Orthologs, genes in other species with common ancestral heritage, have been examined to explain the pathophysiology of HLS; a similar gene within the roundworm, Caenorhabditis elegans, is responsible for the formation of cilia. Current hypotheses place a dysfunction of cilia as the main cause of HLS defects arising from the HYLS1 mutation in humans. Differences between wild type and mutant HYLS1 have been clearly observed; the wild type form is localised to the cytoplasm, while the mutant form is localised to the nucleus and forms small clusters, suggesting that the mutant gene disrupts cellular localisation. The protein encoded by the HYLS1 mutant form is unable to carry out essential targeting of centrioles to the plasma membrane, disrupting ciliary function, which results in ciliopathy. As cilia are located in almost all cells throughout the body, cilial dysfunction causes developmental defects in a range of organs and thus the phenotype of HLS can vary greatly, though brain malformation and polydactyly are most commonly observed. Environmental
Currently, no environmental factors are known to increase the likelihood of HLS development or progression; HLS is caused only by genetic abnormalities. Pathophysiology
The pathophysiology of HLS is abnormal cilia development arising from the inability of the mutated HYLS1 gene to correctly target centrioles to the plasma membrane. Specifically, transition fibres within the transition zone, at the base of the axoneme and adjoining to the plasma membrane, lack proper development. As these structures form the cilial gate, improper development results in a loss of selectivity for protein entry into the ciliary compartment.
Stillbirth and an excess of amniotic fluid (polyhydramnios) are common during pregnancy with a HLS-affected foetus, with cases of up to 8 litres cited compared to the normal 1 litre. Less common symptoms such as abnormally small eyes and a broad nose are also possible. Cause
Genetic
HLS is caused by a genetic missense mutation of the HYLS1 gene, encoding for Hydrolethalus syndrome protein 1, on chromosome 11; a single base change to the amino acid sequence for HYLS1 in exon 6 involves the replacement of aspartic acid 211 with glycine (D211G) in the polypeptide chain. Exon 6 is the only protein coding exon in HYLS1; proper functioning of exons 1-5 ensures regulation and expression of the entire protein. HLS is an autosomal recessive syndrome; development is only possible if both parents carry the defective gene, and in that instance, the risk of the foetus developing the syndrome is 25%. HLS is a member of the Finnish disease heritage, with incidences more common in Finland than the rest of the world; roughly 1 in 20,000 developing foetuses are affected in Finland. Rare cases in other regions have also been documented, often with less severe phenotypes as a result of allele variability across countries, allowing survival of affected offspring for up to several months. Individuals of Finnish descent are advised to undergo genetic testing before attempting to conceive.Prior to the discovery of HLS, the HYLS1 gene was unknown, and similar genes within humans have not been identified.