Comparison of the ABC and ACMG systems for variant classification.

The ABC and ACMG variant classification systems were compared by asking mainly European clinical laboratories to classify variants in 10 challenging cases using both systems, and to state if the variant in question would be reported as a relevant result or not as a measure of clinical utility. In contrast to the ABC system, the ACMG system was not made to guide variant reporting but to determine the likelihood of pathogenicity. Nevertheless, this comparison is justified since the ACMG class determines variant reporting in many laboratories. Forty-three laboratories participated in the survey. In seven cases, the classification system used did not influence the reporting likelihood when variants labeled as "maybe report" after ACMG-based classification were included. In three cases of population frequent but disease-associated variants, there was a difference in favor of reporting after ABC classification. A possible reason is that ABC step C (standard variant comments) allows a variant to be reported in one clinical setting but not another, e.g., based on Bayesian-based likelihood calculation of clinical relevance. Finally, the selection of ACMG criteria was compared between 36 laboratories. When excluding criteria used by less than four laboratories (<10%), the average concordance rate was 46%. Taken together, ABC-based classification is more clear-cut than ACMG-based classification since molecular and clinical information is handled separately, and variant reporting can be adapted to the clinical question and phenotype. Furthermore, variants do not get a clinically inappropriate label, like pathogenic when not pathogenic in a clinical context, or variant of unknown significance when the significance is known.

National clinical Genetic Networks - GENets - Establishment of expert collaborations in Denmark.

Genetic conditions are often familial, but not all relatives receive counseling from the same institution. It is therefore necessary to ensure consistency in variant interpretation, counseling practices, and clinical follow up across health care providers. Furthermore, as new possibilities for gene-specific treatments emerge and whole genome sequencing becomes more widely available, efficient data handling and knowledge sharing between clinical laboratory geneticists and medical specialists in clinical genetics are increasingly important. In Denmark, these needs have been addressed through the establishment of collaborative national networks called Genetic Expert Networks or "GENets". These networks have enhanced patient and family care significantly by bringing together groups of experts in national collaborations. This promotes coordinated clinical care, the dissemination of best clinical practices, and facilitates the exchange of new knowledge.

Cell-Based NIPT Detects 47,XXY Genotype in a Twin Pregnancy.

Background: The existing risk of procedure-related miscarriage following invasive sampling for prenatal diagnosis is higher for twin pregnancies and some women are reluctant to test these typically difficultly obtained pregnancies invasively. Therefore, there is a need for noninvasive testing options that can test twin pregnancies at an early gestational age and ideally test the twins individually. Case presentation: A pregnant woman opted for cell-based NIPT at GA 10 + 5. As cell-based NIPT is not established for use in twins, the test was provided in a research setting only, when an ultrasound scan showed that she carried dichorionic twins. Materials and Methods: Fifty mL of peripheral blood was sampled, and circulating fetal cells were enriched and isolated. Individual cells were subject to whole-genome amplification and STR analysis. Three fetal cells were analyzed by chromosomal microarray (aCGH). Results: We identified 20 fetal cells all sharing the same genetic profile, which increased the likelihood of monozygotic twins. aCGH of three fetal cells showed the presence of two X chromosomes and a gain of chromosome Y. CVS from both placentae confirmed the sex chromosomal anomaly, 47,XXY and that both fetuses were affected. Conclusion: NIPT options can provide valuable genetic information to twin pregnancies that help the couples in their decision-making on prenatal testing. Little has been published about the use of cell-based NIPT in twin pregnancies, but the method may offer the possibility to obtain individual cell-based NIPT results in dizygotic twins.

Family History is Important to Identify Patients with Monogenic Causes of Adult-Onset Chronic Kidney Disease.

