Disease-causing missense mutations in human DNA helicase disorders

Abstract

Helicases have important roles in nucleic acid metabolism, and their prominence is marked by the discovery of genetic disorders arising from disease-causing mutations. Missense mutations can yield unique insight to molecular functions and basis for disease pathology. XPB or XPD missense mutations lead to Xeroderma pigmentosum, Cockayne's syndrome, Trichothiodystrophy, or COFS syndrome, suggesting that DNA repair and transcription defects are responsible for clinical heterogeneity. Complex phenotypes are also observed for RECQL4 helicase mutations responsible for Rothmund-Thomson syndrome, Baller-Gerold syndrome, or RAPADILINO. Bloom's syndrome causing missense mutations are found in the conserved helicase and RecQ C-terminal domain of BLM that interfere with helicase function. Although rare, patient-derived missense mutations in the exonuclease or helicase domain of Werner syndrome protein exist. Characterization of WRN separation-of-function mutants may provide insight to catalytic requirements for suppression of phenotypes associated with the premature aging disorder. Characterized FANCJ missense mutations associated with breast cancer or Fanconi anemia interfere with FANCJ helicase activity required for DNA repair and the replication stress response. For example, a FA patient-derived mutation in the FANCJ Iron-Sulfur domain was shown to uncouple its ATPase and translocase activity from DNA unwinding. Mutations in DDX11 (ChlR1) are responsible for Warsaw Breakage syndrome, a recently discovered autosomal recessive cohesinopathy. Ongoing and future studies will address clinically relevant helicase mutations and polymorphisms, including those that interfere with key protein interactions or exert dominant negative phenotypes (e.g., certain mutant alleles of Twinkle mitochondrial DNA helicase). Chemical rescue may be an approach to restore helicase activity in loss-of-function helicase disorders. Genetic and biochemical analyses of disease-causing missense mutations in human helicase disorders have led to new insights to the molecular defects underlying aberrant cellular and clinical phenotypes.

Figure 1. Clinically relevant missense mutations in FANCJ helicase responsible for Fanconi Anemia complementation group J or associated with breast cancer

See text for details. For a comprehensive listing of FANCJ mutations, see The Rockefeller University-Fanconi anemia mutation database www.rockefeller.edu/fanconi/mutate; also see [23]. FANCJ protein interaction sites for MLH1 [61] and BRCA1 [62] are indicated as well as region of FANCJ shown to interact with BLM helicase [138]. Nuclear localization sequence (NLS) is also designated. Note in the figure the spatial relationships between FA disease-linked or breast cancer associated mutations in FANCJ and domains responsible for helicase activity, protein interactions, or the NLS.

Figure 2. Disease-causing FANCJ-A349P Fe-S domain mutation uncouples DNA translocation from helicase activity

Panel A, FANCJ protein with the conserved helicase core domain and position of the Fe-S cluster. The conserved helicase motifs are indicated by yellow boxes, and the protein interaction domains for MLH1 and BRCA1 are shown by aqua green and blue boxes, respectively. The expanded Fe-S domain shows the locations for conserved cysteine residues in orange, and the A349P missense mutation of a FANCJ patient in bold. Panel B, Biochemical analysis of the purified recombinant FANCJ-A349P protein demonstrated that the amino acid substitution in the Fe-S domain uncouples ATPase and single-stranded DNA translocase activity from its duplex DNA unwinding function. See text and reference [67] for details.

Figure 3. Pathogenic mutation of homologous residue in Fe-S domain of DDX11 and XPD

Amino acid alignment showing start of Fe-S cluster in human DDX11, XPD, FANCJ, and RTEL1 helicase proteins. Mutation of conserved arginine (R263 in DDX11, and R112 in XPD (red)), residing four residues upstream of highly conserved cysteine (green), is responsible for WABS and TTD, respectively. Distinguishing cellular phenotypes and biochemical effects of the DDX11 and XPD mutations linked to WABS and TTD, respectively, are noted. See references [–75] and text for details.

Figure 4. Clinically relevant missense mutations in XPD helicase responsible for Xeroderma pigmentosum, Xeroderma pigmentosum combined with Cockayne syndrome, Trichothiodystrophy, or COFS Syndrome

For discussion of XPD mutations and genetic heterogeneity, see [20]. Note in the figure that some of the XPD mutations linked to TTD reside outside the helicase core domain in the C-terminal extension, whereas all XP- or XP/CS-linked mutations reside within or very near the helicase core domain. XPD-D681N mutation is associated with XP and COFS syndrome.

Figure 5. Clinically relevant missense mutations in WRN helicase-nuclease responsible for Werner syndrome

See text for details. For a comprehensive listing of WRN mutations, see The International Registry of Werner Syndrome www.wernersyndrome.org; also see [91]. The high affinity RPA70 interaction domain of WRN [139] is indicated. WRN exonuclease domain (EXO) and NLS are also shown. See text for details.

Figure 6. Clinically relevant missense mutations in BLM helicase responsible for Bloom’s syndrome

See text for details. For a comprehensive listing of BLM mutations, see The Bloom’s Syndrome Registry www.med.cornell.edu/bsr/; also see [111]. Region of BLM shown to interact with Topoisomerase 3α [140] is shown. Asterisk indicates disease-causing BLM mutations which have been shown to significantly impair or abolish BLM helicase activity.

Figure 7. Clinically relevant missense mutations in RECQL4 helicase responsible for Rothmund-Thomson syndrome, Baller-Gerold syndrome, or RAPADILINO

See text for details. For comprehensive listing of RECQL4 mutations, see [15]. RECQL4-R1021W is associated with both RTS and BGS. Region of RECQL4 shown to interact with SLD2 [128] is shown.