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 patient with epilepsy has a variant at the canonical splice donor site of exon 5 in SCN1A (c.803+1G>A). SpliceAI predicts a high probability of exon 5 skipping. Which functional study would provide definitive evidence of the splicing effect?

  1. A.Western blot of the Nav1.1 protein to assess molecular weight
  2. B.RT-PCR of patient RNA followed by gel electrophoresis and sequencing of the product✓
  3. C.Repeat genomic DNA sequencing at higher depth to confirm the variant
  4. D.Chromosomal microarray to check for a deletion encompassing exon 5

RT-PCR (reverse transcription PCR) of patient mRNA is the definitive functional study for splice-site variants. It directly demonstrates the effect on the mature transcript — exon skipping, intron retention, or cryptic splice site activation. Gel electrophoresis reveals abnormal transcript sizes, and sequencing confirms the aberrant junction. Genomic DNA sequencing identifies the variant but cannot show its effect on splicing. In silico tools like SpliceAI predict the effect but do not constitute definitive evidence.

2. A 65-year-old man presents with progressive tremor, ataxia, and cognitive decline. His grandson was diagnosed with Fragile X syndrome. Genetic testing of the grandfather reveals 95 CGG repeats in FMR1. This clinical presentation is best explained by:

  1. A.Full mutation Fragile X syndrome — the grandfather's repeat exceeds the 200-repeat threshold
  2. B.Fragile X-associated tremor/ataxia syndrome (FXTAS) — RNA toxicity in premutation carriers✓
  3. C.Huntington disease — the CGG repeat is actually located in the HTT gene, not FMR1
  4. D.Alzheimer disease — unrelated to FMR1 but triggered by age-related neurodegeneration

FMR1 premutation carriers (55-200 CGG repeats) produce elevated FMR1 mRNA that is thought to be toxic through an RNA gain-of-function mechanism. In older male carriers, this can cause FXTAS — a progressive neurodegenerative disorder characterized by intention tremor, cerebellar ataxia, and cognitive decline. Unlike full mutation Fragile X syndrome (>200 repeats, gene silencing, absent FMRP), premutation alleles are actively transcribed and produce normal FMRP but excess toxic mRNA.

3. A child with a neurodevelopmental disorder has a variant in a ubiquitously expressed gene, yet the phenotype is restricted to the central nervous system. Which molecular mechanism best explains this tissue-specific effect?

  1. A.The gene is transcribed only in the brain despite being present in all tissues
  2. B.The variant disrupts a brain-specific alternatively spliced exon only✓
  3. C.The protein product is selectively degraded in all tissues except the brain
  4. D.Epigenetic silencing of the gene occurs in all non-neural tissues

Alternative splicing generates tissue-specific isoforms from the same gene. If a variant disrupts an exon that is included only in the brain-specific isoform, other tissues with different splicing patterns are unaffected. This is a well-established mechanism in neurogenetics — for example, variants in brain-specific exons of SCN1A and DYNC1H1 cause purely neurological phenotypes despite the genes being expressed in multiple tissues. Variant interpretation must consider the relevant tissue-specific transcript.

4. A nonsense variant (p.Gln450Ter) is identified in the last exon of a 12-exon gene. Unlike most nonsense variants, this one is predicted to escape nonsense-mediated decay (NMD). Why?

  1. A.Nonsense variants in the last exon always produce fully functional proteins
  2. B.The PTC is in the last exon, so no downstream exon-exon junction exists to trigger NMD✓
  3. C.NMD only degrades mRNAs with frameshift variants, not those with nonsense variants
  4. D.The stop codon is too close to the start codon for NMD to be activated

Nonsense-mediated decay (NMD) requires the premature termination codon (PTC) to be more than 50-55 nucleotides upstream of the final exon-exon junction. Variants in the last exon — or within the last ~55 nt of the penultimate exon — escape NMD because no downstream exon-exon junction exists to trigger the surveillance pathway. These NMD-escaping variants produce stable truncated proteins, which may exert dominant-negative effects rather than causing simple loss-of-function through haploinsufficiency.

5. A genetic report notes that a recurrent pathogenic variant in a disease gene falls at a CpG dinucleotide, which is flagged as a mutation hotspot. A trainee asks what makes CpG sites prone to mutation. Which molecular process accounts for their elevated mutation rate?

  1. A.CpG sites are prone to UV-induced thymine dimers that escape nucleotide excision repair
  2. B.Spontaneous deamination of 5-methylcytosine produces thymine, creating a C>T transition✓
  3. C.DNA polymerase has lower fidelity when replicating through CpG-rich genomic regions
  4. D.CpG sites recruit error-prone translesion synthesis polymerases during S-phase

CpG dinucleotides have a mutation rate approximately 10-fold higher than the genome average because cytosine at CpG sites is frequently methylated to 5-methylcytosine, which spontaneously deaminates to thymine. The resulting G:T mismatch may not be fully repaired before the next round of replication, permanently converting the CpG to TpG. This mechanism makes CpG sites the most common location for recurrent pathogenic missense and nonsense variants in human disease genes.