 10-exon gene is predicted to trigger nonsense-mediated decay (NMD). NMD degrades mRNAs when the PTC is located >50–55 nucleotides upstream of the final exon-exon junction. Since there are 7 more exons downstream of the PTC, this variant clearly meets the NMD rule. NMD prevents production of truncated proteins, typically resulting in effective loss-of-function — the basis for applying the PVS1 criterion.

5. Which of the following variant descriptions is consistent with a frameshift mutation?

  1. A.c.412C>T (p.Arg138Trp) — single nucleotide substitution, amino acid change
  2. B.c.412C>A (p.Arg138Ter) — single nucleotide substitution, stop codon introduced
  3. C.c.412_413del (p.Leu138ProfsTer12) — 2-nucleotide deletion, reading frame shifted✓
  4. D.c.412+1G>A — intronic variant at the splice donor site

A frameshift variant results from an insertion or deletion that is not a multiple of 3 nucleotides, disrupting the reading frame downstream of the variant. c.412_413del is a 2-nucleotide deletion — not divisible by 3 — which shifts the reading frame (indicated by 'fs' in the protein notation) and creates a premature stop codon 12 amino acids downstream (Ter12). Missense (Arg138Trp), nonsense (Arg138Ter), and splice-site variants (c.412+1G>A) are distinct variant classes with different molecular consequences.