DNA size: 9.71kb encoding two exons. This gene has one transcript (splice variant), 82 orthologues, 12 paralogues (www.emsable.org). Sagara et al., reported a splice variant of FZD4 gene which they called FZD4SA, it retains intronic sequence and encodes shorter isoform of only 125 aa. However, its expression is not supported by other experimental evidences.

The FZD4 mRNA transcript is 7383 bp. FZD4-001 ENST00000531380.1: mRNA7383 bp, protein 537 aa.

The gene FZD4 encodes a 537 aa protein with a molecular weight of 59 kDa. FZD4 is a member of the seven transmembrane receptor family consist of 10 receptors that are activated by Wnt family of lipo-glycoproteins. The Wnt/ FZD signaling is involved in a variety of biological processes and its dysregulation have been implicated in cancer development. FZD4 protein contains the N-terminal signal peptide (aa 1-36) that assurances proper membrane insertion of the protein, an extracellular cysteine rich domain (CRD; aa 40-161), which creates the binding site for WNT ligands, a seven-pass transmembrane domain (aa 161-221) that gives rise to three intracellular loops, three extracellular loops and a C- terminal domain (aa 221-537). The CRD domain is necessary to bind WNT ligands or Norrin ligand leading to initiation of distinct downstream signaling pathways. (Schulte G., 2010).

In human, FZD4 is a ubiquitous protein. It is expressed in brain, ovary, liver, pancreas, brain, colon, heart, skeletal muscle, endothelial cells, endometrium, bone marrow, prostate, spleen, breast (www.ncbi.nlm.nih.gov).

FZD4 is localized on the plasma membrane surface. It can be internalized through both constitutive and agonist dependent endocytosis in response to Wnt5a stimulation (Chen W. et al., 2003).

FZD4 is a member of Frizzled gene family involved in neuronal, follicle, cardiomyocyte and retinal vascular development, likewise its dysregulated expression lead to cancer and other diseases. Depending on the cellular contest, FZD4 interacts with different WNT ligands, leading to the activation of Wnt/β catenin signaling and sometimes non canonical Wnt/Ca²⁺ signaling. Wnt/β catenin signaling is activated when WNT ligands bind CDR FZD/Low-density lipoprotein receptor-related protein5/6 ( LRP5/ LRP6) complex, in this case CTNNB1 (β-catenin) degradation complex becomes inactivated, resulting in stabilization of β-catenin that can translocate in the nucleus, where it interacts with LEF1 (TCF/LEF) transcription factor, inducing the transcription of target genes (Clevers H., 2006). Recently, WNT10B/FZD4 interaction in the MCF7 breast cancer cell line suggests an autocrine activation of Wnt signalling in this cell line model (Lazzaroni F. et al, 2016). In melanoma FZD4 binds WNT5A and stimulates tumor invasion through activation of βcatenin signaling (Grossman A. et al., 2013), while in acute myeloid leukemia the interaction between WNT3A and FZD4 induce higher resistance against apoptosis (Tickenbrock L. et al., 2008). WNT2, WNT5A/ WNT5B and WNT11 via FZD4 and FZD6 induced non canonical Wnt signaling activation that regulates cardiomyocite differentiation (Mazzotta S. et al., 2016)
FZD4 is also the only FZD family member that binds selectively a growth factor called NDP (Norrin) and regulates endothelial cells growth during retinal vascular development. In retina, the binding of Norrin with FZD4 conjugated with LRP5 co-receptor and protein TSPAN12 (Tetraspanin-12), results in activation of βcatenin signalling (Schulte G., 2010), alteration in one of this gene is associated with Familial Exudative Vitreoretinopathy.

The FZD4 gene is conserved in chimpanzee, mouse, Rhesus monkey, dog, cow, rat, chicken, zebrafish and frog.

Several types of mutations (missense, nonsense, small deletions) have been reported for the human FZD4 gene and are related to the familial exudative vitreoretinopathy (FEVR). Among these mutations, different heterozygous substitutions have been reported: M342V, W335C, R417, I256V, P33S, G36N, H69Y, M105T, M105V, C181R, C204R, C204Y, C45Y, Y58C, W226X, and G488D (Zhang K. et al., 2011; Kondo H. et al. 2003; Quin et al., 2005) It has also been described a loss of function mutation of FZD4 with nucleotides 1479-1484 deletion in two cases of FEVR, resulting in the lacking of met493 and trp494 that leads to a frameshift and creates a stop codon at residue 533 (Robitaille J. et al.,2002).