# Novel Phenotypic Elements of Type IV Collagenopathy Revealed by the _Drosophila_ Model
## 1 Introduction  
The highly differentiated form of the extracellular matrix is the planar-organized basement membrane (BM) found at the basal side of epithelial and endothelial cells in neural, vascular and adipose tissues, surrounding each individual striated muscle fiber. Major proteins of BMs are type IV collagen, laminin, nidogen or entactin, perlecan and integrins [1]. Nidogens and perlecan are required perinatally but not at earlier phases of development [2]. The BM is anchored to the cell-surface integrin and to the dystroglycan receptors and provides a BM-cytoskeleton linkage [3]. The family of laminins consists of five alpha, four beta and three gamma chains in mammals. They harbor three pairs of type IV collagen genes, _COL4A1_ through to _COL4A6_, with each gene pairin head-to-head genomic organization [4; 5; 6]. Defects in BM components are often linked to human genetic diseases. An example of a laminin disorder is Lma2, merosin-deficient congenital muscular dystrophy (MDC1A and limb-girdle type) arising from lesions in the _LAMA2_ gene, it affects skeletal muscles and the neural system [7; 8]. Mutations of the _LAMB2_ gene cause Pierson syndrome, a form of focal segmental glomerulosclerosis [9]. Nearly 100 years ago, [PERSON] reported about \"hereditary familial congenital hemorrhagic nephritis\" predominantly affecting men [10]. Type IV collagen genes were isolated many decades later, starting with _COL4A1_[11], which was placed at the tip of the long arm of chromosome 13 [12]. Discovery of the rest of the five type IV collagen genes soon followed [13; 14; 15; 16; 17]. Association of the _COL4A5_ gene mutations was demonstrated in patients with X-chromosomal Alport syndrome [18; 19]. [PERSON] syndrome has a digenic inheritance. Only 80% of the patients carried mutations in the X-chromosomal _COL4A5_ gene, whereas the rest had lesions in the autosomal _COL4A3_ locus at 2q36.3 [20; 21]. The glomerular capillaries filter plasma through the glomerular basement membrane consisting of COL4A3, COL4A4 and COL4A5 heterotrimers. This isoform is absent from the glomeruli in patients with X-linked Alport syndrome and that harbor the fetal (COL4A1)\({}_{2}\)COL4A2 trimers, with increased susceptibility to proteolytic attack by collagenases and cathepsins [22; 23]. We identified dominant, temperature-sensitive mutations in the _Drosophila_ type IV collagen gene _col4a1_ exclusively. The _col4a1 +\({}^{\prime}\)_ heterozygotes are viable and fertile at 20 \({}^{\circ}\)C, the permissive temperature; however, they perish under the restrictive condition, 29 \({}^{\circ}\)C. The dominant-negative, antimorphic phenotype suggests the incorporation of mutant COL4A1 monomers into the planar-organized triple helices. The genome of the fruit fly consists of a pair of type IV collagen genes, _col4a1_ and _col4a2_ in a head-to-head orientation, similar to mammals [24]. The mutant phenotype is systemic, affecting the gut [25; 26], the excretory organ Malpighian tubules [27; 28] and the muscles manifested in muscular dystrophy [29]. The recessive phenotype of the _col4a1 \({}^{\prime}\)_- homozygotes leads to lethality in the late embryonic to early larval phases at the permissive temperature [24]. Type IV collagen in _Drosophila_ appears to fulfill a similar function as in mammals by providing BM stiffness and elasticity. Beyond conserved genomic organization, both genes and proteins share a high level of identity and similarity [24]. The clinical spectrum of _COL4A1_-associated defects is wide. Pedigrees of families in the Netherlands were identified as presenting cerebral porencephaly [30]. Mapping placed autosomal dominant type 1 porencephaly at the tip of chromosome arm 13q [31]. These families were assessed for mutations in the _COL4A1_ gene and missense Gly substitutions were demonstrated within the gly-X-Y repeats in the (COL4A1)\({}_{2}\)COL4A2 triple helical domain [32]. Three families were diagnosed with autosomal dominant hereditary angiopathy with nephropathy, aneurysms and muscle cramps (HANAC) [33].