Demonstration of two novel LAMB2 mutations in the original Pierson syndrome family reported 42 years ago

In 1963, Pierson et al. [1963] reported on two siblings with congenital nephrotic syndrome and peculiar eye abnormalities. Despite the existence of a few subsequent case reports probably describing the same condition, it took more than 40 years until its recognition as a separate autosomalrecessively inherited entity, as recently published in this Journal [Zenker et al., 2004b]. The disorder is now known as microcoria-congenital nephrosis syndrome or Pierson syndrome (OMIM 609049). In their original report, Pierson et al. accurately and comprehensively recorded all the characteristic clinical and histopathological features of that condition: the renal disorder which manifests in the newborn period with severe proteinuria, hypoproteinemia, and edema (nephrotic syndrome), followed by early-onset end stage renal failure which—without dialysis—leads to death in the first weeks of life; the histopathological presentation of diffuse mesangial sclerosis on kidney biopsy or autopsy; the ocular changes which are the most discriminative feature, particularly the extreme, non-reactive narrowing of the pupils which is due to aplasia or atrophy of the M. dilatator pupillae. At post-mortem examination, however, the ocular phenotype was already recognized as a far more complex maldevelopment including lens abnormalities, atrophy of the ciliary muscle, corneal, and retinal changes [Pierson et al., 1963]. Homozygous or compound heterozygous mutations of the LAMB2 gene predicting a loss of function have been recently identified as the genetic basis of Pierson syndrome [Zenker et al., 2004a]. The gene encodes the laminin b2 chain, which is specifically expressed at the glomerular and arterial basement membranes, lens capsule, retina, basal laminae of intraocular smooth muscles as well as at the basal lamina of the neuromuscular synapse. Laminins are heterotrimeric basement membrane proteins consisting each of one a-, b-, and g-chain, for example, laminin-11, the a5-b2-g1-trimer, which is the most abundant laminin isoform of the glomerular basement membrane [Miner and Patton, 1999; Tunggal et al., 2000]. In laminin b2-deficient mice, a lethal phenotype of massive earlyonset proteinuria was observed [Noakes et al., 1995b]. It was associated with abnormalities of the retina and the neuromuscular junctions [Noakes et al., 1995a; Libby et al., 1999]. Considering these findings, a primary neurological phenotype of laminin b2-deficiency in mouse and man has been anticipated, but this has not yet been clearly documented, mostly because of the early lethality of the condition in both species [Zenker et al., 2004a,b]. Notably however, there is already a hint at the presence of neurological problems in the original report, where one affected child was described as severely hypotonic with absent reflexes [Pierson et al., 1963]. In order to find out whether the original patients reported by Pierson et al. really had a laminin b2 defect, we intended to study living family members, as no material of the index patients themselves was available any more. A partial pedigree of the family is presented in Figure 1. There was no parental consanguinity. The mother of the index patients is a healthy 79-year-old woman. In early childhood, she had experienced an episode of acute glomerulonephritis probably related to streptococcal infection. She recovered completely and has had no renal problems thereafter. The patients’ father died at age 60 from gastric cancer. He had been otherwise healthy and had no history of a renal or ocular disorder throughout his life. His 77-year-old sister, however, who is still alive and healthy agreed to participate in the study as well as the healthy sister of the index patients, giving a 75% probability to find the presumed paternal mutation. Informed consent was obtained from all participating family members, and an EDTA blood sample was drawn for DNA extraction. The entire coding sequence including exon-intron boundaries of theLAMB2 gene was analyzed by bi-directional direct sequencing as described previously [Zenker et al., 2004a]. We were in fact able to identify two LAMB2 mutations segregating in the paternal and maternal families, respectively. In the patients’ mother who is a suspected obligate carrier, we detected a novel heterozygous mutation c.2067C! G in exon 16. The mutation creates a premature stop at codon 689 (Y689X). The healthy sister of the patients did not harbor the maternal mutation, and she did not show any other relevant sequence variation in the LAMB2 gene, either. However, the paternal aunt turned out to carry another novel heterozygous nonsense mutation c.1122T! A in exon 9 (C374X) (Fig. 1). In the maternal uncle who was available for testing, too, the Y689X mutation was not found. These findings provide circumstantial evidence that the siblings originally reported with Pierson syndrome had been compound heterozygous for the two nonsense mutations, although we did not have the opportunity to prove this directly. Both mutations predict a premature termination codon in the first half of the coding sequence. It is likely that the mRNA carrying these mutations will be degraded by the RNA surveillance machinery through nonsense-mediated decay. We have previously demonstrated that homozygous LAMB2 *Correspondence to: Martin Zenker, M.D., Institute of Human Genetics, University of Erlangen-Nuremberg, Schwabachanlage 10, 91054 Erlangen, Germany. E-mail: mzenker@humgenet.uni-erlangen.de