Unwinding DNA: The Crucial Role of Helicases in Rare Human Diseases

Helicase related artwork from Liv Bethell

Wed, 02/05/2025 - 10:07

Introduction

DNA, the molecule of life, encodes the instructions necessary for cellular function in a linear sequence of nucleotides. Each DNA molecule consists of two complementary strands, where adenine (A) pairs with thymine (T) and cytosine (C) pairs with guanine (G) through hydrogen bonds. This precise base-pairing is fundamental for DNA replication, ensuring genetic information is accurately transmitted during cell division.

For various cellular processes - including DNA replication, transcription, repair, and recombination - the nucleotide sequence must become accessible by unwinding the DNA double helix to expose the bases for processing. But how does a cell efficiently unravel DNA to facilitate these essential functions?

What are Helicases?

Helicases are specialized enzymes responsible for unwinding DNA along its length. In all organisms, from prokaryotes to eukaryotes, helicases break the hydrogen bonds that hold DNA strands together, enabling access to genetic information.

Helicases are classified based on their biological function (replication, repair, transcription, or recombination), their target molecule (DNA, RNA, or DNA/RNA hybrids), their directionality (upstream or downstream), and their structural features. Despite these differences, all helicases share a common mechanism: they use chemical energy in the form of ATP (ATP hydrolysis) to move along the DNA molecule, acting as molecular wedges that force the DNA strands apart.

Helicase’s role during DNA replication

Helicases are pivotal in DNA replication, ensuring precise genome duplication before cell division. Replication begins at specific points in the DNA sequence called origin, where initiator proteins bind to DNA, triggering the process. Each origin generates two replication forks that move in opposite directions.

In bacteria, archaea, and eukaryotes, origin-binding proteins facilitate the loading of helicases onto DNA. In bacteria for example, a protein called DnaA initiates replication by binding and unwinding an A/T-rich region, creating a single-stranded DNA "bubble" where the helicase binds. The primary function of these origin-binding proteins is to load two helicases onto the DNA, which then move in opposite directions, unwinding the parental strands and allowing replication to proceed.

In all three domains of life, helicases are six-subunit complexes (hexamers) that move along single-stranded DNA using chemical energy in the form of ATP (Figure 1). As they move, they act as wedges to separate the parental DNA strands. However, bacterial and eukaryotic helicases differ significantly in their structure and function.

Bacterial helicase (DnaB) is a homohexamer, the same protein that forms the six-unit complex. It binds single-stranded DNA generated by DnaA at the origin.

Eukaryotic helicase like MCM2-7, is a heterohexamer, with each subunit encoded by a different gene but sharing similar sequences. Unlike bacterial helicases, MCM2-7 moves in the opposite direction.

Archaeal helicases resemble eukaryotic MCM helicases, but many archaea possess only a single MCM gene, instead of the multi-gene eukaryotic counterpart.

Despite their differences, helicases across all domains of life share a fundamental role: they unwind DNA, enabling replication to proceed efficiently. Their structural diversity allows them to function in different biological contexts, but their working mechanisms remain conserved.

**Figure 1**. Examples of hexameric helicase from bacteria, human and Archaea bound to DNA.

Helicases and disease

Given the essential role of helicases in preserving genomic integrity, it is not surprising that mutations in these proteins are linked to various human diseases. In particular, mutations in helicase genes are associated with an increased incidence of chromosomal aberrations.

Three human helicase genes - BLM, WRN, and RECQ4 - are directly responsible for Bloom’s syndrome, Werner’s syndrome, and Rothmund-Thomson syndrome, respectively. Interestingly, despite being similar, mutations in these genes result in distinct disorders:

Bloom’s syndrome (BLM mutation): Characterized by increased predisposition to cancer and developmental abnormalities.
Werner’s syndrome (WRN mutation): Marked by accelerated aging and age-related conditions such as cataracts, atherosclerosis, osteoporosis, arteriosclerosis, and type II diabetes.
Rothmund-Thomson syndrome (RECQ4 mutation): Associated with premature aging, a high risk of skin abnormalities, and osteosarcoma.

BLM, WRN, and RECQ4 belong to the RecQ subfamily of DNA helicases, which play a crucial role in genome maintenance, from bacteria to eukaryotes. Structurally, RecQ helicases contain a conserved ATP hydrolysis core, a RecQ C-terminal (RQC) domain, and a Helicase and RNase D C-terminal (HRDC) domain (Figure 2):

The ATP hydrolysis domain (ATPase) powers helicase activity.

The RQC domain binds DNA in a sequence-independent manner, working with the ATP hydrolysis domain to form the functional helicase.

The HRDC domain, present only in BLM and WRN, is the least conserved domain and may mediate unique functional properties.

WRN is unique among RecQ helicases, as it contains an additional domain with DNA-processing capabilities (exonuclease).

RECQ4 lacks the RQC and HRDC domains, instead featuring a unique domain (R4ZBD), which contributes to its distinct functional properties.

**Figure 2.** Domain distribution in 4 helicases belonging to the RecQ family of helicases. Experimental structures for representative proteins have been labelled in the same colour scheme. Structural models have been aligned at the ATPase domain to show their structural similarities and differences.

The structural and functional diversity within the RecQ helicase family explains why mutations in these genes lead to different diseases, each affecting DNA repair, genome stability, and cellular aging in unique ways.

by Cristian Escobar

About the artwork

Liv Bethell (year group 9) a student from Leventhorpe, created this artwork inspired by the helicase molecular machine. Liv enjoyed looking and interacting with protein structures in PDB, this provided motivation to understand shading when working with ribbons to represent the protein structure in the art piece.

��Ϸ��վ