A number sign (#) is used with this entry because of evidence that vertebral, cardiac, renal, and limb defects syndrome-2 (VCRL2) is caused by homozygous or compound heterozygous mutation in the KYNU gene (605197) on chromosome 2q22.

Vertebral, cardiac, renal, and limb defects syndrome-2 (VCRL2) is an autosomal recessive congenital malformation syndrome characterized by vertebral segmentation abnormalities, congenital cardiac defects, renal defects, and mild distal limb defects. Additional features are variable (summary by Shi et al., 2017).

For a discussion of genetic heterogeneity of VCRL, see VCRL1 (617660).

Shi et al. (2017) reported 2 unrelated female patients, one born of consanguineous parents of Lebanese descent (family C) and the other born of unrelated parents from the United States (family D), with VCRL2. Both patients had segmentation defects predominantly affecting the thoracolumbar spine. Both had cardiac defects: patent ductus arteriosus in one patient and hypoplastic left heart in the other. Patient C had microcephaly, low-set ears, hypoplastic kidneys, rhizomelia, talipes, syndactyly, and anterior anus. She died at age 4 months of restrictive respiratory disease due to spondylocostal defects. Patient D had a narrow chest, solitary left kidney with chronic renal disease, short statue, and speech delay. Family history revealed that patient C was 1 of 13 pregnancies, 8 of which were lost in the first trimester. The birth of patient D was preceded by 2 first-trimester losses.

Schule et al. (2021) reported a patient with VCRL2 who was clinically diagnosed with Catel-Manzke syndrome (see 616145). She was prenatally diagnosed with shortened long bones, hypoplastic left heart syndrome, and rocker bottom feet. Postnatal ultrasound showed bilateral hypoplasia of the kidneys. Skeletal abnormalities included bilateral 4th and 5th finger clinodactyly, radial deviation, and polydactyly of the right hand. She also had hallux valgus and lumbrosacral wedge vertebrae. Psychomotor development was normal at 5 years of age.

Szot et al. (2021) reported 4 unrelated probands, including 1 stillborn infant and 1 terminated pregnancy. All 4 patients had limb defects, including shortening of the upper and lower limbs, absent digits and nails of hands and feet, brachymelia, and syndactyly. Three patients had cardiac defects, including tetralogy of Fallot with absent pulmonary valve, atrial and ventricular septal defects, aortic hypoplasia, and mitral valve stenosis. These 3 patients also had dysmorphisms, including wide-spaced eyes, a short neck, microretrognathia, and a broad nose with anteverted nares. Features that were each observed in 1 patient included unilateral absent kidney, vertebral segmentation abnormalities, rib abnormalities, lung hypoplasia, tracheoesophageal fistula, microcephaly, and ventriculomegaly. The 2 patients who were liveborn had developmental delay, and 1 was diagnosed with autism at 6 years of age.

The transmission pattern of VCRL2 in the families reported by Shi et al. (2017) was consistent with autosomal recessive inheritance.

In 2 unrelated patients with VCRL2, Shi et al. (2017) identified homozygous truncating mutations in the KYNU gene (605197.0003-605197.0005). The mutations, which were found by whole-exome sequencing and confirmed by Sanger sequencing, segregated with the disorder in the families. In vitro functional expression studies showed that the mutations essentially abolished KYNU enzymatic activity. Analysis of plasma from patient D showed increased levels of the upstream metabolite 3HK and decreased levels of the downstream metabolites NAD and NAH(H). Studies in mice, which have different niacin levels compared to humans, indicated that the congenital malformations found in humans resulted from deficient NAD levels rather than increased 3HAA. Shi et al. (2017) noted that NAD is a cofactor with broad cellular effects, including ATP production, macromolecular biosynthesis, redox reactions, energy metabolism, DNA repair, and modulation of transcription factors, all of which play an important role in embryogenesis. Shi et al. (2017) theorized that niacin supplementation could be of benefit in such patients.

In a 5-year-old girl with VCRL2 who was clinically diagnosed with Catel-Manzke syndrome, Schule et al. (2021) identified homozygosity for deletion of exon 5 of the KYNU gene. Exon 5 contains 62 bp (c.374_436del, NM_00119924.1) and the deletion is predicted to result in a frameshift and premature termination (p.125_145delLeu146TyrfsTer). The deletion breakpoints (chr2:142,954,376 and chr2:142,955,239, GRCh38), encompassing 835 bp (605197.0006), were confirmed by CGH array and long-range PCR. SNP array showed that the homozygous mutation arose from maternal uniparental isodisomy of chromosome 2.

In 4 unrelated patients with VCRL2, Szot et al. (2021) identified compound heterozygous or homozygous mutations in the KYNU gene (see, e.g., 605197.0007-605197.0010). Yeast with a homozygous knockout for bna5, the yeast ortholog of human KYNU, were transformed with plasmids containing KYNU with each of the mutations or with wildtype KYNU. Transfection with each of 4 mutant KYNU plasmids resulted in smaller yeast mass compared to wildtype when grown in niacin-free culture media. NAD levels were reduced in the yeast transformed with any of the 6 mutant KYNU plasmids compared to wildtype. Szot et al. (2021) concluded that the compromised growth and/or NAD content of the bna5 knockout yeast transfected with each mutant KYNU was a result of impaired KYNU function.

Shi et al. (2017) found that Kynu-null mice were viable and normal. Plasma analysis showed increased 3HK levels, but normal NAD levels. The authors noted that mice have increased niacin levels compared to humans and that mouse embryos may receive niacin from their mothers, resulting in a buffering effect on genetic-based NAD deficiency. These findings suggested that the congenital malformations found in humans with increased 3HK levels and decreased levels of NAD resulted from the deficient NAD levels. Indeed, further studies in mutant mice born to mothers on a niacin-free diet showed that NAD deficiency due to lack of Kynu resulted in multiple defects, including defects in vertebral segmentation, heart defects, small kidney, cleft palate, talipes, syndactyly, and caudal agenesis. Supplementation of Kynu-null mouse embryos with niacin during gestation restored NAD levels and prevented the disruption of embryogenesis.