Monogenic causes of chronic kidney disease (CKD) are more prevalent in adults than previously thought, as causative gene variants are found in almost 10% of unselected patients with CKD. Even so, genetic testing in patients with adult-onset CKD is uncommon in clinical practice and the optimal criteria for patient selection remain unclear. A family history of kidney disease emerges as one marker associated with a high diagnostic yield of genetic testing. We present 3 cases of adult-onset CKD with underlying monogenic causes exemplifying different modes of inheritance. Case 1 is a 60-year-old male with slowly progressive CKD initially ascribed to hypertension and diabetes despite a family history with several affected first-degree relatives. A pathogenic MUC1 variant was found, and thus we identified the first Danish family of MUC1-associated autosomal dominant tubulointerstitial kidney disease. Case 2 is a 40-year-old female with nephrocalcinosis, nephrolithiasis, and unexplainable hypercalcemia consistent with vitamin D intoxication. The family history indicated autosomal recessive inheritance, and genetic testing revealed 2 pathogenic CYP24A1 variants in compound heterozygous form associated with idiopathic infantile hypercalcemia. Case 3 is a 50-year-old male with microscopic hematuria, proteinuria, and hearing loss. Electron microscopy of renal biopsy showed thin basal membrane syndrome, and the family history indicated X-linked inheritance. A novel missense variant in COL4A5 was identified, suggesting an atypical late-onset form of X-linked Alport syndrome. This case series illustrates the heterogeneous presentations of monogenic kidney disease in adults and emphasizes the importance of family history for initiating genetic testing to identify underlying monogenic causation. Moreover, we discuss the potential impact of genetic diagnostics on patient management and genetic family counseling.

Functional megalin is expressed in renal cysts in a mouse model of adult polycystic kidney disease.

BACKGROUND: Autosomal dominant polycystic kidney disease (ADPKD) is characterized by the progressive growth of cysts and a decline of renal function. The clinical feasibility of the number of potential disease-modifying drugs is limited by systemic adverse effects. We hypothesize that megalin, a multiligand endocytic receptor expressed in the proximal tubule, may be used to facilitate drug uptake into cysts, thereby allowing for greater efficacy and fewer side effects. METHODS: The cyst expression of various tubular markers, including megalin and aquaporin 2 (AQP2), was analysed by immunohistochemistry (IHC) of kidney sections from the ADPKD mouse model (PKD1RC/RC ) at different post-natal ages. The endocytic function of megalin in cysts was examined by IHC of kidney tissue from mice injected with the megalin ligand aprotinin. RESULTS: Cyst lining epithelial cells expressing megalin were observed at all ages; however, the proportion decreased with age. Concomitantly, an increasing proportion of cysts revealed expression of AQP2, partial expression of megalin and/or AQP2 or no expression of the examined markers. Endocytic uptake of aprotinin was evident in megalin-positive cysts, but only in those that remained connected to the renal tubular system. CONCLUSIONS: Megalin-expressing cysts were observed at all ages, but the proportion decreased with age, possibly due to a switch in tubular origin, a merging of cysts of different tubular origin and/or a change in the expression pattern of cyst lining cells. Megalin expressed in cysts was functional, suggesting that megalin-mediated endocytosis is a potential mechanism for drug targeting in ADPKD if initiated early in the disease.

Screening for Fetal Aneuploidy and Sex Chromosomal Anomalies in a Pregnant Woman With Mosaicism for Turner Syndrome-Applications and Advantages of Cell-Based NIPT.

Background: Cell-free NIPT and cell-based NIPT are risk-free testing options using maternal blood samples to screen for fetal aneuploidies, but the methods differ. For cell-free NIPT, the fetal fraction of cell-free DNA in plasma is analyzed with a high background of maternal DNA. In contrast, for cell-based NIPT, a limited number of the rare, intact fetal cells are isolated for the genetic analysis. This case demonstrates the differences regarding testing for fetal sex-chromosomes anomalies (SCAs) between these two tests. Materials and Methods: A pregnant woman with mosaicism for Turner syndrome opted for NIPT in first trimester. For the cell-free NIPT analysis, DNA extraction, genome-wide massive parallel sequencing, and data analysis were carried out as described by the kit manufacturer (Illumina©, San Diego, CA, USA). For cell-based NIPT, the first sample gave no result, but the woman consented to repeat cell-based NIPT. After whole genome amplification and STR analysis, fetal DNA from three individual fetal cells was subjected to chromosomal microarray (aCGH, Agilent oligoarray, 180 kb). Results: Fetal fraction was 7%, and cell-free NIPT showed 2 copies of chromosomes 13, 18, and 21 and a decreased proportion of chromosome X, suggestive of fetal Turner syndrome. In contrast, the cell-based NIPT result showed no aneuploidy and two X-chromosomes in the fetus. Conclusion: cell-based NIPT may provide a non-invasive testing option to screen for SCAs in women with mosaicism for monosomy-X in blood, where cell-free NIPT cannot discriminate whether the X-loss is maternal or fetal.

Total Anomalous Pulmonary Venous Connection in Mother and Son with a Central 22q11.2 Microdeletion.

