Genetic testing for autosomal dominant hypercholesterolemia in Norwegian patients and relatives

Introduction and methods

The prevalence of heterozygous familial hypercholesterolemia (FH) is 1/200-1/500 in most Western countries [1,2]. In Norway, a prevalence of 1/313 has been estimated [3].

Autosomal dominant hypercholesterolemia (ADH) is caused by mutations in the LDLR, APOB, and PCSK9 genes. More than 2300 different loss-of-function mutations in the LDLR gene have been reported that result in FH [4-6]. A few missense mutations have been reported in the APOB gene and cause familial defective apolipoprotein B-100 (FDB) [3,7]. And a few gain-of-function mutations have been found in the PCSK9 gene that are causal of FH3 [8].

Patients with ADH have a high risk of coronary heart disease (CHD) [9,10]. To reduce the risk of CHD, identification and treatment at an early age of these patients is therefore necessary [11]. This study presents the data of diagnostic screening for mutations in LDLR, APOB, and PCSK9 genes in unrelated hypercholesterolemia patients and their family members from Norway.

Genetic testing for ADH was performed in 29,449 unrelated adult index patients with a total serum cholesterol of at least 6 mmol/L from 1993 to 2020. In addition, 14,230 relatives were screened. Molecular testing was done at Unit for Cardiac and Cardiovascular Genetics, Oslo University Hospital, Norway. Sanger sequencing of the LDLR gene (NM_000527.4) with at least 20 bp flanking intronic region was done. For the APOB gene (NM_000384.3), a 101 bp fragment spanning nucleotides c.10537-c.10637 in exon 26 was sequenced. This exon contains codon 3527, which is the only codon where mutations are definitely causal of FDB [12,13]. PCSK9 gene (NM_174936.4) with flanking introns was sequenced and in addition multiplex ligation-dependent probe amplification (MLPA) analysis of the LDLR gene was performed in those with total serum cholesterol of ≥8 mmol/L without an underlying mutation in the LDLR gene or APOB exon 26. The mean age at the time of genetic testing of index patients was 47.4 ± 18.2 (SD) years and 36.3 ± 18.3 (SD) years of relatives.

Main results

• A pathogenic mutation was found in 2829 (9.6%) index patients. 2818 Patients were heterozygotes and 11 were compound heterozygotes or homozygotes.

• 5993 (42.1%) Relatives were positive for a pathogenic mutation; identifying on average 2.1 affected relatives of index patients.

• In 8811 patients and relatives with a pathogenic heterozygous mutation for ADH, 8281 (94%) had a mutation in the LDLR gene, 478 (5.4%) in the APOB gene, and 52 (0.6%) in the PCSK9 gene.

• 259 Different mutations were identified in the LDLR gene, including 29 novel ones. There were 123 missense mutations, 30 large and 38 small deletions or duplications, 28 nonsense mutations, 31 mutations in the flanking intron sequences, 7 mutations in the promoter region, and two silent mutations that affect RNA splicing. In addition, 2 missense mutations were found in the APOB gene and 3 missense mutations in the PCSK9 gene.

• Of all heterozygous mutations, the c.[313+1G>A] mutation in intron 3 of the LDLR gene was identified in 18.8% of ADH patients, the p.(C231G) mutation in the LDLR gene in 10.9%, p.(P685L) in the LDLR gene in 7.4%, and the LDLR p.(D221N) mutation in 6.5% of patients.

• The prevalence estimate of affected patients with a heterozygous ADH mutation was 1/602.

Conclusion

References

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