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.
Variations and related pathologies
Mutations in KIF7 have also been noted in patients that present a similar phenotype to HLS and the characteristic HYLS1 A to G transformation; homozygous deletion of the KIF7 gene causes a variant form of HLS, HLS2. KIF7 encodes a structural factor vital to cilial transport, and is also implicated in other developmental disorders, such as Joubert syndrome (JS). Additionally, mutations in HYLS1 are no longer explicitly connected to HLS in humans. Homozygous mutations removing the stop codon in exon 4 of HYLS1 result in a different genomic sequence disruption to the missense mutation of HLS, and phenotypically present as JS. The ‘molar tooth sign’ of the brain, an anomaly in which cerebellar volume is reduced but cerebellar shape is retained, resembles the molar tooth and is used to identify JS. JS presents with mutations in more than 30 genes, whilst the HYLS1 mutation is the sole cause of HLS, but is also present in the HLS2 variant form with the mutated KIF7 gene. Diagnosis
HLS can be readily diagnosed during pregnancy through the use of ultrasound, which will often reveal hydrocephaly and an abnormal structure of the brain. Precise examination via ultrasound or at birth is necessary to rule out Meckel syndrome, Trisomy 13, or Smith–Lemli–Opitz syndrome, which present with similar physiological defects. HLS can be detected at the end of the first trimester, approximately 13 weeks gestation. Treatment
No cure or treatment option for individuals with HLS currently exist.