👤 Faten Hadj Kacem

Familial hypercholesterolemia (FH) is a genetic disorder characterised by elevated plasma LDL-cholesterol, predisposing to premature atherosclerotic cardiovascular disease. Most cases follow an autosomal dominant pattern (ADH) caused by pathogenic variants in LDLR, APOB or PCSK9. In contrast, the rare autosomal recessive form (ARH) results from biallelic mutations in LDLRAP1, leading to defective LDL receptor-mediated endocytosis. Despite the high rate of consanguinity in Tunisia, LDLRAP1 variants have not yet been reported in this population. In this study, Whole Exome Sequencing of two consanguineous Tunisian families, identified distinct pathogenic variants. In the first family (FH-A), a recurrent LDLR splice-site variant (c.1845+1G>A) was detected in both heterozygous and homozygous states, consistent with an autosomal dominant inheritance pattern. In the second family (FH-B), a novel homozygous LDLRAP1 missense variant (c.161G>A; p.Gly54Asp) was identified, confirming autosomal recessive inheritance. In silico analyses using MutationTaster, DynaMut2, MUpro, DDGun, NetSurfP-2.0, ConSurf and PyMOL predicted that the p.Gly54Asp substitution destabilises the PTB domain of LDLRAP1 by disrupting key hydrogen bonds and hydrophobic interactions, thereby likely impairing LDLR internalisation. According to ACMG guidelines, this variant is classified as likely pathogenic. Clinically, ARH patients exhibited early-onset xanthomas and an unusual quadricuspid aortic valve (QAV). Targeted analysis of valvulogenesis genes (NOTCH1, GATA4, NKX2-5, TBX5, AGTR1, BMP2) revealed no co-segregating pathogenic variants, suggesting that QAV may result from embryonic LDL accumulation disrupting Notch1 signalling rather than a monogenic defect. Comparison with other ADH Tunisian families carrying the same LDLR mutation showed phenotypic variability, likely influenced by genetic modifiers, treatment response and environmental factors. These findings provide the first evidence of LDLRAP1-associated ARH in Tunisia and highlight the genetic heterogeneity of FH, emphasising the importance of integrating molecular, structural and functional analyses for accurate diagnosis, personalised management and early prevention. Show less

17β-hydroxysteroid dehydrogenase type 3 (17β-HSD3) converts Δ4-androstene-3,17-dione (androstenedione) to testosterone. It is expressed almost exclusively in the testes and is essential for appropriate male sexual development. More than 70 mutations in the HSD17B3 gene that cause 17β-HSD3 deficiency and result in 46,XY Disorders of Sex Development (46,XY DSD) have been reported. This study describes three novel Tunisian cases with mutations in HSD17B3. The first patient is homozygous for the previously reported mutation p.C206X. The inheritance of this mutation seemed to be independent of consanguineous marriage, which can be explained by its high frequency in the Tunisian population. The second patient has a novel splice site mutation in intron 6 at position c.490 -6 T > C. A splicing assay revealed a complete omission of exon 7 in the resulting HSD17B3 mRNA transcript. Skipping of exon 7 in HSD17B3 is predicted to cause a frame shift in exon 8 that affects the catalytic site and results in a truncation in exon 9, leading to an inactive enzyme. The third patient is homozygous for the novel missense mutation p.K202M, representing the first mutation identified in the catalytic tetrad of 17β-HSD3. Site-directed mutagenesis and enzyme activity measurements revealed a completely abolished 17β-HSD3 activity of the p.K202M mutant, despite unaffected protein expression, compared to the wild-type enzyme. Furthermore, the present study emphasizes the importance of genetic counselling, detabooization of 46,XY DSD, and a sensitization of the Tunisian population for the risks of consanguineous marriage. Show less