Irregularities of X-linked Inheritance - PDF

Irregularities of X-linked inheritance Pseudoautosomal inheritance Male lethality in X-linked dominant conditions X chromosome inactivation in manifesting female carriers Lecture content Irregularities of sex-linked inheritance patterns Pseudoautosomal inheritance Male lethali...

Irregularities of X-linked inheritance Pseudoautosomal inheritance Male lethality in X-linked dominant conditions X chromosome inactivation in manifesting female carriers Lecture content Irregularities of sex-linked inheritance patterns Pseudoautosomal inheritance Male lethality in X-linked dominant conditions X chromosome inactivation in manifesting female carriers Pseudoautosomal inheritance Pseudoautosomal regions (PAR) of X and Y chromosomes Homologous regions of X- and Y-chromosomes, located on subtelomeric ends PAR1, 2.7 Mb on tips of short arms PAR2, 330 kb on tips of long arms PAR1 contains 24 genes (including SHOX, implicated in disease) PAR2 contains 5 genes During male meiosis, cross-over is restricted to PAR one obligatory cross-over/meiosis in PAR1 (note: males with deletion of PAR1 are sterile) less frequent in PAR2 (1% of meioses) PAR genes exhibit pseudoautosomal inheritance pattern Pseudoautosomal dominant inheritance Disease-causing allele is on Y chromosome in affected males marked Y, but on X chromosome in all other affected individuals. Individuals with red asterisk (*) result from X-Y crossover in their father. Leri-Weill dyschondrosteosis Skeletal dysplasia: short stature, abnormal shortening of forearms and lower legs, misalignment of wrist OMIM number starts with 1 indicating autosomal dominant inheritance SHOX = short stature PD = pseudoautosomal homeobox: dominant transcription factor expressed by chondrocytes Gene location on X or Y https://www.omim.org/entry/127300 Male lethality in X-linked pedigrees For some X-linked dominant conditions, absence of normal allele is lethal before birth affected male embryos abort spontaneously affected newborns are all female female-to-female transmission (50% probability) Rett Syndrome rare neurodevelopmental disorder affecting females progressive loss of motor skills and speech gene MECP2; expressed in all cells, especially CNS neurons; regulator of transcription; essential for embryonic development, especially maturation of CNS mostly de novo, rare cases inherited Female carriers of X-linked disorders Female carriers of X-linked disorders may be symptomatic or asymtomatic depending on: the inheritance pattern (dominant or recessive) escape from X chromosome inactivation skewed X chromosome inactivation X chromosome inactivation (XCI) Lyonization (Mary Lyon, 1950’s) Dosage compensation mechanism to ensure that X- linked genes are expressed at same level in females and males. Random silencing of one of two X chromosomes in female somatic cells during early embryogenesis; choice is maintained throughout mitotic cell divisisons Inactivated X condenses to become Barr body within nucleus of female somatic cells X inactivation is an epigenetic mechanism Controlled by X inactivation center (Xic) containing lncRNA genes Initiated by transcription of Xist from X chromosome to be inactivated A. Xist is transcribed from the X inactivation centre (Xic); Xist RNA coats entire X chromosome in cis and silences gene expression through epigenetic modification of histones and DNA. B. Xist is controlled by two other lncRNAs transcribed from opposite DNA strand, one acting negatively (Tsix), the other positively (Jpx). Xa = active X chromosome Xi = inactive X chromosome Lee et al 2012 Science 338, 1435-1439 Chromosomes from a mouse cell. Confocal images from a combined RNA-DNA FISH Red indicates the visible Xist RNA on an experiment for Xist in female mouse fibroblast cells. inactivated X chromosome (Xi). Yellow= probe for Xist gene (positive on both Xa and Xi) Image: Ng K et al. EMBO Reports 2007, 8: 34 Red= probe for Xist transcript (positive only on Xi) Image: Reinius et al 2010, BMC Genomics Escape from XCI 15-25% of X chromosome genes escape inactivation and are expressed from both active and inactive X chromosomes. Some genes escape consistently (including all PAR1 genes, some PAR2 genes). Other genes escape variably; the degree varies between genes, tissues, developmental stages, and individuals → likely contributes to phenotypic variability Before differentiation, the paternal X (Xp) and the maternal X (Xm) chromosomes are active. As cells differentiate, random X inactivation is initiated by coating one X chromosome with Xist RNA (pink cloud). This will become the inactive X, while the other X remains active (green chromosome). Since the process is random, either the Xp or the Xm is inactivated in a given cell, resulting in mosaicism in females. Some genes escape X inactivation, i.e., are expressed from both the Xi and the Xa (yellow bars) in all cells. Other genes escape from X inactivation in a subset of cells in a given tissue resulting in mosaicism of escape patterns. Berletch et al. Hum Genet. 