In this clinical report, we describe a male infant and his mother, who had similar congenital heart defects. They were both diagnosed neonatally with total anomalous pulmonary venous connection (TAPVC) in combination with other heart defects. Neither of the two had any other organ malformations or dysmorphic facial features. SNP-array identified a central 22q11.2 microdeletion in the male infant and his mother as well as in the maternal grandmother and maternal aunt. The mother and the maternal aunt additionally harbored a 15q11.2 BP1-BP2 microdeletion. The maternal grandmother was unaffected by heart disease. However, heart computed tomography scan of the maternal aunt revealed a quadricuspid aortic valve. Additionally, the maternal grandmother and the maternal aunt both had significant learning disabilities. Rarely, TAPVC has been described in patients with the common 22q11.2 microdeletions. However, to the best of our knowledge, TAPVC has not previously been reported in patients with this small central 22q11.2 microdeletion. Haploinsufficiency of TBX1 was originally thought to be the main cause of the 22q11.2 microdeletion syndrome phenotype, but TBX1 is not included in the atypical central 22q11.2 microdeletion. Previous reports have suggested an association between TAPVC and the 15q11.2 BP1-BP2 microdeletion. Our report does not support this association as the maternal aunt, who harbors both microdeletions, is unaffected by TAPVC, and the male infant affected by TAPVC does not harbor the 15q11.2 BP1-BP2 microdeletion. Our findings support that genes located in the central 22q11.2 region are important for heart development and that haploinsufficiency of these genes plays a crucial role in the development of the rare heart defect TAPVC.

Phenotypic heterogeneity and mosaicism in Xia-Gibbs syndrome: Five Danish patients with novel variants in AHDC1.

Xia-Gibbs syndrome (XGS) is a neurodevelopmental disorder characterized by intellectual disability, developmental delay, seizures, hypotonia, obstructive sleep apnoea and mild facial dysmorphism. Heterozygosity for loss-of-function variants in AHDC1, encoding the AT-hook DNA binding motif containing protein 1, were discovered in 2014 as the likely genetic cause of Xia-Gibbs syndrome. We present five patients with Xia-Gibbs syndrome caused by previously unreported variants in AHDC1. Two of the patients share a frameshift variant: c.2849del (p.(Pro950Argfs*192)) in AHDC1. Despite sharing this variant, the two patients show remarkable phenotypic differences underscoring the clinical heterogeneity of Xia-Gibbs syndrome. In addition, we present a case of Xia-Gibbs syndrome caused by mosaicism for an AHDC1 variant.

Mosaicism for copy number variations in the placenta is even more difficult to interpret than mosaicism for whole chromosome aneuploidy.

OBJECTIVE: To compare mosaicisms in prenatal chorionic villus samples (CVSs) with corresponding postpartum placental samples. METHOD: We collected placentas from 15 consecutive cases of mosaicism detected in CVSs and obtained five standardized samples on each placenta after delivery. All pre- and postnatal placental samples were uncultured and analyzed by high-resolution chromosomal microarray. RESULTS: Ten cases of mosaicism for whole chromosome aneuploidy (mWC) and five cases with mosaicism for (sub)chromosomal copy number variations (mCNVs) were included. In 5/10 mWC cases and in 4/5 mCNV cases the prenatally detected aberration was confirmed in the postpartum placenta. Three postpartum placentas revealed various complex aberrations differing from the prenatal results: (1) mosaicisms for different deletions/duplications on 9p and 9q in all samples (prenatal: mosaic 5.3 Mb duplication on 9p24), (2) different regions with deletions/duplications/loss of heterozygosity on 1p in all samples (prenatal: mosaic 2.3 Mb 1p36 duplication), and (3) mosaicism for a duplication on 5q and a deletion on 6p in one out of five samples (prenatal: mosaic trisomy 7). CONCLUSION: CNVs constitute a complex subgroup in placental mosaicism. Counseling of these couples after chorionic villus sampling should not focus on the specific CNV involved, but on the nature of mosaicism and the option of amniocentesis and ultrasound.

Clinical genetic diagnostics in Danish autosomal dominant polycystic kidney disease patients reveal possible founder variants.

