chr4·hg38·190.2 Mb

Chromosome 4

The chromosome of contradictions — home to both the most dreaded single-gene disease in medicine and the largest gene deserts ever discovered in the human genome.

Physical Properties

Length190.2 Mb

Centromeresubmetacentric

p-arm49.7 Mb

q-arm138.5 Mb

GC content38.2%

Genomic Features

Protein-coding genes757

Gene density4.0 / Mb

CpG islands12,068

EPIC v2 probes46,063

Notable

Largest geneGRID2 (1.5 Mb)

Disease associations

Huntington disease

Wolf-Hirschhorn syndrome

Achondroplasia

· Contains the Huntington disease gene HTT at 4p16.3

· Home to TET2, a key epigenetic regulator

· KIT/PDGFRA cluster drives GIST tumors

Genomic Architecture

Isochore structure — Dominated by L1 and L2 (AT-rich) isochores, with only scattered H1 isochore islands. The paucity of GC-rich isochores explains both the low gene density and late replication timing.

Segmental duplications — The 4p16 subtelomeric region shares ~99% sequence identity with chromosome 10q26, including the D4Z4 tandem repeat array. This paralogous duplication has clinical implications for FSHD diagnosis.

Gene deserts — Contains what are believed to be the largest gene deserts yet discovered in the human genome, paradoxically among the most conserved noncoding sequences across mammals and birds.

Evolutionary History

Ancestral chromosome — Human arrangement represents the ancestral state. The pericentric inversion distinguishing HSA4 from chimpanzee chromosome 3 occurred in the chimpanzee lineage.

Deep synteny — Portions correspond to ancestral mammalian chromosomes (MAM) 1, 3, and 7. MAM7 showed remarkable conservation, with 95% of its length unaffected by intrachromosomal rearrangements for 76 million years.

Deep Cuts

The Huntington paradox — The normal huntingtin protein is essential — complete knockout is embryonic lethal in mice. The mutant protein gains a toxic function rather than losing the normal one.

D4Z4 on chromosome 10 — Near-identical array (~99% identity) on 10q26 never causes FSHD. The sole difference is a single polymorphism in the pLAM region creating/destroying a polyadenylation signal.

The ADH evolutionary moment — The ADH4 variant emergence ~10 Mya coincides with the transition to terrestrial lifestyles — "the drunken monkey hypothesis."

§ Deep dive

Isochore structureDominated by L1 and L2 (AT-rich) isochores, with only scattered H1 isochore islands. The paucity of GC-rich isochores explains both the low gene density and late replication timing.

Segmental duplicationsThe 4p16 subtelomeric region shares ~99% sequence identity with chromosome 10q26, including the D4Z4 tandem repeat array. This paralogous duplication has clinical implications for FSHD diagnosis.

Gene desertsContains what are believed to be the largest gene deserts yet discovered in the human genome, paradoxically among the most conserved noncoding sequences across mammals and birds.

Evolutionary breakpointsA pericentric inversion distinguishes human chromosome 4 from the chimpanzee homolog (PTR3), with breakpoints at HSA4p14 and 4q21.3.