# Novel Phenotypic Elements of Type IV Collagenopathy Revealed by the _Drosophila_ Model
## 6 Conclusions  
The basement membrane is likely to be the primordial form of the extracellular matrix and type IV collagen the key molecule in the transition from uni- to multicellularity [51]. The visco-elasticity of the BM is provided by the type IV collagen network, with defects in this component compromising the network's elasticity and function. Type IV collagen proteins are highly conserved structurally and functionally during evolution, including the mammalian-_Drosophila_ relationship [51]. The ubiquitous human and mammalian BM consists of (COL4A1)\({}_{2}\)COL4A2 heterotrimers and is distributed throughout the body. Glycine substitutions trigger systemic disease [1]. The genotype-phenotype relationship was established through 93 _COL4A1_ and 12 _COL4A2_ mutations. All of the lesions were dominant with nearly equal representation in males (48%) and females (52%). Inherited incidences were recorded at 47% and sporadic incidences at 53%. As observed in other types of collagens, triple helical Gly substitutions were the most prevalent class of mutations (68 out of 93 in _COL4A1_) and frequent substitutions were charged amino acids; Arg (30), Glu (11) and Asp (10) [52].  
Expressivity of the _COL4A1_ mutation-associated phenotypic elements often depends on genetic context in humans, mice and flies. In French families recapitulating key features of HANAC, different heterozygous missense mutations in the _COL4A1_ gene were demonstrated [34]. In a Spanish family, tortuosity of the retinal arteries and retinal hemorrhage were identified, but the lesion was not associated with muscle cramps, renal or brain anomalies, although the p.G510R mutation was previously detected in a French family with HANAC syndrome [35]. Crossing of the _Col4a1_ splice site mutant C57 BL6/J mice with the 129/SvEvTac and CAST/Eij inbred strains showed that the F1 progeny were phenotypically almost indistinguishable from the wild-type [42]. Similarly, in the isoallelic p.G552D1 and p.G552D2 mutants, the survival rates upon heat selection differed slightly. The line with the p.G552D1 lesion presented survival rates of 26.9% for pupae and 5.3% for adults, whereas the survival rates for the p.G552D2 line were 5.7% and 0.50%, respectively [24; 29].  
Here, we provided data on the systemic phenotype of the _col4a1_ mutations and demonstrated the shifting of \(Z\)-discs, mitochondrial fusion and loss of sarcomeres in intestinal visceral muscle fibers. Genetic, genomic and proteomic data suggest that the fruit fly harbors all of the genetic elements of the BM, although the number of genes coding for BM components is apparently reduced in comparison with mammals, increasing the genetic tractability of the fly mutants' defective genes coding for proteins of the BM that are structurally and functionally conserved during evolution, including type IV collagens [53]. The dominant _col4a1_ mutations therefore develop BM-specific phenotypes which are more severe than those observed in human and mouse models. Dominant temperature-sensitivity of the _col4a1_ alleles is a lethal condition, which cannot be maintained in mammals. Nevertheless, the _col4a1_-associated phenotypic elements provide numerous biomarkers that can be measured quantitatively during the screening of large numbers of drugs in high-throughput _Drosophila_ screens, aimed at alleviating symptoms of type IV collagenopathy.  
Conceptualization and methodology, M.M.; validation, A.A.K., N.S-P., M.M.; formal analysis, A.A.K., N.S-P., M.M.; resources, Z.B., V.T.; writing--original draft preparation, M.M., K.C., Z.B.; supervision and project administration, M.M.  
This research was funded by the Hungarian Scientific Research Fund OTKA, grant number NN 108283 to M.M. and by the UNKP-18-3 New National Excellence Program of the Ministry of Human Capacities, grant number UNKP-18-3-IV-SZTE-23 to A.A.K. and UNKP-18-3-IV-SZTE-43 to N.S.-P.  
The authors declare no conflict of interest.