A 24-generation-old founder mutation impairs splicing of RBBP8 in Pakistani families affected with Jawad syndrome

Jawad syndrome is a multiple congenital anomaly and intellectual disability syndrome with mutation in RBBP8 reported only in two families. Here, we report on two new families from Pakistan and identified a previously reported variant in RBBP8, NM_002894.3:c.1808-1809delTA. We could show that this mutation impairs splicing resulting in two different abnormal transcripts. Finally, we could verify a shared haplotype among all four families and estimate the founder event to have occurred some 24 generations ago.

Genome-wide linkage and sequence analysis challenge CCDC66 as a human retinal dystrophy candidate gene and support a distinct NMNAT1-related fundus phenotype

To uncover the genotype underlying early-onset cone-rod dystrophy and central nummular macular atrophic lesion in 2 siblings from an endogamous Arab family, we performed targeted next-generation sequencing (NGS) of 44 retinal dystrophy genes, whole-exome sequencing (WES) and genome-wide linkage analysis. Targeted NGS and WES in the index patient highlighted 2 homozygous variants, a CCDC66 frameshift deletion and a novel missense NMNAT1 variant, c.500G>A (p.Asn167Ser). Linkage and segregation analysis excluded the CCDC66 variant and confirmed the NMNAT1 mutation. Biallelic NMNAT1 mutations cause Leber congenital amaurosis with a central nummular macular atrophic lesion (LCA9). The NMNAT1 mutation reported here underlied cone-rod dystrophy rather than LCA but the fundus lesion was compatible with that of LCA9 patients, highlighting that such a fundus appearance should raise suspicion for biallelic mutations in NMNAT1 when in the context of any retinal dystrophy. Although Ccdc66 mutations have been proposed to cause retinal disease in dogs, our results and public databases challenge CCDC66 as a candidate gene for human retinal dystrophy.

A common NYX mutation in Flemish patients with X linked CSNB

AIMS

The Schubert-Bornschein type of complete congenital stationary night blindness (CSNB) is a genetically heterogeneous retinal disorder. It is characterised by a non-progressive disease course, often associated with high myopia and nystagmus. So far, mutations in two genes, NYX (nyctalopin) and GRM6 (metabotropic glutamate receptor 6) have been associated with this form of CSNB. The purpose of this study was to identify the genetic defect in affected male patients from Flemish families with complete CSNB.

METHODS

Probands with CSNB from three large Flemish families underwent ophthalmological examination. DNA was extracted from peripheral blood, and the coding region of NYX along with parts of the 5'UTR and 3'UTR and intronic regions covering the splice sites were PCR amplified and sequenced.

RESULTS

In the affected individuals of three Flemish families with the complete form of CSNB a novel NYX mutation, c.855delG was identified. This deletion is predicted to lead to a frameshift mutation, p.Asp286ThrfsX62 causing a premature stop codon.

CONCLUSION

Previously, both single families with different mutations in NYX as well as different families with an identical mutation, suggestive of a founder mutation, have been described. The c.855delG deletion in NYX seems to be a common mutation associated with CSNB in the Flemish population from Belgium. Thus, we suggest performing diagnostic testing for CSNB in the Flemish population initially directed towards the identification of this mutation. Subsequent screening for other mutations in NYX or GRM6 could be performed as a second step.

Geroderma osteodysplasticum hereditaria and wrinkly skin syndrome in 22 patients from Oman

Excessive skin wrinkling and cutis laxa are seen in many genetic conditions and overlapping features can make a clinical diagnosis difficult. Here we report on 22 Omani patients from 11 consanguineous families with the diagnosis of wrinkly skin syndrome (WSS, OMIM 278250) or geroderma osteodysplasticum hereditaria (GO, OMIM 231070). The WSS phenotype evolves during early childhood and includes a generalized and excessive skin wrinkling, dental problems, herniae, foot deformities, hip dislocations, growth retardation, and a large anterior fontanelle. The facial gestalt is characterized by a broad nasal bridge, hypertelorism, and downslanting palpebral fissures. We were unable to differentiate between WSS and cutis laxa with growth and developmental delay (CLGDD, OMIM 219200) suggesting that both can be considered as one entity. Distinct hallmarks of GO were skin wrinkling limited to the dorsum of hands and feet and to the abdomen, normal fontanelles, maxillary hypoplasia, bowed long bones, and osteopenia with frequent fractures. In contrast to the attenuation of the skin phenotype with age in WSS, adult patients with GO appeared prematurely aged. A serum sialotransferrin type 2 pattern was found in all four WSS patients tested. Apolipoprotein CIII (a marker for O-glycosylation) was normal suggesting that WSS is frequently associated with a N-protein glycosylation defect, probably at the level of processing (CDG-II). All four investigated GO patients showed normal sialotransferrin patterns. The known loci for cutis laxa and WSS on 2q31, 5q23-q31, 7q11, 11q13, and 14q32 were excluded. We suggest that WSS and GO are distinct entities with overlapping features.