BACKGROUND: Autosomal dominant polycystic kidney disease (ADPKD) is the most common heritable kidney disease. ADPKD leads to cysts, kidney enlargement and end-stage renal disease. ADPKD is mainly caused by variants in PKD1 and PKD2, with truncating PKD1 variants causing the most severe phenotype. This study aimed to characterize variants in Danish patients referred for screening of genes related to cystic kidney disease. METHODS: 147 families were analysed for variants in PKD1, PKD2 and GANAB using next generation sequencing and multiplex ligation-dependent probe amplification. If a variant was identified, relatives were analysed for the specific variant using Sanger sequencing. RESULTS: A pathogenic or possibly pathogenic variant was identified in 87% (103/118) of patients suspected to suffer from ADPKD, according to the requisition form. In total, 112 pathogenic or possibly pathogenic variants were observed, of which 94 were unique; 74 (79%) in PKD1 and 20 (21%) in PKD2, while 41 variants were novel. No variants in GANAB were observed. Ten recurrent variants were observed in 26 (26%) families. These were either PKD2 variants (N = 6) or non-truncating PKD1 variants (N = 4). Five of these were likely founder variants. CONCLUSIONS: The distribution of pathogenic or possibly pathogenic variants in the Danish ADPKD population is similar to that in other populations, except that recurrent truncating PKD1 variants appear to be rare, i.e. founder variants tend to be variant types associated with a mild phenotype. Patients with a mild phenotype may remain undiagnosed, consequently the frequency of founder variants and prevalence of ADPKD may be underestimated.

PAX2 variant associated with bilateral kidney agenesis and broad intrafamilial disease variability.

Pathogenic variants in PAX2 have previously been associated with renal coloboma syndrome. Here we present a novel variant c.68T>C associated with bilateral kidney agenesis, minimal change nephropathy, ureteropelvic junction obstruction, duplex kidney with hydronephrosis of upper pole system and bilateral kidney hypoplasia within the same family. Additionally, two family members were found to have optic nerve abnormalities further supporting the impact of the PAX2 variant. This is the first report of a PAX2 variant associated with bilateral kidney agenesis.

National data on the early clinical use of non-invasive prenatal testing in public and private healthcare in Denmark 2013-2017.

INTRODUCTION: In Denmark, non-invasive prenatal testing (NIPT) has been used since 2013. We aimed to evaluate the early clinical use of NIPT in Danish public and private healthcare settings before NIPT became an integrated part of the national guidelines on prenatal screening and diagnosis in 2017. MATERIAL AND METHODS: NIPT data were collected between March 2013 and June 2017 from national public registries and private providers. Results from follow-up samples (chorionic villi, amniotic fluid, postnatal blood or fetal tissue) were included from The Danish Cytogenetics Central Registry and indications and outcome from The Danish Fetal Medicine Database. RESULTS: A total of 3936 NIPT results were included in the study from public hospitals (n = 3463, 88.0%) and private clinics (n = 473, 12.0%). The total number of prenatal tests was 19 713 during the study period: 20% were NIPT analyses (n = 3936) and 80% invasive procedures (n = 15 777). Twenty-five percent of NIPTs in the private clinics were performed before gestational week 11+0 , whereas NIPT in public settings was used only after combined first trimester screening (P < .001). Regardless of indication, the national public sensitivity was 96.9% (95% CI 82.0%-99.8%) for trisomy 21, 100% (95% CI 46.3%-100%) for trisomy 18, 100% (95% CI 5.5%-100%) for trisomy 13, and 87.0% (95% CI 74.5%-92.4%) for any fetal chromosomal aberration. Forty-seven true-positive NIPT results included cases of common aneuplodies (trisomy 21, n = 31; trisomy 18, n = 5; and trisomy 13, n = 1), sex chromosomal aberrations (n = 7) and atypical chromosomal aberrations (n = 3). One false-negative NIPT result occurred (trisomy 21). Of 47 cases, 21 (45%) cases with a true-positive NIPT result resulted in live births by choice; 11 of these children had Down and 4 had Edwards syndrome. CONCLUSIONS: The total number of NIPT analyses was low compared with the number of invasive procedures in the implementation period. In contrast to the generally high termination rate after a positive result following invasive testing in Denmark, a high proportion of true-positive NIPT results from the public setting resulted in live births. NIPT may be an important risk-free alternative to invasive testing for a minority of women in the public setting who wish to use prenatal genetic testing for information only and not for reproductive decision-making.

Cell-based non-invasive prenatal diagnosis in a pregnancy at risk of cystic fibrosis.

