Identification of disease causing genes in families with skeletal and ophthalmological disorders

Abstract

Hereditary and congenital anomalies are common in Pakistan and are a significant cause of morbidity and mortality. Generally, the monogenic disorders occur due to heterozygous dominant or homozygous recessive mutations. Here, the recessive mutations causing rare diseases usually appear due to increased inbreeding. The current study was aimed to identify the causative variants segregating with autosomal recessive malformation in Pakistani families. Five families with rare hereditary skeletal and ophthalmological disorders were recruited from Southern Punjab, Pakistan. DNA of the available affected and unaffected subjects was extracted from peripheral blood for molecular study. SNP genotyping carried to find the candidate chromosomal region and exome sequence was done to find candidate variants causing diseases in the recruited families and identified mutations validated by Sanger sequencing. These families were resolved by identifying five different mutations: (i) In family 1, a novel homozygous deletion c.463-477del p.(155-159del) in CHST11 was observed to cause osteo chondrodysplasia with malformed digits and skeletal defects. This finding confirms the role of CHST11 in skeletal morphogenesis but also indicate that CHST11 mutation has variable manifestations and includes a range of digit malformations and skeletal defects. (ii) In family 2, a homozygous deletion encompassing MYADML2 and PYCR1 genes was observed to causes new syndromic phenotype with unusual skeletal malformation with features of cutis laxa. The unusual clinical features included lack of cranial symmetry, displaced fused sagittal sutures, maxillary hypoplasia, mandibular prognathia, teeth abnormalities, delayed bone maturity, multiple joints dislocation, malformed patellae, intracondylar fissures in hummers, scapular dyskinesis, lumbar lordosis, protruding chest, clavicles were prominent clevicles, long, limbs, reduced 5th digital rays, and transvers creases in digits. Additionally, features of cutis laxa were present in all affected sibs. Mutations in PYCR1 caused autosomal recessive cutis laxa (ARCL) and all patients had common features of ARCL whereas MYADML2 is a gene of unknown function and has not been associated with disease. This study reported a possible phenotype related to mutation in MYADML2 gene that causes impaired bone patterning and maturation. The study findings provide a start point for future studies on the function of MYADML2 protein. Discovery of new patients requires confirming the MYADML2 mutation related Abstract xv phenotype. (iii) In family 3, a homozygous variant in GHR gene c.442G>C was implicated in proportionate dwarfism Laron type. This molecular report confirmed the diagnosis of Laron syndrome and suggestive for to develop the diagnostic tests for carrier testing, prenatal and premarital screening for genetic counselling of the families. (iv) In family 4, a homozygous two-base pair deletion c.4054_4055delCT in COL18A1 was implicated in Knobloch syndrome with unusual features like chest abnormality, low sweating, slow bleeding in wounds, low sensitivity to pain and abnormal skin pigmentation. However, the range of clinical symptoms evident in the present family is surprising and taken together with other studies; this study further expands the phenotypic manifestation of KNO1. (v) In family 5, a homozygous deletion C.2861_2862delAA in coding region of PRG4 was detected in patients that had camptodactyly-arthropathy-coxa vara-pericarditis syndrome with new clinical features like brachydactyly, small nails, scoliosis, and barrel shaped chest. Conclusively, the study results might be helpful to establish genotype-phenotype correlation of the CACP. In conclusion, the study results may prove helpful for understanding the puzzling nature of the inherited malformation. This understanding may lead to novel strategies by which the rare disorders can be promptly diagnosed, reduced or prevented. Study could be beneficial for clinicians and it would be highly informative to setup the centers for genetic counseling as well as equally useful for various medical centers and institutions.