Osteogenesis imperfecta (OI) comprises a group of inherited disorders characterized by bone fragility and increased susceptibility to fractures. Historically, the laboratory confirmation of the diagnosis OI rested on cultured dermal fibroblasts to identify decreased or abnormal production of abnormal type I (pro)collagen molecules, measured by gel electrophoresis. With the discovery of COL1A1 and COL1A2 gene variants as a cause of OI, sequence analysis of these genes was added to the diagnostic process. Nowadays, OI is known to be genetically heterogeneous. About 90% of individuals with OI are heterozygous for causative variants in the COL1A1 and COL1A2 genes. The majority of remaining affected individuals have recessively inherited forms of OI with the causative variants in the more recently discovered genes CRTAP, FKBP10, LEPRE1,PLOD2, PPIB, SERPINF1, SERPINH1 and SP7, or in other yet undiscovered genes. These advances in the molecular genetic diagnosis of OI prompted us to develop new guidelines for molecular testing and reporting of results in which we take into account that testing is also used to 'exclude' OI when there is suspicion of non-accidental injury. Diagnostic flow, methods and reporting scenarios were discussed during an international workshop with 17 clinicians and scientists from 11 countries and converged in these best practice guidelines for the laboratory diagnosis of OI.

Vascular Ehlers-Danlos Syndrome (vEDS), also known as EDS type IV, is considered to be an autosomal dominant disorder caused by sequence variants in COL3A1, which encodes the chains of type III procollagen. We identified a family in which there was marked clinical variation with the earliest death due to extensive aortic dissection at age 15 years and other family members in their eighties with no complications. The proband was born with right-sided clubfoot but was otherwise healthy until he died unexpectedly at 15 years. His sister, in addition to signs consistent with vascular EDS, had bilateral frontal and parietal polymicrogyria. The proband and his sister each had two COL3A1 sequence variants, c.1786C>T, p.(Arg596*) in exon 26 and c.3851G>A, p.(Gly1284Glu) in exon 50 on different alleles. Cells from the compound heterozygote produced a reduced amount of type III procollagen, all the chains of which had abnormal electrophoretic mobility. Biallelic sequence variants have a significantly worse outcome than heterozygous variants for either null mutations or missense mutations, and frontoparietal polymicrogyria may be an added phenotype feature. This genetic constellation provides a very rare explanation for marked intrafamilial clinical variation due to sequence variants in COL3A1.