7q21.3

Lung cancers

Neoplastic processes often result from combinatorial activity of developmental genes (Abate-Shen, 2002). Dysregulated expression of homeobox-containing genes of the distal-less family, arranged as three bigenic pairs in mammals (DLX1/2, DLX3/4 and DLX5/6; Kraus and Lufkin, 2006), has been reported to correlate with distinct oncogenic mechanisms.
DLX6 along with DLX5 is a direct MYC oncogene inducer, responsible for neoplastic initiation in many cancers, including lymphoma and lung cancers (Xu and Testa, 2009).

Breast cancers and their bone metastases

DLX6, together with DLX5, is upregulated in lung and bone metastatic cells derived from primary breast tumors in human - a pattern associated with tumour agressivity and thus, poor prognosis and increased relapses (Morini et al., 2010). Transcriptional profiling in search for prognosis markers has identified DLX6 as an upregulated candidate for high-grade astrocytomas (Phillips et al., 2006).

Dysmorphologies

DLX6 is often regarded as a functional substitute of DLX5 and autonomous regulation as been seldom observed; one rare such situation being the Endothelin-1→Endothelin-Ra→Dlx6→Hand2 signalling cascade which specifies lower jaw identity in the mouse embryo (Charité et al., 2001). As such, malformations described for DLX5 are commonly regarded as involving DLX6. In most mouse mutant models, severe phenotypes result from dual invalidation of Dlx5 and Dlx6. Malformative processes implying DLX6 will thus be simultaneously described on the DLX5 gene card.
Split hand-foot malformation (SHFM) type 1 with sensory-neural hearing loss (SHFM1D; MIM:220600). This malformative syndrome affects hands and feet alike, resulting in moderate to severe median ray deficiency with syndactily. Among the described six non-syndromic SHFM loci, one spans the DLX5/DLX6 bigenic cluster (Scherer et al., 1994; Crackower et al., 1996). Numerous reported mutations spare DLX5 or DLX6 open reading frames, suggesting it may rather be their common regulatory elements which is impacted (Robledo et al., 2002 ; Lo Iacono et al., 2008). However recently, two rare familial cases of SHFM1 have been demonstrated to result, with highest probability, from intragenic missense mutations of two critical glutamine residues in the third helix of the DLX5 homeodomain (Q178P reported in Shamseldin et al., 2012; and Q186H characterized in Wang et al., 2014). In the first case, a causal link between defective DLX5/DLX6 expression and the pathogenic mechanism impairing limb development remains to be elucidated. In the second case, the mutated DLX5 has been demonstrated to fail at transactivating its bona fide MYC target. Such an observation is not unexpected as the mutation affects Q50, the most conserved residue of all homeoproteins (see diagram), which numerous biochemical studies have demonstrated to be responsible for the specificity of the DNA recognition at the TAATT homeo-element (for review, Galliot et al., 1999).
Other pathogenetic processes: on a further note, SHFM cases have often been reported to include hearing loss, a trait consistent with a developmental role demonstrated for Dlx5/Dlx6 during ear formation in mouse embryogenesis (Acampora et al., 1999; Merlo et al., 2002; Robledo and Lufkin, 2006; Chatterjee et al., 2010; Frenz et al., 2010). Moreover, both genes are major targets of two regulator genes whose deficiencies are responsible for a related pathogenic condition, the auriculo-condylar syndrome (ACS, Rieder et al., 2012).
Anorectal malformation associated with SHFM has been reported in a family with a missense mutation in the P63 gene, a known direct upstream regulator of DLX5/DLX6 during morphogenesis (Su et al., 2013). Whether DLX5/DLX6 expression is dysregulated in this condition, and whether this trait can be functionaly associated with the phenotype, remains to be elucidated.

Trinucleotide repeats

The first DLX6 exon harbours a trinucleotide repeat region of 11 to 20 CAG triplets in normal, heterozygous subjects. This CAG repeat is highly polymorphic (Pfeffer et al., 2001). While no obvious phenotype was associated with this newly discovered polymorphism in the investigated cohort, such repeat length variations are critical determinants of colon carcinogenesis and neurodegenerative disorder when occurring in the androgen receptor and huntingtin genes, respectively.

Rett syndrome

DLX6 and DLX5 (OMIM #600028) have been controversial candidates for neurodevelopmental defects progressively afflicting young girls suffering of Rett syndrome (OMIM #312750). This late onset disorder features fatal motor abnormalities, seizures, autism and mental retardation. While the genomic sequence of the DLX5/DXL6 locus remains unaffected in all reported cases, it is a direct target of the transcriptional regulator methyl-CpG-binding protein 2 (MeCP2), which has been strongly associated to this syndrome by linkage analysis (Horike et al., 2005). While still debated (Horike et al., 2005; Schüle et al., 2007; LaSalle, 2007; Miyano et al., 2008), initial MeCP2 deficiency is considered as causing defective neurogenesis through dysregulated expression of DLX5/DLX6, due to altered chromatin state at this target locus (Horike et al., 2005; Lilja et al., 2013). Mouse mutagenesis has substantiated this hypothesis by pinpointing GABA (γ-aminobutyric acid)-releasing neurons as a major cellular target expressing Dlx5 and Dlx6, whose deficiency impairs neurogenesis in MeCP2 null mutant (Chao et al., 2010).

Osteoporosis

Mouse mutational studies have demonstrated a role for Dlx5 and Dlx6 as a major determinant of chondrogenesis and chondrocyte hypertrophy in the endochondral skeleton, throughout embryogenesis and adulthood (Samee et al., 2007; Samee et al., 2008; Samee et al., 2009). These observations pave the way for a better understanding of human osteoporosis, in particular in patients with dysfunctional regulation of bone-remodeling hormonal levels (Prall et al., 2013).

Reproductive tract

Dlx5 and Dlx6 are involved in the development and function of the reproductive tract. The dual mouse mutant for Dlx5 and Dlx6 displays abnormal urethra formation (Suzuki et al., 2008), reduced testicular steroidogenesis with feminization (Nishida et al., 2008), and early ovarian follicular depletion (Bouhali et al., 2011). A human mutation in a genomic region including DLX5 and DLX6 has been associated to a case of familial premature ovarian failure (Caburet et al., 2012).

Teratology

With regards to pharmacologically-induced teratogenesis, dysregulation of Dlx5/Dlx6 gene expression has been demonstrated to be a major step during craniofacial embryopathy induced by two compounds :
i) retinoic acid, a vitamin A derivative found in the RoAccutane ® drug, which prevents the induction of Dlx5/Dlx6 in all animal models investigated (Vieux-Rochas et al., 2007; Vieux-Rochas et al., 2010); this discovery has given strong insight into the aetiology of teratologic impact of fetal exposure to RoAccutane medication in man. Analyses of retinoic acid-induced embryopathy in mouse neurulas have demonstrated that retinoic acid exposure prevents proper induction of both Dlx5 and Dlx6 by endothelin-1 signalling. This disruption has been found to be finely tuned during a surprisingly short timeframe spanning a critical period of neurulation. This exposure creates a functional invalidation of Dlx5/Dlx6-controlled cranio-facial morphogenesis (reviewed in Gitton et al., 2010);
ii) the food contaminant ochratoxin A, a fungal toxin demonstrated to prevent Dlx5 activation in exposed mouse embryos, which later develop craniofacial malformations (Wei and Sulik, 1993; Napoletano et al., 2010). Although a causal link between Dlx5, Dlx6 and the toxin remains to be functionally demonstrated, this observation may account for teratogenesis observed in human embryos maternally exposed to the toxin (Hope and Hope, 2012; Thrasher et al., 2012).