Introduction
Interallelic complementation is defined as the ability of two differentially mutated alleles to function better together than either can on its own. Despite its near universality in lower organisms [1], its potential to contribute to clinical heterogeneity in human disease is seldom considered. Evidence of interallelic complementation at clinically relevant loci is limited to biochemical and cell-based studies of a handful of metabolic disorders with defects in enzymes including propinyl-CoA carboxylase [2], argininosuccinate lyase [3], galactose-1-phosphate uridylyltransferase [4], and methylmalonyl CoA mutase [5].
Compound heterozygotes are individuals carrying two different mutant alleles of the same gene. In the absence of a dominant (wild-type [wt]) allele, genetic interactions between recessive alleles (referred to here as “biallelic” effects) could result in different phenotypic outcomes including interallelic complementation. Although amelioration of disease symptoms by interallelic complementation would create an ascertainment bias in the clinic, the lack of evidence concerning interallelic complementation or other biallelic effects in human disease is likely caused by the difficulty in distinguishing such effects from environment and genetic background.
XPD encodes one of the two helicase components of basal transcription/DNA repair factor IIH (TFIIH), a ten-subunit, multifunctional complex that is essential for multiple processes, including basal transcription initiation and DNA damage repair via the nucleotide excision repair (NER) pathway [6,7]. Alterations in XPD resulting in defective TFIIH function are associated with UV-sensitive, multisystem disorders including xeroderma pigmentosum (XP), XP combined with Cockayne syndrome (CS), and trichothiodystrophy (TTD) [8–10]. XP is marked by sun-induced pigmentation anomalies and a greater than 1,000-fold elevation in skin cancer risk. Severe cases can also present with growth retardation and primary neurodegeneration [11]. CS and TTD, on the other hand, are segmental progeroid disorders characterised by progressive post-natal growth failure and primary demyelination resulting in severe neurodysfunction, but without a clear cancer predisposition [12–15]. Patients with TTD additionally display hallmark sulphur-deficient brittle hair and nails and scaling skin [13], resulting from a basal transcription defect in specific cell types [16,17]. A related disorder with the cancer predisposition of XP combined with the neurodevelopmental complications of CS (XPCS), although rare, has also been described [18].
Many XPD mutations are associated with an exclusive disease phenotype (e.g., XPDR722W with TTD and XPDR683W with XP) and are thus viewed as causative of the corresponding syndromes. Alleles not associated exclusively with one disorder are considered “likely null” alleles [19,20]. Some of these alleles fail to support viability in a haploid Schizosaccharomyces pombe yeast strain with a null mutation in the XPD homologue rad15 and are thus considered devoid of significant biological activity [19]. This classification of alleles as either causative or null currently defines what we refer to as a “monoallelic” paradigm of XPD disease. However, the identification in recent years of XP complementation group D patients with atypical disease presentation, including symptoms of both XP and TTD [8], casts doubt on the ability of such a monoallelic paradigm to explain clinical heterogeneity in compound heterozygotes.
Previously, we generated a TTD mouse model (XPDR722W) that phenocopies the human syndrome [15,21]. Here we report the generation of additional mutant Xpd alleles that fail to support viability on their own but nevertheless ameliorate TTD-associated premature segmental ageing, cutaneous features, cellular DNA repair capacity, and UV survival when present in a compound heterozygote state.