2011;130(2):237-45 An additional layer of variability in escape patterns results from differences between individuals. Escape from XCI Figure 11.20 Genes that escape X-inactivation. Columns in the rectangle show results of systematic RT- PCR tests for expression of X-linked genes in nine independent somatic cell hybrids. Each hybrid contained a single inactive human X chromosome. Blue bars identify genes that were expressed from the inactive X chromosome in a particular hybrid, and yellow bars mark genes that were not expressed. Many genes, scattered all along the X chromosome, escape inactivation in one or more of the hybrids. Some genes, such as XIST, escape inactivation in all nine hybrids, whereas others show a more patchy pattern of inactivation. Cen, centromere. [From Carrel L & Willard HF (2005) Nature 434, 400– 404.]  Skewed X chromosome inactivation Generally, XCI is random, producing a 50:50 ratio of maternal and paternal X alleles inactivated Skewed XCI occurs when inactivation of one X chromosome is favored over the other leads to an uneven proportion of cells with each chromosome inactivated (deviation from 50:50 ratio) ratio of 65:35 (or 35:65) is considered skewed due to negative selection (e.g., preferential inactivation of chromosome bearing lethal allele) or a stochastic event? Fioretto et al. Ther Adv Musculoskelet Dis. 2020;12:1759720X20918456 Implications for X-linked disorders For genes subject to XCI, inactivation may be skewed with preferential silencing of normal allele or pathogenic variant → variable symptoms in female carriers Example: Duchenne muscular dystrophy (DMD) Degenerative myopathy primarily affecting boys progressive muscle degeneration and weakness elevated serum creatine kinase levels onset in early childhood (2-3 years); loss of ambulation by age 13y, cardiomyopathy by age 18y; death by early 30’s due to cardiac and respiratory failure Inheritance XR, 100% penetrance Duchenne muscular dystrophy Gene: DMD Protein: dystrophin cytoplasmic protein of myofibers located beneath sarcolemma, part of a complex (DGC, dystrophin glycoprotein complex) linking the cytoskeleton to the extracellular matrix, essential for integrity and stability of sarcolemma during contraction-induced stress Pathogenic variants: null variants → absent dystrophin 80% deletion or duplication of one or more exons → premature Figure 1. Min et al 2019 Ann Rev Med 70:239-255 stop codon 20% SNVs or small indels Figure 1 legend Min et al 2019 Ann Rev Med 70:239-255 a) Skeletal muscle is composed of thousands of multinucleated myofibers. Myofibers are held together in groups called fascicles. (b) The dystrophin-glycoprotein complex (DGC) resides on the sarcolemma of the myofiber and acts as an anchor. The N terminus of dystrophin connects with actin filaments and the C terminus interacts with the DGC, providing stability and integrity to the muscle cell. (c) Dystrophin protein structure. The N terminus of dystrophin contains the primary actin-binding domain, whereas the C terminus contains the dystroglycan-, dystrobrevin-, and syntrophin-binding sites. The N and C termini are essential for dystrophin function. The central rod domain acts like a spring between the two ends. The 24 spectrin-like repeats in the rod domain can be shortened to create a functional but less flexible dystrophin. (d ) The exon structure of the dystrophin gene, showing the 79 exons. The open reading frame (ORF) compatibility is shown by the shape of the adjacent exons. The exons are color coded to match the major functional dystrophin protein domains in panel c. The exons within the mutational hotspot regions are indicated in red. Min et al 2019 Ann Rev Med 70:239-255 Manifesting female carriers Skeletal muscle biopsy dystrophin immunofluorescence Most DMD female carriers are asymptomatic Normal 8% display symptoms of muscle disorder with variable severity from individual to individuals → reduced or absent dystrophin expression Affected male Female carrier  Manifesting female carriers Normal X inactivated → no product Balanced (50:50) inactivation of X bearing normal or mutant DMD Normal X active Skewed X-inactivation with → product preferental silencing of X bearing normal DMD X inactivation in X-linked disorders Gene X-linked Inheritance Male phenotype Female carrier Gene subject to XCI disorder phenotype KDM6A Kabuki Dominant ID, congenital Affected No syndrome 2 malformations FMR1 Fragile X Dominant Developmental Affected; milder Yes syndrome delay, ID, behavioural phenotype than in Skewed XCI with preferential problems males inactivation of pathogenic allele OFD1 Joubert Recessive ID, congenital Unaffected No syndrome-10 malformations DMD Duchenne Recessive Progressive muscle Mostly unaffected; Yes muscular degeneration and mild symtoms in 8% of Skewed XCI with preferential dystrophy weakness carriers inactivation of normal allele ID = intellectual disability Data from Brandy et al. Brain Sci. 2021 Jul 9;11(7):904. Note: X inactivation blurs the distinction between X-linked dominant and X-linked recessive inheritance Reading Strachan and Read. Human Molecular Genetics Chapter 5 Patterns of inheritance Chapter 10 Gene regulation and the epigenome (section 10.4 for X inactivation) Thompson and Thompson. Genetics in Medicine 7 Chapter 3 The Human Genome: Gene Structure and Function (page 103 for X inactivation) Chapter 7 Patterns of single-gene inheritance