OBJECTIVE: We aimed to develop cell-based NIPT for cystic fibrosis (CF) and test a pregnancy at risk of two common pathogenic variants. METHOD: A pregnant woman carrying monozygotic twins opted for prenatal testing as she and her partner were heterozygote carriers of F508del (c.1521:1523del). The partner was also positive for the CFTR-related variant R117H (c.350G>A). Fetal trophoblasts from maternal blood were enriched and isolated using antibodies and a capillary-based cell-picking instrument. Multiplex PCR-based fragment length analysis was performed on the extracted fetal DNA for STR-genotyping, fetal gender and F508del variant status. The R117H variant status was tested using SNaPshot analysis. RESULTS: The fetal origin of the isolated cells was verified by detection of two paternally inherited STR alleles and an Y chromosome marker, while no maternal DNA contamination was detected. The direct variant analysis detected F508del heterozygosity and the SNaPshot analysis for R117H detected only the normal allele. Thus, the results showed that the fetuses were healthy carriers of F508del, concordant with the findings of conventional prenatal testing. CONCLUSION: Cell-based NIPT could accurately state the fetal variant status and distinguish fetal trophoblasts from maternal cells. In the future, cell-based NIPT may provide an accurate less invasive alternative to chorionic villous sampling.

[Patients with a kidney disease can benefit from a specific genetic diagnose].

This review describes clinical characteristics, mode of inheritance, and molecular genetic testing for the following monogenic kidney diseases: polycystic kidney disease, Alport syndrome, autosomal dominant tubulointerstitial kidney disease, and nephronophthisis. The same is described for steroid resistant nephrotic syndrome, kidney stones and congenital anomalies of the kidney and urinary tract, which in some cases have a monogenic cause. Knowledge of possibilities within molecular genetic testing may help more kidney disease patients to receive a specific diagnosis.

A Gene Implicated in Activation of Retinoic Acid Receptor Targets Is a Novel Renal Agenesis Gene in Humans.

Renal agenesis (RA) is one of the more extreme examples of congenital anomalies of the kidney and urinary tract (CAKUT). Bilateral renal agenesis is almost invariably fatal at birth, and unilateral renal agenesis can lead to future health issues including end-stage renal disease. Genetic investigations have identified several gene variants that cause RA, including EYA1, LHX1, and WT1 However, whereas compound null mutations of genes encoding α and γ retinoic acid receptors (RARs) cause RA in mice, to date there have been no reports of variants in RAR genes causing RA in humans. In this study, we carried out whole exome sequence analysis of two families showing inheritance of an RA phenotype, and in both identified a single candidate gene, GREB1L Analysis of a zebrafish greb1l loss-of-function mutant revealed defects in the pronephric kidney just prior to death, and F0 CRISPR/Cas9 mutagenesis of Greb1l in the mouse revealed kidney agenesis phenotypes, implicating Greb1l in this disorder. GREB1L resides in a chromatin complex with RAR members, and our data implicate GREB1L as a coactivator for RARs. This study is the first to associate a component of the RAR pathway with renal agenesis in humans.

A mild form of Stickler syndrome type II caused by mosaicism of COL11A1.

Stickler syndrome, a clinically as well as molecularly heterogeneous connective tissue disorder, is predominantly inherited in an autosomal dominant manner and is considered complete penetrant. Previously, mosaicism in Stickler syndrome has been reported in only a few cases. We describe a child with Stickler syndrome due to a novel splice site mutation in COL11A1. Initially, Sanger sequencing of both parents showed normal test results for the mutation. Due to mild phenotypic traits, the father was tested again using a more sensitive method (NGS), and was found to have low-grade mosaicism in various tissue samples (range 7-22% of the DNA). Therefore, we recommend using sensitive genetic testing when mosaicism is suspected. Furthermore, we support previous suggestions of parental testing even when the parents of an affected patient do not have obvious phenotypic signs of Stickler syndrome.

Mandibulofacial Dysostosis with Microcephaly: Mutation and Database Update.

