syndrome, with a recurrence rate of 50 percent when inherited from an unaffected mother. Fortunately, only about 1 percent of our genes find expression from one or other parent.812
Multilocus imprinting disorders with maternal effect genes (including NLRP2, NLRP7, and PADI6) can affect oocytes and resulting offspring, who may manifest with atypical imprinting disorders.813, 814 Multilocus imprinting disturbance in methylation may affect growth and development. Epigenetic effects are evident in sperm, oocyte, and zygote genomes.815, 816 It is no surprise then, that mutations in NLRP genes may result in early miscarriages, hydatidiform moles, and apparent infertility.813 Most imprinted genes express in the placenta, and loss of imprinting can affect placental weight, fetal growth, and development,817–821 and the regulation of placental hormones.821
Potential imprinting disturbances at the sperm, oocyte, or zygote stages are associated with ART and preimplantation procedures. Cogent evidence exists of an increased incidence of imprinting disorders following ART.822–828 In the most extensive report to date, Hattori et al.822 in a nationwide study in Japan, reported on 931 patients with imprinting disorders. These included 117 cases of Beckwith–Weidemann syndrome, 67 with Silver–Russel syndrome, 520 with Prader–Willi syndrome, and 227 with Angelman syndrome. Most were conceived through ART including intracytoplasmic sperm injection. They noted a 4.46‐ and 8.91‐fold increased frequency of Beckwith–Weidemann syndrome and Silver–Russel syndrome respectively. Cortessis et al.,828 in a meta‐analysis of 23 studies on ART and the occurrence of imprinting disorders, reported significant odds ratios of 4.7 for Angelman syndrome, 5.8 for Beckwith–Weidemann syndrome, 2.2 for Prader–Willi syndrome, and 11.3 for Silver–Russel syndrome.
Mutations in imprinted genes that occur after fertilization can result in somatic mosaicism.829 An interesting example is represented by discordant monozygotic twins in which only one has the disorder (Beckwith–Weidemann syndrome)830–832 or Silver–Russel syndrome,829, 833 pointing to epigenetic disturbances in early development.829
About 1.7 percent of births in the United States result from ART.834 Although the frequency of imprinting disorders is increased, the actual risks are very low, but should be discussed.
Genotype–phenotype associations
DNA mutation analysis has slowly clarified genotype–phenotype associations requiring extensive databases and definitive phenotyping835, 836 (see Chapter 14). Notwithstanding this limitation, mutation analysis does provide precise prenatal diagnosis opportunities and detection of affected fetuses even with compound heterozygosity. Simple logic might have concluded that genotype at a single locus might predict phenotype. For monogenic disorders this is frequently not the case. Allelic combinations of missense, nonsense, and compound heterozygous mutations within different genes could result in overlapping clinical phenotypes as exemplified for the Kabuki syndrome and Schinzel–Giedion syndrome.837 Now that clinical diagnostic criteria have been established838 and two genes (KMT2D and KDM6A) recognized, syndrome identification has been facilitated.839 Additional novel pathogenic variants continue to be discovered.840 It appears that hyperinsulinism, long halluces, large central incisors, and hypertrichosis are more common in Kabuki syndrome due to KDM6A mutations,841, 842 while the classic Kabuki facial features and renal/palatal anomalies are more commonly found with KMT2D mutations.839, 843 In the autosomal dominant Marfan syndrome (due to mutations in FBN1), family members with the same mutation may have severe ocular, cardiovascular, and skeletal abnormalities, while siblings or other close affected relatives with the same mutation may have mild effects in only one of these systems.844 In Gaucher disease with one of the common Ashkenazi Jewish mutations, only about one‐third of homozygotes have significant clinical disease.845 At least two‐thirds have mild or late‐onset disease or remain asymptomatic (see Chapter 21). Compound heterozygotes for this disorder involving mutations p.L444P and p.N370S have included a patient with mild disease first diagnosed at 73 years of age, while another requiring enzyme replacement therapy was diagnosed at the age of 4 years.846
In cystic fibrosis, a strong correlation exists between genotype and pancreatic function but only a weak association has been noted with the respiratory phenotype847 (see Chapter 15). Although individuals who are homozygous for the common cystic fibrosis mutation (ΔF508) can be anticipated to have classic cystic fibrosis, those with the less common mutation (p.R117H) are likely to have a milder disease.848 On occasion, an individual who is homozygous for the “severe” ΔF508 mutation might unexpectedly exhibit a mild pancreatic‐sufficient phenotype. Illustrating the complexity of genotype–phenotype associations is the instance noted by Dork et al.849 of a mildly affected ΔF508 homozygote whose one chromosome 7 carried both the common ΔF508 mutations and a cryptic p.R553Q mutation. Apparently, a second mutation in the same region may modify the effect of the common mutation, permitting some function of the chloride channel850 and thereby ameliorating the severity of the disease. Modifying genes in cystic fibrosis are being increasingly recognized851–853 (see Chapter 15).
The extensive mutational heterogeneity in hemophilia A854–856 is related not only to variable clinical severity but also to the increased likelihood of antifactor VIII antibodies (inhibitors) developing. Miller et al.857 found about a fivefold higher risk of inhibitors developing in hemophiliac males with gene deletions compared with those without deletions. In Netherton syndrome, a severe autosomal recessive ichthyosis that affects skin, hair, and immune system, upstream mutations in the SPINK5 gene correlate with more severe phenotypes.858
The many mutations and wide phenotypic range seen in neurofibromatosis type 1 is well known, and characterized by variable expressivity and age‐dependent clinical features. This variability makes phenotype prediction difficult. Among the few thousand constitutional variants in the NF1 gene, recurrent pathogenic missense variants at p.Met1149, p.Arg1276 or p.Lys1423 have been associated with a Noonan‐like phenotype.859 Moreover, these authors also found that mutations at p.Arg1276 was associated with spinal neurofibromas, and that mutations at both p.Lys1423 and p.Arg1276 were associated with a high prevalence of cardiovascular abnormalities, including pulmonic stenosis.
Chronic intestinal pseudo‐obstruction (CIPO), also known as megacystis–microcolon–intestinal–hypoperistalsis syndrome, is a severe debilitating visceral myopathy involving enteric smooth muscle.860–863 Mutations in the ACTG2 gene account for about 44–50 percent of cases. We noted in a whole‐exome sequencing study a mutational hotspot in the ACTG2 gene (Figure 1.3).863 We subsequently determined somatic ACTG2 mosaicism,864 further complicating genotype–phenotype determination.
Figure 1.3 Shown are the 16 pathogenic variants reported in the ACTG2 gene and the number of times each mutation was observed in 45 probands.863
Japanese authors have assembled mutation data for the NOTCH3 gene and recognized three mutations as major contributors to the phenotype of CADASIL.865 They also recognized gender differences in symptomatology (worse in