7q21.3

SHFM1,SHFM1D

Lung cancer

The DLX5 gene was reported to be overexpressed in the great majority of human non-small cell lung cancers examined by Kato et al., 2008. Furthermore, immunohistochemical studies revealed that positive immunostaining for DLX5 correlated with tumor size and poorer prognosis.
A DLX5 transcript isoform has been shown to be strongly overexpressed in a large panel of primary lung cancer samples, providing a reliable prognosis marker (Kato et al., 2008). In this study, the detected isoform (1.8 kb on Northern blot using a probe spanning exon 3 and 3UTR) was claimed to be found only in the placenta, among 23 normal adult tissues. It should be observed that other studies have reported numerous expression sites in adult human, including bone (osteoblasts and marrow), ear, tooth, fat and brain. With regards to function, down-regulation of DLX5 through RNA interference compromised the growth or survival of two lung cancer cell lines, suggesting that controling DLX5 expression levels might be clinically relevant (Kato et al., 2008).

Lymphoma

DLX5 was found to be highly expressed in 3 of 7 (42%) patient-derived T-cell lymphomas compared with that observed in nonmalignant lymph node samples (Tan et al., 2008). In addition, these investigators found repeated upregulation of Dlx5 in T-cell lymphomas from transgenic mice in which the Lck promoter was used to drive expression of a constitutively active form of Akt2 in the thymus. Dlx5 was overexpressed due to a novel chromosome inversion that placed the T-cell receptor beta (Tcrb) enhancer region near the Dlx5 locus.

Breast cancer

Both DLX5 and DLX6 were found to be upregulated during metastasis formation after intravenous injection of MDA-MB-231 breast cancer cells. The in vitro treatment of MDA-MB-231 cells with endothelin 1, a peptide associated with breast cancer invasive phenotype, resulted in a switch from DLX2 to DLX5 expression. Mutually exclusive expression of DLX2 and DLX5 was found in both MDA-MB-231 cells and human breast cancer specimens. This evidence suggested that DLX genes are involved in human breast cancer progression, and that expression of DLX2 and DLX5 genes might serve as prognostic markers (Morini et al., 2010).

Astrocytoma

Transcriptional profiling in search for prognosis markers has identified DLX5 as an upregulated candidate for high-grade astrocytomas (Phillips et al., 2006).

Various cancers

DLX5 mRNA is abundantly expressed in many cancer cell lines derived from malignant tissues of breast, brain, lung, skin, and ovarian cancer patients, whereas expression of DLX5 was low or undetectable in tumor cells from patients with leukemia or with colorectal, prostate, and kidney cancers (Tan et al., 2010).

Dysmorphologies

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). However, 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, one rare familial case of SHFM1 has been demonstrated to result, with highest probability, from an intragenic missense mutation of the DLX5 homeodomain (Q178P; Shamseldin et al., 2012). In the latter case, a causal link between defective DLX5/DLX6 expression and the pathogenic mechanism impairing limb development remains to be elucidated (Lango Allen et al., 2014).
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; Brown et al., 2010).
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.

Rett syndrome

DLX5 and DLX6 (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., 2007), 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 indirectly prevents the induction of DLX5/DLX6 in human (Lammer et al., 1985; Coberly et al., 1996) and in all animal models investigated (Vieux-Rochas et al., 2007), which share a wide range of jaw and ear malformations;
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).