Mandibulofacial dysostosis with microcephaly (MFDM) is a multiple malformation syndrome comprising microcephaly, craniofacial anomalies, hearing loss, dysmorphic features, and, in some cases, esophageal atresia. Haploinsufficiency of a spliceosomal GTPase, U5-116 kDa/EFTUD2, is responsible. Here, we review the molecular basis of MFDM in the 69 individuals described to date, and report mutations in 38 new individuals, bringing the total number of reported individuals to 107 individuals from 94 kindreds. Pathogenic EFTUD2 variants comprise 76 distinct mutations and seven microdeletions. Among point mutations, missense substitutions are infrequent (14 out of 76; 18%) relative to stop-gain (29 out of 76; 38%), and splicing (33 out of 76; 43%) mutations. Where known, mutation origin was de novo in 48 out of 64 individuals (75%), dominantly inherited in 12 out of 64 (19%), and due to proven germline mosaicism in four out of 64 (6%). Highly penetrant clinical features include, microcephaly, first and second arch craniofacial malformations, and hearing loss; esophageal atresia is present in an estimated ∼27%. Microcephaly is virtually universal in childhood, with some adults exhibiting late "catch-up" growth and normocephaly at maturity. Occasionally reported anomalies, include vestibular and ossicular malformations, reduced mouth opening, atrophy of cerebral white matter, structural brain malformations, and epibulbar dermoid. All reported EFTUD2 mutations can be found in the EFTUD2 mutation database (http://databases.lovd.nl/shared/genes/EFTUD2).

A novel FBN1 variant in a large Marfan family with high penetrance of aortic dissection or rupture.

INTRODUCTION: Marfan syndrome is an autosomal, dominantly inherited disorder of the connective tissue. We report the clinical data and results of a genetic analysis of a large Danish Marfan family. METHODS: Sanger sequencing of FBN1 was initially performed on genomic DNA from the index patient. Subsequently, four affected family members and three non-affected family members were tested for the variant identified in the index patient. RESULTS: A novel variant (c.701G>T) in the FBN1 segregated with Marfan features in the family. CONCLUSION: In the majority of the family members, this novel variant seems to cause a uniform and very detrimental set of disease characteristics including fatal aortic dissection. FUNDING: not relevant. TRIAL REGISTRATION: not relevant.

Segmental overgrowth syndrome due to an activating PIK3CA mutation identified in affected muscle tissue by exome sequencing.

Mosaic PIK3CA-mutations have been described in an increasing number of overgrowth syndromes. We describe a patient with a previously unreported segmental overgrowth syndrome with the mutation, PIKCA3 c.3140A>G (p.His1047Arg) in affected tissue diagnosed by exome sequencing. This PIK3CA-associated segmental overgrowth syndrome overlaps with CLOVES syndrome and fibroadipose hyperplasia but is distinct from each of these entities.

Cobalamin and haptocorrin in human milk and cobalamin-related variables in mother and child: a 9-mo longitudinal study.

BACKGROUND: Measurement of milk cobalamin is hampered by the high content of the cobalamin-binding protein haptocorrin, and limited data are available relating trustworthy measures of milk cobalamin to cobalamin status in healthy mothers and their children. OBJECTIVES: The objectives were to explore the concentration of cobalamin and haptocorrin in foremilk and hindmilk during the first 9 mo of lactation and to relate these results to biomarkers of an impaired cobalamin status of mother and child. DESIGN: Milk samples from 25 mothers were collected at 2 wk, 4 mo, and 9 mo postpartum for the measurement of cobalamin and haptocorrin. Plasma samples from a larger cohort of lactating mothers (n = 107) and their infants (n = 108) were collected at the same time points for the measurement of cobalamin, holotranscobalamin, total transcobalamin, total haptocorrin, and methylmalonic acid. RESULTS: Median (range) concentrations of cobalamin in hindmilk were 760 (210-1880), 290 (140-690), and 440 (160-1940) pmol/L at 2 wk, 4 mo, and 9 mo, respectively; the respective haptocorrin concentrations were 25 (9-102), 22 (4-100), and 180 (30-460) nmol/L. We found slightly lower values in foremilk. A decrease in milk cobalamin at 4 mo was associated with decreases in plasma cobalamin (P , 0.0001) and holotranscobalamin (P , 0.0001) in the infants. Strong positive associations in paired maternal-infant cobalamin concentrations were found at all time points. CONCLUSIONS: Foremilk and hindmilk contained comparable amounts of cobalamin and haptocorrin, but marked changes were observed during 9 mo of lactation. At 4 mo, low concentrations of milk cobalamin mirrored biochemical changes in infants, which suggests an impaired cobalamin status and indicates that nutrition from only mother's milk may not be sufficient for the supply of cobalamin from this age. This trial was registered by the Danish Data Protection Agency at www.datatilsynet.dk/english as 2008-41-2185.